# Talk:Mirror

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## Mirrors in Mythology and/or Pop Culture

I find this to be an area where this article is severely lacking, and there's no other source that I could find on Wikipedia that would givethis is wrong me the information - specifically, the world behind mirrors, the use of mirrors to defeat enemies, etc. If someone could find the time to get information and put it up here, I would be most appreciative - it's one of the most difficult things to search the web for, because "mirror" aniorwghwererhlhewlrhirewhlehewuerwguerwqgreekrljgewrewruwghruiwgriutge4irgiurerkjwegrwerjjhgwgrjwgqrjgergwjrjwgjwrewjhgrjhwgrjewgrhjewgrjhwgrhwqrgjhwegr venjex programlso refers to a copy of a website on a different server. User:138.236.226.102 (attributed later)

The story about Medusa being defeated by using a mirror to avoid being turned to stone by her gaze is one example. --David Edgar 08:50, 24 October 2005 (UTC)

I've forgotten who did it, but there's that old story that... was it Gallileo? Managed to set a fleet of ships alight using polished bronze shields or mirrors to reflect the light. They tested the theory on Mythbusters and came to the conlcusion that although it is physically possible, the logistics of it make it pretty much impossible. Gemfyre 03:24, 15 January 2007 (UTC)

So you want to include a myth you don't remember based on something that was dispelled? Interesting. --173.15.125.237 (talk) 15:50, 10 May 2011 (UTC)
Actually, the story has been well documented, and other "myth-busters" have had different results over the ages.
In the book Burning Mirrors, Diocles first describes parabolic mirrors around 200 BC. During the Roman siege of Syracuse, from 212 to 214 BC, Archimedes is said to have burned several of the sieging ships with a large array of bronze mirrors, in which the focus could be adjusted. Archimedes was well known as a designer of weapons for the King of Syracuse, and he came up with some amazing creations, including some of the largest catapults ever, as well as a pulley system for lifting enemy ships right out of the water. Many of these far-out inventions are mentioned by scholars like Pliny the Elder, and Polybius.
The first to give a detailed description of Archimedes mirrors was Lucian of Samosata and Galen, around 100 BC. Anthemius of Tralles gives another description of the mirrors, and so does Johannes Zonaras.
In 514 AD, Proclus used a similar array of mirrors to destroy the ships of Vitellius. Proabaly the most famous of the description of Archimedes' mirrors comes from Johannes Tzetzes. Johannes compiled his information from the works of Cassius Dio and Diodorus Siculus. Johannes writes:
When Marcellus withdrew them [his ships] a bow-shot, the old man [Archimedes] constructed a kind of hexagonal mirror, and at an interval proportionate to the size of the mirror he set similar small mirrors with four edges, moved by links and by a form of hinge, and made it the center of the sun's beams – its noon-tide beam, whether in summer or in mid-winter. Afterwards, when the beams were reflected in the mirror, a fearful kindling of fire was raised in the ships, and at the distance of a bow-shot he turned them into ashes. in this way did the old man prevail over Marcellus with his weapons.
This has be exhaustively discussed by scientists since then. One of the first "myth-busters" was Comte de Buffon in 1747. Buffon realized that the real problem was creating a parabolic mirror with an adjustable focal length. He created such a mirror array, using 168 mirrors, measuring 10 inches by 8 inches each. He performed various tests. At 66 feet away, he found that only 40 mirrors were needed to ignite a creosoted board. At 150 feet, it took 128 mirrors to burn up an untreated (higher reflective) pine board. At 20 feet away, it only took 45 mirrors to melt six pounds of tin.
In 1975, Dr. Ioannis Sakkas used 60 men, each holding a mirror, to burn up a ship that was 160 feet away. The newspaper article reports that the ship caught fire instantly. In 2002, a group of German scientists had 500 men, each holding a mirror 45 cm in diameter, focus the energy on a ship that was 160 feet away. The energy focused was over 100 KW, and the ship caught fire within seconds. Zaereth (talk) 19:01, 10 May 2011 (UTC)

## One-way mirrors

Concerning one-way mirrors: I read "somewhere" that you can tell if a mirror is one-way by putting a finger to the reflecting side. If the reflection of your finger doesn't touch your finger, but is a fraction of an inch offset, then it's a one-way mirror. Is this true, or just an urban legend? -- Zoe

Urban legend, I think. The distance between your finger and its reflection is exactly equal to the thickness of the glass, since you are touching the front of the glass and it's mirrored on the back - and that should be the same for both conventional and one-way mirrors. Sometimes you can detect a one-way mirror by making a "cup" with your hand around your eyes as you peer into it closely, since the "one-way" effect depends on it being darker on one side than the other. -- Someone else 02:47 Dec 9, 2002 (UTC)
The distance between your finger and its reflection is twice the thickness of the glass! Patrick 11:35 Dec 9, 2002 (UTC)

I've noticed that at least one dictionary says that one way mirrors can sometimes be called two way mirrors. O.K., but what should an intelligent person say? Dhodges 01:19, 31 Jan 2004 (UTC)

Yes, surprisingly it seems to be the same!--Patrick 09:42, 31 Jan 2004 (UTC)
Yes, a one way mirror is simply a mirror that is lit from the side you are observing from, or one without an extra cover on the reverse side. silvarbullet1 02:19, 10 June 2006 (UTC)

A beam can't be parallel on its own, it has to be parallel with something, like another beam.--ZorroIII 13:12, 2004 Sep 5 (UTC)

## No such thing as a one-way mirror!

There is no such thing as a one way mirror. It it did exist, it would allow energy to flow from cold objects to hot objects in contradiction to the laws of thermodynamics. Imagine a cold room separated from a hot room by a one way mirror. The mirror reflects the energy from the hot room back inwards, while allowing the energy from the cold room to pass into the hot room. You could run an engine (or thermocouple) off the temperature difference. Voila! Free energy! Perpetual motion! Reversal of entropy!

• Mirrors reflect....light.
Light and heat are all the same thing, moron. CGS 20:14, 20 Feb 2004 (UTC).
Yeah. Right. Too late to get a refund on that thermodynamics course? That grammar course? That finishing school? -- Nunh-huh 20:53, 20 Feb 2004 (UTC)
I'm a computer scientist, not a physicist, and I've never studied thermodynamics but I even I know that light is a form of electromagnetic radiation and that heat can travel by electromagnetic radiation. Travel through a one way mirror, for example. What's wrong with my grammar? CGS 10:16, 21 Feb 2004 (UTC).
The phrase "X and Y are all W" would pull most people up short: generally it would be said that "X and Y are both W". It's not frankly wrong, just unidiomatic. Fine for a talk page, and wouldn't have provoked comment if not preceeded by a false assertion and followed by a rude epithet. -- Nunh-huh 21:38, 21 Feb 2004 (UTC)

What does exist is a partial mirror. The reflective coating allows some percentage of incident light to pass through, some percentage is reflected back (and some percentage is absorbed). When one side of such a mirror is dark and the other side is light, one side sees through, the other side sees their own reflection.

• And these partial mirrors are called one-way mirrors. Etymology is not meaning. -- Nunh-huh 20:07, 20 Feb 2004 (UTC)

One-way windows, I believe! Arthurvasey (talk) 11:43, 18 June 2012 (UTC)

The article once said (before I deleted this paragraph):

Note that a one way mirror that only allows light to pass in one direction does not exist. It would allow energy to flow from cold objects to hot objects in contradiction to the laws of thermodynamics. Imagine a cold room separated from a hot room by such a mirror. The mirror reflects the energy from the hot room back inwards, while allowing the energy from the cold room to pass into the hot room. You could run an engine (or thermocouple) off the temperature difference. The result would be free energy, perpetual motion, reversal of entropy!

It's certainly something interesting to think about, but somehow I think that trying to explain one-way mirrors (really pretty simple) in terms of thermodynamics (too few people have a correct understanding of it) is not very helpful. (EditHint: Perhaps move to a physics article -- perhaps http://wikibooks.org/wiki/Thermodynamics ?)

And this explanation in particular is incorrect. It wouldn't violate thermo if the mirror uses the temperature differential to extract energy *from* the 2 rooms (or obtained energy in some other way) (dumping some heat in the cold room), then used that energy to preferentially send visible light in only one directions. Are television cameras impossible ?

There's no such thing as an empirical law: One cannot proscribe anything based on inductive edict, much less consider it. And yes, there is such a thing as a "one-way mirror"; it's called a matter and|or energy attractor. (Despite the name, it can also repel, as seen in the accelerating Hubble flow.) As I explained in my treatise Refutation of Thermodynamic Laws submitted to the free_energy eGroup over a year ago, which no one could refute, and following posts a tunable optic and sonic black hole can and does violate the increasing entropy law. Of course it's easier to narrow the kinds that go one way for a given object/system, but cases already exist. lysdexia 09:09, 20 Oct 2004 (UTC)

From the point of view of the effect that people would like to achieve a true one way mirror is entirely possible, the only thing that the laws of thermodynamics prohibit is a passive one way mirror. There is nothing that would prohibit an active one way mirror that had an external power supply.

I'd like to see some info also added here about chromatic mirrors. ie, mirrors that selectively reflect/transmit based on wavelength. unfortunately I don't seem to have the time... --Morbid-o 15:50, 20 Jun 2005 (UTC)

I've readded a phrase about the thermodynamic implications of a one-way mirror since I think it's very relevant.192.129.3.196 12:27, 29 April 2006 (UTC)
It's enough for the true one-way (visible light) mirror to heat up and radiate infrared to be thermodynamically correct, isn't it? It's only a problem if you want the whole spectrum to be blocked from one way and allowed from another. First of these properties is a black body fantasy already; the other is exhibited by vacuum. 130.233.105.30 (talk) 08:58, 5 March 2015 (UTC)
The light absorbed by the mirror, which is turned into heat, does not transmit through the mirror, nor is reflected off the mirror. Therefore, it is inconsequential to the image. The glass itself is opaque to the infrared, blackbody wavelengths. The typical cut-off for glass is around 2000 nm or so, depending on the type of glass. At most, the amount of spectrum that can transmit through the glass is between 170 and 3000 nm for quartz, and less for other types. The amount of light that does transmit is inversely proportional to the amount reflected, minus the absorbed energy, and this is true for both directions. It is impossible to make a mirror that acts like a diode, in which total transmission is allowed in one direction and total reflection is produced in the other. Transmissivity is the same from both directions. This is simply a side-effect of the massless, boson nature of light, which doesn't react to incoming traffic in the same way that a fermion does. As a consequence, any transmissive mirror will be a two-way mirror, with exactly the same transmission characteristics from either direction. The only way to make it a "one-way mirror" is to lower the eye's visual acuity to the transmission on one side, by increasing the light reflected on that side, and to do the opposite on the other side by darkening the room, reducing the reflections and enhancing the transmission. Zaereth (talk) 09:40, 5 March 2015 (UTC)
Another way to think about it is that a window is a two-way mirror, with only 4% reflectivity from the first surface (less from the second surface). In the day, it's very good at keeping people from looking in, but in the dead of night I close the curtains, because I can't see out but everyone outside can see in. A two-way mirror (one-way mirror, output coupler, etc...) is simply a window with higher reflectivity. Zaereth (talk) 01:25, 13 March 2015 (UTC)

## mirror-like objects

I've seen some interesting ... um ... mirror-like objects. Instead of trying to evenly put a layer of some incredibly precise thickness all over the glass, it has (when you look closely at once side) small silvery metal polka-dots all over the glass (like halftone). That makes it easy to understand that *exactly* the same percentage of light that is transmitted one way is also transmitted the other way (it's the percentage of unobstructed clear glass). But the percentage of light *reflected* one way is *different* than the percentage of light reflected the other way. (When you look closely at the *other* side with a magnifying glass, you see exactly the same pattern of polka-dots, but they are black paint). Someone painted large advertisements on the windows of local city buses in a similar way. On the outside the polka-dots are all different colors -- from a distance they blur together into some advertising image. On the inside of the bus, the windows just look heavily tinted until you look closely and see the all-black polka-dot pattern.

... or is it just so dark inside the bus that the sunlit scene outside swamps the colored dots ?

-- DavidCary 03:17, 12 Jun 2004 (UTC)

## glass is easier to scratch than aluminum

The article currently says "Front silvered mirrors, where the reflecting surface is placed on the front surface of the glass, have a better image quality ..." which is excellent, but then it goes on to say "... but are easily scratched and damaged."

I don't think that's the real reason.

I always thought aluminum was *stronger* than glass. That's why people make mostly-aluminum airplanes, instead of mostly-glass airplanes -- right ? So wouldn't it be *more difficult* to scratch a slab of glass protected by a front coating of aluminum ?

-- DavidCary 02:01, 12 Jun 2004 (UTC)

"Stronger" is perhaps too vague a word. Glass is harder than aluminum, but aluminium is tougher than glass. (See [1]) In other words, a car built of glass would shatter under stress, but at least it would resist being keyed. :) --Arteitle 08:25, Aug 18, 2004 (UTC)

## Mirror Manufacturing

There seems to be plenty on the "what mirrors are made of" but little on the how they are made. I'm interested in the heat processes used in the manufacturing of mirrors. What kind of ovens do they use? How much heat do they need to see and for how long?--68.213.45.251 13:50, 1 Oct 2004 (UTC)David

I agree. I've reorganized the manufacturing section, but there's a lot to be added (e.g. details on different coatings, why choose Aluminum over Silver or vice versa, shaping non-planar mirrors, etc). --Dan Griscom (talk) 03:55, 1 January 2008 (UTC)

Some information about magnetron sputtering technology could be relevant too. — Preceding unsigned comment added by Jolanil (talkcontribs) 14:42, 5 August 2013 (UTC)

Cecil Adams in an old Straight Dope column ([2]) mentions the existence of a mirror which doesn't reverse the image. He says "I'm told that somebody has designed a mirror that uses a complex combination of concave and convex surfaces (although presumably still basically spoonlike) to produce a "right-reading" image--a mirror, in other words, that shows us not a "mirror image" of ourselves, but rather the appearance we present to the rest of the world." Does anyone have any information on these mirrors, or an external link?

-- Jackdavinci 23:32, 11 Oct 2004 (UTC)

Yep, and they're not nearly as complex as he supposed. The True Mirror is just a pair of regular flat mirrors joined at a 90 degree angle. This lets one see himself or herself just as others would, which is possible because this is reversing the image left-to-right, which normal mirrors don't do. Cecil was somewhat unclear about that in his article. --Arteitle 05:43, Oct 12, 2004 (UTC)
I think something about such mirrors needs to be in the article. -Rolypolyman (talk) 17:23, 22 January 2008 (UTC)

## Applications - Image

Girl looking at herself in the mirror.

Firstly, I'm not sure this image is necessary. It looks to me like someone is basically using it to advertise. Also, given the potential copyvio, as well as anyone who might innocently visit the site stamped on the picture, I think this image needs to go. --Morbid-o 15:48, 20 Jun 2005 (UTC)

ok, moved here for now. --Morbid-o 15:40, 23 Jun 2005 (UTC)

A few lines about physics would be nice...it's not obvious why light,an electromagnetic wave, reverses its direction when hitting a mirror.

Stefan Udrea 15:48, 11 October 2005 (UTC)

## Practical Tips

It would be nice to see some practical tips on buying mirrors - like how to test the quality of a mirror.

I usually stand a foot away and look at objects about 3 feet behind me and at the level of my feet - if these slightly distant objects which are also at an angle appear clear, I consider the mirror to be of a decent quality.

Of course, there are multiple ways of testing mirrors. One is the 'granny tips' method (as above), and there's technical methods too - I'm sure mirror manufacturers have some quality testing procedures. Mirrors might also come with same quality rating or reflection rating.

Sam

It would be nice to see something about the first mirrows made by men. It seems that during Ancient Roma times, all the mirrows of the upperclass Roman people were imported directly from China by way of the Silk Road (as well as the silk itself and other luxury things). That is, Roman people did not know how to make one of them by themselves... but Chinese people knew. 81.203.157.106 20:12, 14 June 2006 (UTC)

Jeremiahmccarver (talk) 04:46, 31 January 2014 (UTC)You might mention that bronze mirrors were also mentioned in the book of Exodus, reflecting the culture of the time. Source: Exodus 37:8Jeremiahmccarver (talk) 04:46, 31 January 2014 (UTC)

Yes, metal mirrors were most likely the first mirrors available, and are mentioned in many sources. Even the ability to coat glass with metals has been around for thousands of years. Amalgams were the best ways to coat glass, because the coating has an amorphous structure, which has much better reflectivity than crystalline metals. This process was used for coating armor and other objects long before it was applied to glass, not just in China, but in Europe as well. It is not uncommon for a technology to develop independently, without each having knowledge of the other, and this is quite likely with mirror coatings as well.
The real problem was making a piece of glass with a smooth enough surface and an even thickness. For a long time, the only practical way to do this was by blowing a glass bubble, and then cutting out a small, circular section, giving a small convex/concave circle of glass with a smooth enough surface to produce a decent reflection. The earliest glass mirrors were small, handheld mirrors, typically only a few inches in diameter (3 to 5), and had a convex surface. These were often fitted into small cases or paddle-shaped handles. This produced a distorted image, where "objects in the mirror were larger than they appear," but this was better than polished metal mirrors.
The real innovation came from Italy. This was not from amalgams, but from the glassworker's new process for creating flat, smooth sheets of plate-glass, allowing the creation of flat glass mirrors of nearly any size. Zaereth (talk) 10:47, 31 January 2014 (UTC)

## One-way mirrors/glass

File:Electromagnetic spectrum.JPG
Electromagnetic spectrum with visible light highlighted

The article says:

Contrary to popular belief, one-way mirrors that function well between equally lit rooms do not exist. The laws of physics do not allow for real, passive one-way mirrors (i.e. that do not need external energy); if such a device was possible, one could break the second law of thermodynamics, and make energy flow from a cold object to a hot one, by placing such a mirror between them."

Essentially a reference to Maxwell's demon. When I first read it, I thought "darn, I guess it's not possible". But now that I think of it, I think the sentence is too simplistic. The sentence alludes to externally-powered devices, which seems plausible, but maybe aren't even necessary? We're only concerned with light here, not all forms of radiation, so as long as the energy is still moving equally in both directions, the light can be going in only one. If the light in one direction was absorbed by objects on the other side, and re-emitted as blackbody radiation which traveled backwards through the mirror, for instance, that might work. Anyway, it seems like an oversimplification. — Omegatron 05:25, 9 July 2006 (UTC)

The statement is generally correct, but perhaps a bit oversimplified. Thermodynamics makes it impossible to make a true broadband passive one-way mirror. I recall seeing the proof of this years ago in an undergrad physics class, so the claim is a standard one for which references could be found if necessary. You seem to be thinking of light and blackbody radiation as two distinct things. All blackbodies at greater than absolute zero will emit some visible light, and at high temperatures they can emit quite a lot of visible light. If you separate two blackbody cavities that are initially at the same temperature with a "one-way mirror", you won't have thermal equilibrium. To achieve equilibrium, one of the cavities would have to become at least slightly warmer than the other, which would violate the laws of thermodynamics. You can imagine a solution in which the mirror transmits infrared better in the direction in which it reflects visible light, so that the energy flows balance. This probably still won't work, though, because the choice of blackbody temperature is arbitrary. The mirror must not violate the laws of thermodynamics for any choice of temperature for the two blackbody cavities. Both cavities could be "white hot" (emitting strongly in the visible), or even hot enough that their emission peaks in the ultraviolet. The mirror has to allow equal energy flow between two cavities at the same temperature, regardless of what that temperature is.
It is certainly possible to make a narrowband "one-way" optical device, however. A Faraday isolator can do this over a very narrow range of wavelengths, such as a laser line. I haven't thought about how this avoids violating thermodynamics. I presume there's a simple explanation that makes use of the extremely narrow bandwidth.--Srleffler 04:17, 17 July 2006 (UTC)
I'm well aware that you can't make a device that works perfectly and for all frequencies. That's what the article says and I think that its status as a "standard undergrad physics problem" is the reason it's being brushed off as impossible without actually thinking about it.
For a useful mirror, you only need to transmit/block visible light. As far as the electromagnetic spectrum is concerned, this is a narrowband device. So whatever's working for the Faraday isolator could work for this, too.
For comparison, imagine that the boxes are separated by a perfect, Maxwell's demon, broadband one-way mirror, but only halfway. Energy can freely flow in both directions through the other half of the passageway. Does this violate any laws? What if the passageway is then blocked off completely, but only for half of the spectrum?
Blackbody radiation is a statistical spectrum of many different frequencies, some of which will always be able to flow through the theoretical mirror, regardless of temperature. I have a hunch (just a hunch) that the energy will "find a way" to even out in every situation. The second law also doesn't prohibit a temporary decrease in entropy, as long as the long-term trend is towards equilibrium. There are a lot of loopholes.
I'm not talking about an ideal, perfect device that works for all frequencies. I'm talking about a practical, useful device that works for only light, or another selected portion of the spectrum. There are no other assumptions. The device might not be 100% efficient at blocking/transmitting light. 99% is fine for a useful device. There are no assumptions about how the mirror itself absorbs and re-emits heat, etc.
It's not a very good analogy (since energy is eventually traveling from a very hot object to an enclosed area that is much cooler), but consider a greenhouse while thinking about it. — Omegatron 12:36, 17 July 2006 (UTC)
It turns out that a Faraday isolator is not a counterexample. See, for example
Rayleigh, On the magnetic rotation of light and the second law of thermodynamics, Nature (London), Vol. 64, p. 577 (Oct. 10, 1901).
This paper is cited and discussed here. It turns out that the fact that a faraday isolator does not return the rejected light to its source is important. Any optical setup that allows the rejected light to be returned to its source would also open up a "leak" through the isolator. Faraday isolators don't violate the laws of thermodynamics, and it appears that they would not do so even if they were broadband devices.
Regarding your argument above, and your "hunch". The problem is this: the only way nature can "find a way" to even out the energy flow is by varying the temperature of one of the two blackbodies, because that's the only variable there is. The spectrum of a blackbody is determined solely by its temperature. Any such temperature variation could be used to violate the second law. I now don't think that it matters how narrow the spectrum of the one-way mirror is. Any true, passive, one-way transmission would violate the second law.
Please remove the "dubious" tag. This is an interesting issue, and I'll be happy to continue discussing it here, but the position you are proposing is original research, and so should not be reflected in the article (unless some new evidence comes up). Please do not restore to my last version of the text, just remove the dubious tag.--Srleffler 15:17, 17 July 2006 (UTC)
?? But your own reference says that it's possible. "With such an isolator, I could construct a one-way window such that I could see you but you not see me." — Omegatron 02:35, 18 July 2006 (UTC)
Crucial detail: It's a one-way window, not a one-way mirror. This goes to the core of Rayleigh's argument. You can have a device that only transmits light in one direction. You just can't have one that does that and returns the rejected light to its source.--Srleffler 05:27, 18 July 2006 (UTC)
Aha. All I was concerned with is the one-way transmission. So the one-way window is absorbing the incident radiation in one direction and re-radiating the energy into both chambers? — Omegatron 06:00, 18 July 2006 (UTC)
There are several different designs. Some Faraday isolators absorb the rejected light. Others emit it out of separate apertures in the sides of the device. Rayleigh's contemporaries thought that you could just set up mirrors to take the rejected light from the latter type of device, and inject it back into the chambers. Rayleigh showed that if you did this it would cause the isolator to "leak".--Srleffler 12:11, 18 July 2006 (UTC)
Because it would allow light to "backflow" through the "exhaust port" and end up in the other chamber, right? It's a little hard to understand their description without an image. In my mental image, the non-absorptive device would still be violating laws.
Maybe we should work on improving the Faraday isolator article. — Omegatron 13:48, 18 July 2006 (UTC)
Yes. If the "exhaust port" directs light back to the source it came from, light from that source can follow that path in the opposite direction. Because of the "one-way" properties of the Faraday rotating element, though, that light (which enters through the "exhaust" port) ends up going to the other source, so that the Faraday isolator no longer prevents transmission of light in that direction. No laws are violated. To really consider the nonabsorptive device properly, you have to consider all four ports, and include the environment that the two exhaust ports dump light into. Clearly the isolator will only function as expected if the environment outside the exhaust ports is darker (or colder) than the sources on the other two ports. If the exhaust environment is warmer than one of your sources, you get a net flow of energy in through the "exhaust" port. Note that if you just ignore what happens to the light dumped out the "exhaust" ports, you are essentially treating them as an infinite sink for energy. You then no longer have an isolated system, and the second law of thermodynamics does not apply.--Srleffler 03:39, 19 July 2006 (UTC)

## Extraordinary section - should go in a Lewis Carroll museum or start a WP museum

Has anyone noticed that ridiculously long passage about "how we think we ought to see ourselves" in mirrors? Does this make sense to anyone? It seems like utter babble and ranting about absolutely nothing of any scientific quality. Someone with a greater knowledge of this subject should edit it. Saeghwin 04:35, 4 August 2006 (UTC)

I had never read this article before but, now that I have, I have to agree with Saeghwin, it is an absolutely amazing piece of nonsense. Lewis Carroll would have been proud of it! Yes, it needs someone with a bit of expertise in writing on such topics to produce a few pithy lines that would substitute this WP curio and then we could start a WP museum page and this could go in proudly as the first exhibit. - Ballista 05:12, 4 August 2006 (UTC)

## X ray mirrors

Should the large para about nuclear bombs detonation actually be on this page about mirrors?. Maybe it would fit better at nuclear bomb--Light current 16:32, 7 August 2006 (UTC)

## Cut from page for refinement

Imagine you had a complete copy of your body that you could manipulate into different positions, and imagine that it is directly in front of you and facing the same direction as you, so that you are looking at its back. If you twist the copy around the vertical axis, as if it were turning to face you, and then compare that with your reflection in the mirror. The reflection will be different from the model because everything that should be on the left will be on the right.

But imagine instead that you twist the model about a horizonal axis, as if it were doing a handstand. The model would be upside down, facing you. If you compare this to yourself, and the reflection, then left and right are all correct. Your wedding ring, eye patch, and false leg are all on the correct side, be that east west north or south, but something is glaringly wrong about the reflection compared with the model, the reflection's feet are down at the bottom, where its head should be! Or, you could just keep the model in front of you so that you are looking at its back, and compare that with the reflection. Now left, right, up and down are all correct, but the reflection has its back where its front should be.

The model represents the way the 'real you' ought to look. If some other person looks at you, what would they see?. So you compare the reflection against what you think you 'ought' to see. If you think you ought to see what you look like when you're doing a handstand, then the reflection is upside down, and if you think you ought to see your own back, then the reflection is flipped front and back. But most people want to see themselves from the front, whilst standing up, they think their reflection ought to look like they would look if they turned around, and they think that left has been flipped with right.

In some sense what has 'really' been flipped is front and back. If you were to describe the body with co-ordinates, east/west north/south up/down, and the mirror has been facing south whilst you are looking north, then the difference is in the north/south direction; front and back. So I don't think my reflection looks a bit wrong because left has flipped with right, I think it looks hideously deformed because it has a face where the back of its head should be.

--Light current 16:40, 7 August 2006 (UTC)

Well done & thanks, 'Light current' - It's good to have taken this lot out for reworking - however, I feel there is still need for greater clarity in what's left in the article. I feel that most of it's in need of a pretty hefty rewrite, all round but it needs to be done by someone with greater technical know-how than I have, for technical accuracy. - Ballista 16:56, 7 August 2006 (UTC)

I certainly agree that far more clarity is needed possibly involving a complete rewrite of this section. I dont feel confident or knowledgable enough either ATM, so I just thought I'd start the ball rolling by removing the worst bits! Feel free to hack the rest of it about. Maybe we should take the rest of it out pro tem?-- Ill try to help where I can 8-)--Light current 17:26, 7 August 2006 (UTC)

Thank you for editing this. Saeghwin 17:55, 7 August 2006 (UTC)

Maybe the above could be used on psychology of vision or some such page? 8-)--Light current 18:03, 7 August 2006 (UTC)

## Images in plane mirrors

Rays from each and every point on the body travel to the mirror and get reflected into the subjects eye from the point you are directing your eye towards. There is no swapping over of the rays in transit to/from the mirror (you'd need a pinhole or a lens to do that). Therefore, the left side of your body forms a vitual image behind the mirror and to the left. The right side of your body is shown on the right, the top at the top, and the bottom.... etc. Think of your self as rotating in front of the mirror: When your body is horizontal, your 'left' side is at the bottom, right at top etc. You wouldnt expect it any other way would you?

This is therefore a NON PROBLEM and not really a paradox. Maybe all mention of it should be deleted and ref made to the optics pages.--Light current 17:50, 7 August 2006 (UTC)

## Not salvagable

cut from page ass not really salvagable. Maybe someone else can mak it make sense?

For an object with approximate reflection symmetry, a reflection in some mirror plane corresponds to a combination of:

• a translation if the mirror is parallel to the symmetry plane of the object, and otherwise a rotation about the line of intersection of the two planes by an angle which is twice the angle between the two planes
• a reflection in the approximate symmetry plane of the object (due to the assumption this is a minor change)

We can apply this to the image in a mirror of, say, a standing person, because people have approximate bilateral symmetry. The image is the most realistic if it is still vertical, i.e., if the rotation is about a vertical axis. This is the case if the mirror is vertical. In this case the image of the person is in normal standing orientation and vertically in a normal position, at a horizontally different position and with an orientation rotated about a vertical axis, the latter except if the mirror is parallel to the approximate symmetry plane of the person.

In particular, if one looks at one's image in a vertical mirror in left-right orientation, the image corresponds to a rotation by 180° about the vertical axis in the mirror, combined with a reflection in one's approximate symmetry plane.

#### Top to bottom reversal

Why does the mirror reverse left to right and not top to bottom? The answer is that it actually does reverse top to bottom.

The mathematical or geometrical version of the question is: "why does a chiral object (such as a right hand or glove) appear as an object of opposite chirality (left hand or glove) in the mirror?" The answer is that chirality of the three-dimensional space is dictated by the choice of the directions of the three axes. When the direction of one axis is reversed, as is the case in a mirror image, the chirality (or "handedness") of space changes to the opposite one. If two mirrors are set side by side (with, say, a 90° angle between them), the axes in the doubly reflected image are inverted twice and the "handedness" of the image is not changed. In such a double mirror, a right hand looks like a right hand. This set-up lets you see how you really look, but most people find it very difficult at first to use a mirror like this for shaving.

--Light current 17:56, 7 August 2006 (UTC)

If you stand in front of a mirror and point to the left the image also points to the left. However, if you point towards the mirror the image points back at you, so the image is actually reversed in and out rather than left and right, although it all seems to be a matter of perception. See Herefor a better explanation.Richerman (talk) 15:03, 10 January 2008 (UTC)

## Safety and easier viewing section

Most of the stuff under this heading doesn't seem to match the heading - it needs tidying up.

--Stevenayre 12:58, 6 September 2006 (UTC)

## Technology: Television and Projectors

With HDTV becoming more of a staple of consumer electronics, I thought that it was worth noting the information that I inserted under this heading. The two paragraphs are as follows:

Microscopic mirrors are a core element of many of the largest high-definition televisions and video projectors. The most common technology of this type is Texas Instruments' DLP, citation needed. A DLP chip is a postage stamp-sized microchip whose surface is comprised of an array of millions of microscopic mirrors. The picture is created as the individual mirrors move to either reflect light toward the projection surface (pixel on), or toward a light absorbing surface (pixel off).
Other projection technologies involving mirrors include LCoS. Like a DLP chip, LCoS is a microchip of similar size, but rather than millions of individual mirrors, there is a single mirror that is actively shielded by an liquid crystal matrix with up to millions of pixels. The picture is formed as light is either reflected toward the projection surface (pixel on), or absorbed by the activated LC pixels (pixel off). LCoS-based televisions and projectors often use 3 chips, one for each primary color.

As far as leading the Technology section; I'm sure it seems a little egotistical of me to put it at the top of the list, but I felt that being consumer driven technology rather than medical or military, it would be relevant to a greater number of readers. --Atomicskier 00:38, 29 December 2006 (UTC)

## Two-way mirror

The article does not mention two-way mirrors. —The preceding unsigned comment was added by Frap (talkcontribs) 08:27, 29 January 2007 (UTC).

It does - one-way mirrors.-69.87.200.211 12:19, 15 May 2007 (UTC)

## White surface vs. mirror reflectivity

If an ordinary mirror only reflects about 80% of the light, what is the range of overall reflectivity of visible light of ordinary white surfaces? Seems like important and relevant comparative info, for situations where imaging is not important (or not desired).-69.87.200.211 12:19, 15 May 2007 (UTC)

It looks like white paper can reflect up to at least 90%, so white surfaces seem to easily be as reflective, or more, compared to ordinary mirrors.-69.87.200.211 12:56, 15 May 2007 (UTC)

Yes, but there is a bit difference between specular reflection and diffuse reflection. Timb66 23:27, 15 May 2007 (UTC)

I think most people would be surprised to learn that a good white surface reflects more total light than an ordinary mirror! And, I think the 80% figure is so important that it needs a source reference, which hopefully would give a range for typical household mirrors. Also, it would be good to give a range of reflectance for ordinary black surfaces -- I'm guessing it is quite a bit more than the ~0% an ordinary person would assume? (If you are wondering why the Mirror article has to cover all these matters, well, I can't find anywhere that does -- so we need to pick somewhere, and gather the appropriate info, and link to there.)-69.87.204.241 17:17, 16 May 2007 (UTC)

## Reflectance Table

• Colors
70-80% White
70-80% Light cream
55-65% Light yellow
45-50% Light green
45-50% Pink
40-45% Sky-blue
40-45% Light grey
25-35% Beige
25-35% Yellow ocher
25-35% Light brown
25-35% Olive green
20-25% Orange
20-25% Vermilion red
20-25% Medium grey
10-15% Dark green
10-15% Dark blue
10-15% Dark red
10-15% Dark grey
• Materials
95% Mirror
80% Plaster
65-75% White enamel
60-75% Glazed white tiles
60% Maple
60% Birch
40% Light oak
15-20% Dark oak
15-20% Dark walnut
15-40% Concrete
5-25% Red brick
2-10% Carbon-black
6-8% Clear glass
• Lighting design
60-90% Ideal Ceilings
35-60% Ideal Walls
30-50% Ideal Countertops

-69.87.203.133 01:08, 25 May 2007 (UTC)

## Signal Mirror?

I was just looking for a reference to signal mirrors--used by sailors or outdoorsmen to alert rescuers to their location. This topic might be a useful addition. Fagiolonero 02:29, 10 August 2007 (UTC)

## Magic

What does a mirror have to do with a magic trick?--Kingforaday1620 22:33, 17 August 2007 (UTC)

## One-way Glass

Would it be possible for someone fluent with Optical Coatings to have a look at One-way Glass? It would be helpful to have the input from an expert on what the actual reflective substances are in this case and how they are applied to the glass. Also, I admit that I tend to understand its use from a Security point of view and balance is required in the article. Exit2DOS2000TC 07:50, 3 September 2007 (UTC)

## History of the Mirror

There is absolutely nothing on this at all! Johnbod 19:20, 7 September 2007 (UTC)

• I came here to make exactly the same comment. Yes, a history section is sorely needed in this article. Matt 03:09, 7 November 2007 (UTC). —Preceding unsigned comment added by 86.150.101.125 (talk)

I started it based on what was in How It's Made episode 305. Cburnett 21:06, 2 December 2007 (UTC)

Kudos to the string of recent anonymous users who greatly expanded the history section! Glad I could start it.  :) Cburnett (talk) 05:33, 13 December 2007 (UTC)

## Can you put a mirror Online?

Is it possible to have a mirror online where you can see yourself?

Thanks, Lisa —Preceding unsigned comment added by Thinklisa (talkcontribs) 17:12, 31 October 2007 (UTC)

Nope. A computer monitor cannot actively reflect light, only generate it. Of course, the front surface of the monitor can be reflective but the amount of reflectivity cannot be controlled. You could horribly fake it with a web cam but I'm sure that's not what you're wanting. Cburnett 20:48, 2 December 2007 (UTC)

## Mirrors and Animals

I have restored the list of animals in this section which was deleted by an anonymous IP address without explanation. I have left out the African Grey Parrot which seems to be a later addition to the original list and is most likely a joke. It is certainly not mentioned in the New Scientist article which is given as a reference for this section and it is flatly at odds with the text which specifies large brained animals.

^The magpie is not a large brained animal so that statement is false because magpie are one of only a few species of bird that indeed ca recognize themselves in mirrors. —Preceding unsigned comment added by 70.228.80.248 (talk) 20:05, 23 September 2010 (UTC)

Spinningspark (talk) 01:38, 17 December 2007 (UTC)

It may not be a joke - I was just listening to a radio documentary which mentioned that some birds as well as animals could recognise themselves in reflection. This was verified by placing coloured "stickers" on the bird in a position where they could only see it in the mirror. On seeing the reflection, they would remove the offending mark, and stop their attempts when the reflection was marker free.
If I find a citation for this, I'll post it. - Paul (talk) 14:56, 5 September 2008 (UTC)
It would be interesting to make the experiment without a mirror, but with two birds. Maybe it's only a signal for another bird to clean itself?--TeakHoken213.150.232.3 (talk) 15:07, 29 May 2012 (UTC)

## Non-optical mirrors.

There exist acoustic mirrors and atomic mirrors. They should be mentioned.
1. As for X-ray mirrors and the picture of the explosion due to reflection of X-rays, it looks as a confusion rather than a way of fusion. The subseciton about reflection of X-rays in the nuclear bomb is not supported by literature. The same should be addressed to the article Nuclear_weapons_design which uses the same picture without to justify it. Even worse: references

indicate that the efficient reflerction of X-rays is possible only at the grazing incidence, with the grazing angle of order of 10-6 radian. The figure indicates the revlection at angle of order of 45 degrees, this looks as either a joke or an error; it is 6 orders of magnitude out of reality. X-rays reflected from a solid angle 10-12 steradian should not have any implosive effect.
2. Let us remove the doubtful figure and provide links to other wiki-articles about reflection of invisible waves. dima (talk) 22:45, 7 April 2008 (UTC)

## Retroreflector

Retroreflectors include spherical mirrors, of which there exists a very common biological example, the tapetum lucidum of the many animals that display eyeshine. Please add appropriate mention of that to this article. --Una Smith (talk) 03:34, 2 June 2008 (UTC)

## Whats with this paragraph?

"The softness of old mirrors is sometimes replicated by contemporary artisans for use in interior design. These reproduction antiqued mirrors are works of art and can bring color and texture to an otherwise hard, cold reflective surface. It is an artistic process that has been attempted by many and perfected by few." Whats with the last sentence in this paragraph? I think that is merely opinion and there is no fact supporting it.209.194.173.227 (talk) 19:50, 3 December 2008 (UTC)

## Water

Isnt water the first mirror or the first way to see a reflection? —Preceding unsigned comment added by 151.202.101.178 (talk) 01:01, 2 January 2009 (UTC)

It used to say so. I don't know when or why that part was removed. I'll restore the text if I can find it. 86.134.47.11 (talk) 04:20, 25 January 2009 (UTC).

## Non-Reversing Mirror

If you'd like to help edit a new article on the non-reversing mirror (a.k.a. the mirror of the future)... just kidding, but who knows?... you can find it here. Thanks! Synesthetic (talk) 21:11, 5 February 2009 (UTC) =]

## Questionable sentences

In the face to face mirror section the last two sentences don't seem to make any sense. I'm tempted to just remove them, but thought I'd check to see if anybody knew what they are supposed to mean. Anybody got a clue about this?

Also, the manufacturing section seems pretty focused on the tin/mercury method. Perhaps it might be good to describe silvering, aluminum coating, and other techniques.

The second line in the "instruments" section is not accurate. Silver is by far more reflective than aluminum at shorter wavelengths. Silvered mirrors can be more than 99% reflective in the visual range, compared to aluminum, which is around 80%. (However, both can be enhanced and further protected with dielectric coatings.) Compare that to the less than 50% of a standard house mirror.) Silvered mirrors are still used extensively in lasers, and were the first mirror to be over 99% reflective. I think the quickest way to look this up is in the Edmunds Optics catalog or the CVI Laser catalog, but I'll look for more info.Zaereth (talk) 01:11, 23 September 2009 (UTC)

## Silver bonding

The article currently claims "The Tin(II) chloride is applied because silver will not bond with the glass". It seems to bond well when using Tollens' reagent and an aldehyde, a method which is not mentioned. And what is the "chemical activator"? Icek (talk) 17:35, 12 March 2010 (UTC)

What bonds to glass is often dependant on the type of glass used. As far as I know, silver will adhere to most common substrates. The first silver coated mirrors were made using a mixture of silver and nitric acid, (silver nitrate). The substrate was placed in the highly explosive solution, and the silver would precipitate into a thin film on the glass. The most common method today is vacuum deposition. Zaereth (talk) 17:47, 12 March 2010 (UTC)

## Misuse of sources

This article has been edited by a user who is known to have misused sources to unduly promote certain views (see WP:Jagged 85 cleanup). Examination of the sources used by this editor often reveals that the sources have been selectively interpreted or blatantly misrepresented, going beyond any reasonable interpretation of the authors' intent.

Please help by viewing the entry for this article shown at the cleanup page, and check the edits to ensure that any claims are valid, and that any references do in fact verify what is claimed. Tobby72 (talk) 14:33, 18 September 2010 (UTC)

## history of the mirror

in 1985 Mr Jack B. Ludwig published an article in Glass Digest (15th of Sept. 1985) with the title "do you know the history of mirrors?". His sources are not known but the contents of his article do not correspond entirely to the article presented in Wikipedia: Mr Ludwig states that the invention of combining glass with a metal foil to make mirrors has been attributed to Flemish glassmakers who made this discovery at the beginning of the Renaissance.194.78.5.197 (talk) 13:48, 17 December 2010 (UTC)

Well, I am not familiar with the foil method of making mirrors. I'm more familiar with the amalgam methods, (which the earliest known use for mirrors started about 1500 years ago), silver nitrate methods, and vacuum deposition methods. However, it's hard to argue with Pliney, who wrote in his encyclopedia about glass mirrors using gold foil, and that was written nearly 2000 years ago. Zaereth (talk) 20:33, 17 December 2010 (UTC)

## Mirror image

The article section "Mirror image" is somewhat poor. It's too informally written and inaccurate. As it reads, "If one looks in a mirror, one's image reverses (e.g., if one raises one's right hand, his left hand will appear to go up in the mirror.)". First of all, mirrors only invert depth. If you raise your right hand, which I'll call your A-hand, the mirror shows you raising your right "A"-hand (hell, the mirror can't possibly swap your hands!), only it's shown from your side. Only depth is inverted, as if you move your hand forwards towards the mirror, it's shown as moving backwards (towards you) in the mirror.

We tend to have a broken sense of "reversing" because we live in mostly flat environments and love to stay on our feet, which means the only way to face each other is to rotate 180º in the Z axis. We call that "not reversed", even though we are looking at space in an opposite direction, causing inversion. If we rotated in another axis to look at each other, e.g. by standing on our heads, we'd cause inversion in another axis. Mirrors don't do this: you just happen to what's on left on the left, and what's on the right on the right, and so on, but we aren't used to see ourselves from our own side, without any reversion. 217.125.117.197 (talk) 14:19, 8 June 2011 (UTC)

A mirror inverts an image in depth. This phenomenon is no different than the Hollow-Face illusion. Only a concave mirror would give a truely reversed image. Zaereth (talk) 17:11, 8 June 2011 (UTC)

## 180 degree reflection ?

Is it contended that the phenomenon of specular reflection allows an 180 degree angle of reflection? That doesn't sound possible. How about 2 90 degree reflections in series?WFPM (talk) 18:49, 13 December 2011 (UTC)

I'm not sure I understand the question. When discussing mirrors, the normal direction of reflection, (ie: perpendicular to the mirror, so that the light from a laser is reflected directly back at it), is usually referred to as a 0 degree angle of incidence. Any angle of relection that is different is usually measured from this 0 degree line. A 90 degree angle of incidence would imply that the light is moving straight across the surface of the mirror, like shining a laser across the mirror's plane. Typically, any reflection between about 80 and 90 degrees will produce total reflection off of the air/glass interface. 180 degree reflection would imply transmission. Perhaps, if you ar able to rephrase the question a little, I may be able to help point you to the answer you're looking for. Zaereth (talk) 19:27, 13 December 2011 (UTC)

Thank you! It's just that I can't visualize an individual atom being able to either 180degree rotate or else absorb and retransmit any light at an angle of 180 degrees from incidence. I can see it with 2 angles of reflection such as to add up to 180degrees, which must be the specular reflection property. And since the silver reflection property is a external surface? reflection phenomenon, I don't see how it is accomplished.WFPM (talk) 21:11, 13 December 2011 (UTC)

OK, I think I understand now what you're asking. When light is reflected off a mirror, it doesn't change direction 180 degrees from its original path. Instead, it's reflected back along its original path with no change in orientation. In other words, if I shine a laser at a mirror, the light on the left side of the mirror returns along it's original path, as does the light on the right side of the beam. The beam has inverted in the field of depth, but there is no change in orientation, so its returning along a 0 degree angle.
Two mirrors placed at a 45 degree angle of incidence would give such a reflection. This type of reflector is called a retroreflector. If I shine my laser at a mirror with 45 degrees angle of incidence, the beam reflects off of it at another 45 degree angle, (for a total of 90), but has inverted in it's field of depth. When the beam reflects off the second mirror, it makes another 90 degree turn, (for a total of 180) and inverts the field of depth back to normal. In the case of an image, your image's right hand is on your own left-hand side, and the logo on the t-shirt reads normal. However, the image (or laser-beam) do not return on their original paths. Zaereth (talk) 22:54, 13 December 2011 (UTC)
Yes I've seen some of those 90 degree angled mirror reflections. And they are intriguing, but don't involve an 180 degree angle of reflection. But at the atomic reflection level you're asking me to believe in a 180 degree angle of reflection, which I can only imagine as follows: In your OK paragraph you say that on one side of the atom the light is reflected back 180 degrees, and on the other side it goes forward in the same backwards direction, which would imply that the first side was traveling in the same direction and at the same velocity as the incoming light!!!. I've always thought that in order to be reflected a light had to pass around and behind the atom and then leave at an equal angle direction. That would preclude your left side reflection theory and only permit your right side return theory, and thus would increase all reflection values by 180 degrees. I've never seen it explained like that and I wonder if you would agree with me?WFPM (talk) 01:33, 14 December 2011 (UTC)
I moved your comment out of the middle of mine, so as not to be confusing to other users. Perhaps I am still misunderstanding the question. You asked about specular reflection, but you can't define specular reflection on the atomic scale. Specular reflection requires an entire wave-front and a smooth surface, neither of whichis possible with individual photons or atoms. When talking about reflection on the atomic scale, it is usually in terms of absorption of the light-energy (photon) by the atom, and reemission in a direction equal to, but opposite of the angle of incidence. If the angle of incidence is 0, the reflected angle is also 0. If the angle is 45, then the reflected angle is also 45, but on the opposite side of the normal. This is discussed in books like Physics for scientists and engineers or Principles of lasers. Zaereth (talk) 03:26, 14 December 2011 (UTC)
I'll think about it some more. But it sounds to me like you're defining specular and 180 degree reflection rather than explaining it. And thanks for the comment and reference.WFPM (talk) 10:03, 14 December 2011 (UTC)
Ok, yeah, I did some checking through my books. Not many give the mechanics of reflection, but a few go into it in with very little detail. Because this is on such small scales, it must be explained with quantum mechanics, because classical mechanics doesn't work. However, quantum mechanics has to be explained in classical mechanical terms, so no explanation will really make sense. In fact, all of the experts, right down to Bohr, seem to agree that if you understand quantum mechanics, then you haven't been paying attention. I don't understand it myself, which is why I'm just repeating in my own words what the books say. In the words of Richard Feynman, from: QED: The strange theory of light and matter, "What I am going to tell you about is what we teach our physics students in the third or fourth year of graduate school... It is my task to convince you not to turn away because you don't understand it. You see my physics students don't understand it... That is because I don't understand it. Nobody does."
At any given point a photon emits an energy wave, which spreads equally in all directions. However, destructive interference cancels this wave out in all directions except one; in the direction of the light wave. This is what causes light to propagate in a rectilinear fashion (in a straight line), and this is referred to as transmission. When a photon encounters an individual atom, what occurs is diffraction. This is because the wavelength of visible light is typically much larger than the atom. However, if the photon encounters a mass of atoms bound into a structure, this mass may exceeed the size of the wavelength. In this case, the photon is absorbed by the atom and reemitted in all directions. Depending on the physical properties (vibrational characteristics, harmonic resonance, etc...) of not only the atom individually, but also with the surrounding atoms in unison, the sphereical wave is canceled out in all directions, except one of a possible three, (not counting emission by fluorescence, phosphorescence or heat). Constructive interference causes the photon to be reemitted in the most probable direction that the wave is travelling. This is either the direction of transmission, the direction of refraction, or the direction of reflection.
The exact direction depends on the angle of incidence. In the case of reflection, if the surface roughness is larger than the wavelength, the angle of reflection will not equal the angle of incidence. This is called diffuse reflection. If the surface roughness is smaller than the wavelength of the light, then the angle of incidence equals the angle of reflection. This is called specular reflection. However, in the terminology of optics, none of these angles ever referred to in the obtuse, (eg: none ever exceed 90 dgrees). A good example of this is a sandy beach. In visible light, the reflection is diffuse, because the surface roughness is larger than the wavelength. However, in microwaves, the reflection off the beach is specular, because the grains of sand are much samller than the wavelength of microwaves. Zaereth (talk) 21:41, 16 December 2011 (UTC)
I'd like to agree with you but I'm afraid I can't. Take light from a star for instance. Without obstructions I can theoretically see it from any position on a sphere with a radius the same distance as mine. And that cant be due to a spread out photon. The photon I see must have come directly from the star. And it must have been emitting a multitude of photons in order for one or a few to get into my eye. And when I think about light I think about the image of the Whirlpool Galaxy, with all the stars and light and obstructing media detail. And in the upper right hand part of the image you can see a few remote stars with a halo of light around them like it was a more remote galaxy shining its light through the chaotic milieu of the Whirlpool. So think about that. There has to be direct line of sight transmission of light energy. And after I lose a few games of computer chess I occasionally try to make some sense out of Feynman's QED and he's got me believing that what I have to do to understand about light is to understand what happens within a beam of light. And I have to worry about reflections and/or refractions related to anything placed within the impact area of the beam. And I don't let him get me involved in all the infinitude of other possible paths and their associated probability values. And I'm still trying to figure that much out. And with regard to light energy, I'm afraid I'm like Dr Gustave Le Bon says in his "The evolution of matter", "It is possible for greater minds to conceive of energy without connection to matter; but most of us have to have to associate our concepts with something with which we are familiar". That's a paraphrase because I can't find the book, but it's on page 13. Regards WFPM.WFPM (talk) 01:23, 17 December 2011 (UTC)

## How does a mirror actually work?

This article seems to miss a very important section: "how does a mirror actually work?". I was searching for some description of what happens on the atom level and below, but couldn't find anything. What makes a material reflective? What factors determine which wavelengths are reflected and which are not? I think it would be good to add a section on the actual physics of mirrors. --Tbleher (talk) 07:23, 10 January 2012 (UTC)

I couldn't find much about the actual physics of reflection, but what I could find I described in the section directly above this one. See my last comment there. Zaereth (talk) 20:42, 10 January 2012 (UTC)
It's easier to describe at a wave level than at a photon level. The key is the conductivity of the film of metal. When electromagnetic waves encounter a conductive material, the changing electric fields set up a changing current in the material, which then generates an electromagnetic wave propagating the other way. While detailed description of that effect probably belongs more in an article on electromagnetics than it does here, I agree that a brief description along these lines could be usefully added here, if someone can find it in a reliable source. --Trovatore (talk) 21:57, 10 January 2012 (UTC)
I agree that, when dealing with optics, it's almost always easier to describe as a wave than as photons. However, when you get down to the atomic level, you almost have to bring photons into the picture. There needs to be something to transfer energy from the wave to the atom and back again. It's probably useful to know the quantum explanation, especially for people who work with lasers (quantum electronics). The above explanation came from a book called Physics for scientists and engineers.
If you have a source for the conductivity explanation, it may also be useful, but I'm not sure if it would cover all mirror types, or 9if it would only cover metallic coatings. There are also dielectric mirrors which use non-conductive, transparent coatings combined with thin-film interference to produce the reflections. There are also other processes involved in metallic coatings, such as the absorption and re-emission as heat (phonons) of certain wavelengths. Gold, for instance, is an excellent reflector of infrared, but will absorb much of the wavelengths shorter than red. Silver is an excellent reflector of visible light, and out-performs aluminum in the visual range, but its reflectance drops to nothing beyond 310nm.
I also agree that something should be added about the physics of all of this, but am not sure if it would belong here, the reflectivity article, or somewhere else. I'll need to research it more, find some more sources, and get back to it sometime in the future, (if no one beats me to it). Zaereth (talk) 22:57, 10 January 2012 (UTC)
I would avoid talking about the quantum stuff too much. To describe the observed behavior in terms of photons, you can't just talk about an individual photon being reflected. You have to do a Feynman path integral over all the possible photon paths, which somehow magically interfere to produce just what the classical wave description would give you. That gets you into awkward issues of interpretation; you'll have to describe it differently depending on whether you like Copenhagen, many-worlds, or transactional. To me that seems a little outside the scope of an article called mirror. --Trovatore (talk) 23:03, 10 January 2012 (UTC)
Agreed. For an article like this, we probably shouldn't get much deeper than the scale of surface roughness and wavelength. Zaereth (talk) 23:20, 10 January 2012 (UTC)
An important question you can ask about the functioning of mirrors is whether the reflection process involves a single incidence of reflection of up to 180 degrees, or else involves 2 incidences of reflection which add up to that same amount. It would seem to be impossible for an atom to directly reflect back a light back 180 degrees by some physical transformation process. However if the ray were to be deflected so as to pass around the impacted atom it could be argued that the path of the exit angle could equal the angle of incidence.WFPM (talk) 19:47, 6 February 2012 (UTC) Such a path would be interesting in that it involves a similar to parabolic path around a focal point except that there is a loop around the focal point. Does anybody know a mathematical formula for such a looped focal point path? I can visualize a planar hyperbolic path that leaves a small (but disconnected) loop around the focal point, but can't visualize a connection between the loop and the other path of the hyperbola.WFPM (talk) 22:46, 6 February 2012 (UTC)

It's described here: Reflection_(physics) under the topic Mechanism. --TeakHoken213.150.232.3 (talk) 06:40, 30 May 2012 (UTC)

## thickness

I miss some information on the actual thickness a mirror needs to have before it can reflect light. A few wavelenths? Half a wavelenght? Which wavelenght would it be relevant, the one corresponding to the speed of light in the mirroring material?

I remember extremely thin layers of gold look green (!?), but couldn´t find the numbers to it.

And how thick is the silver on the back of a normal bathroom mirror? Would a film of the minimum thickness (which wouldbe cheapest) be stable? --129.13.72.198 (talk) 17:02, 18 June 2012 (UTC)

That's a simple set of questions with some complex answers. The first question, in regards to how thick a coating needs to be, with dielectric coatings, the thickness will be in some direct proportion to the wavelength of the light (ie: 1/2 or 1/4 of the center wavelength, etc...), and there will generally be a multitude of different layers with different thicknesses, to achieve the desired reflectance.
However, with metallic coatings, the thickness has less to do with the wavelength of the light, and more to do with the density of the material. As I understand it, (and I may be misinterpreting things a bit here), light will "penetrate" deeper into the surface of a lighter metal, and shallower into a heavier metal. For maximum reflectance, the coating mut be thicker than this penetration. Any less, and some of the light will transmit through the coating. For instance, with aluminum, this minimum thickness is around 100 micrometers (oops, I meant nanometers; the thickness of metallic coatings is usually smaller than the wavelength of light). Above this thickness, and the coating will be opaque (no light gets through, only reflected or absorbed). Below this thickness, and the coating is partially opaque (some light gets through, so it is transmitted, reflected, and absorbed). With denser materials like silver or gold, this minimum thickness is usually smaller.
To answer your second question, yes, thin layers of gold are said to be green. I don;t know why this is, except to say that it is common for many materials to be one color in heavy concentrations, but another in light concentrations. Perhaps someone else can answer as to why this occurs in gold, but I cannot.
Your normal bathroom mirror will generally be coated with nickel or beryllium. This is typically done because it's cheap and easy to do. Nickel plating is usually done with electroplating techniques, which requires no highly specialized equipment, such a vacuum pumps and precise temperature control. Like with vacuum deposition, electroplated mirrors have an amorphous structure, which provides better reflectance than crystalline metals. However, precise control over the coating thickness is not possible, and the thickness may vary quite a bit over the mirror. Nickel has higher absorption than other common metals, which is why it's never used in optics, but is fine for a house mirror.
The exact thickness depends on the manufacturer, but, often to save money in materials costs, the thickness is reduced to allow somewhere between 75% and 50% transmittance, making it a two-way mirror. Then the coating is painted with black paint, which serves the same purpose of keeping one side of the two-way mirror dark, so that on the other side all you see is your reflection.
Mirrors for use in optics have much stricter criteria for construction. The thickness of the coating can be precisely adjusted to allow for exact transmission/reflectance characteristics. It can also be adjusted to affect polarization and phase shift. The coating must be carefully matched to the substrate to account for the differences in thermal expansion, which create bimetallic bending effects, (which can be quite detrimental in items like lasers or telescopes.
I have only begun to graze the surface of the information involved with mirror construction, but I hope that helps answer your question. Zaereth (talk) 20:36, 18 June 2012 (UTC)
I did do a little looking, because the question about gold intrigued me a little. The book The Colour of Metal Compounds gives some interesting, yet highly technical, descriptions of why metals change colors in differnt concentrations. Thin layers of gold sometimes appear green, but when mixed with soda-lime glass to make stained glass, it turns a very deep red. Cobalt turns blue, and neodymium purple. Chromium, although deep red in sapphire, is bright green in glass. Uranium turns pink. I don't have time to really go into the book much ddeper, but it appears to be available on google books, if you want to look. Zaereth (talk) 00:59, 19 June 2012 (UTC)
Interestingly, I did come across a rule from the National Institute of Standards and Technology (NIST) which regulates the reflectiveness of automotive mirrors, confirming what I said above. The rear-view mirror in a car is only required to be 35% reflective for daylight driving, and 4% for night. Zaereth (talk) 04:34, 28 July 2013 (UTC)

## Two-way vs. One-way mirror terminology

I removed the distinction between two-way and one-way mirrors and changed the primary term to one-way mirror. Please note that right now the "two-way mirror" article is a redirect to "one-way mirror", which was the result of a move request made after extended argument. In normal use the term "one-way mirror" does not refer to the physically impossible kind of mirror that only ever works one way, but to the mundane mirror that works differently depending on surrounding light levels, so it didn't seem to make sense to suggest the terms had fundamentally different meanings. --114.145.176.47 (talk) 13:44, 8 March 2013 (UTC)

In my opinion, there are more suitable images to show what a mirror is than the image with the coin and optical flat. The problem is that it's unclear there's a reflection. It appears to be a can of shoe polish under a transparent surface that has two coins on it. Why? Other than the overprint, the coin is symmetrical, and there's nothing else around the coin. It feels like someone just had to pick the geekiest image and use it to explain the ${\displaystyle \lambda }$/20 flatness and less than 31.6 nanometer surface deviation. That's all very nice, but you could also just show a person looking in a mirror. You know, a mirror, something that you would point to if a space alien asked what a mirror is. WP:BB Above the geeky coin stuff, I'm adding: [[File:Barack Obama takes one last look in the mirror, before going out to take oath, Jan. 20, 2009.jpg|thumb|[[Barack Obama]], the [[Presidency of Barack Obama|current]] [[President of the United States]], looks in a mirror before taking [[Oath of office of the President of the United States|the oath of office]] at [[United States Capitol|the Capitol]] on 20 January 2009.|alt=Barack Obama looking in a mirror]]. --82.136.210.153 (talk) 09:56, 4 August 2014 (UTC)

Agreed, good call. Should we avoid using an image featuring a distractingly recognisable person, though? I thought there was a line in the MOS about this but can't find it. --McGeddon (talk) 10:03, 4 August 2014 (UTC)
A mirror, reflecting a vase
To avoid starting an edit war, I am taking this to talk. I added my "geeky" image because I thought the article could use a photo of a first-surface mirror, and the penny was only added for size. (The idea was to picture it showing how the angle of the shadow (angle of incidence) equals the angle of the reflected light.
I do think a more aesthetic image should be used for the opening pic. I am by no means a fan of Obama, but replaced it because the IP lacked any edit summary of why they removed it. However, perhaps it is too political. I do not know why the original image was removed, but maybe we should go back to it. Zaereth (talk) 23:40, 29 August 2014 (UTC)
Right — just to clarify my own biases, my personal view of Obama is not especially positive, but he doesn't make my blood boil either. I see him as a competent and well-intentioned man who happens to have some philosophical views with which I sharply differ.
So I don't really have such a strong political reason to object to his picture per se. Rather, I think it looks at least questionable to shoehorn him into an article so unrelated to his personal notability. If the motive for including him was political (which I can't know, but it looks at least suspicious), well, I do have an objection to that. Even if the motive was not political, I think you can't really blame readers for thinking it was, and that's a problem in itself. --Trovatore (talk) 00:20, 30 August 2014 (UTC)
I understand. Do you see any objection to adding this image of the vase back to the article, or do you know of a better one? This was the lede image for a long time, and I don't know when or why it was removed. Zaereth (talk) 00:31, 30 August 2014 (UTC)
Can't see any problem with the vase. --Trovatore (talk) 01:47, 30 August 2014 (UTC)

## Missing: plastic mirrors

I don't know where to start, but I cannot find any information at all on plastic mirrors and plastic reflectors. Are they simply metal coated plastic or are they an admixture of substances? Rursus dixit. (mbork3!) 09:26, 7 February 2015 (UTC)

Plastics in general do not have smooth enough surfaces for specular reflection (although some, like spectralon, produce superb diffuse-reflections), so a metallic coating is usually used. Most plastics are not chemically stable enough for the same coating chemicals used for glass mirrors. Likewise, they are not thermally stable enough for vacuum deposition. Plastics are usually electroplated to produce a mirror finish. I always remember those plastic eggs that my mom's nylons came in. Fun to play with as a kid, but the chrome coating could easily be peeled off the surface of the plastic. Because plastics do not conduct electricity, electroplating is done by coating it with carbon first, and then electroplating, producing a mirror-like coating on the plastic. Zaereth (talk) 09:44, 7 February 2015 (UTC)

## Text needs integrating

I have moved the following slab of text to here from the article as it was dumped into the "History" section with no regard for the existing content, thereby destroying the overall flow of the section. It needs to be properly integrated.

Mirrors have been used since prehistoric times, but mostly consisted of small, poorly reflective-surfaces, such as water or polished rocks. After humans learned to smelt metals from ore, metallic mirrors of solid copper or bronze plates became available, although these were usually reserved for the very wealthy and provided poor color rendering. When methods of manufacturing steel became available in the Middle Ages, mirrors of bright, polished steel became the most popular and most affordable form of mirrors until the mid-1800s, but crystalline-metal mirrors often produce poor or distorted reflections, were prone to rust and tarnishing, and good flatness became more difficult to achieve as mirrors were made larger. Glass mirrors, coated in a layer of hot lead, are reported to have been invented in the Middle East around 100 AD, but the glass needed to be very thin to prevent cracking from the heat. Most glass was heavily tinted with iron-oxide, turning it a blue-green color. To produce a smooth enough surface and uniform thickness, the glass needed to be blown into bubbles and then coated on the inside with hot lead. Then circular sections of the bubble were cut away, producing small, convex mirrors which were typically only a few inches in diameter. These glass mirrors were often used as jewelry or as curiosities, producing distorted images and reflecting more of the surroundings than the person looking into them. It was not until the 1500s that glass-makers in Italy perfected a method for manufacturing flat panes of white glass that were smooth, uniform, and clear enough to produce undistorted reflections. The Italians coated them with the heat-reduction of mercury-tin amalgams, producing an amorphous metallic-coating that had superior reflectivity to crystalline metals. The "Venetian mirrors" were very expensive and were limited to a foot or two in size by the glassblower's technology. However, in the 1800s, methods of mass-producing large, flat panes of glass had been perfected and, by combining them with heatless, chemical-coating technologies, glass mirrors of nearly any size became much more affordable to the average person.<ref>''The Mirror: A History'' by Sabine Melchior-Bonnet -- Routledge 2001 Page I--70</ref> — Preceding unsigned comment added by 217.44.208.185 (talk) 21:44, 7 March 2015 (UTC)
I agree. I was thinking the same thing, but have no time right now. Winter has been very mild and my work load high, so I'll get back to this when I have the time and energy to do it properly. I do think the current history-section focuses too much on coatings, when advancement in both glass and metal-working technologies worked hand-in-hand to advance mirror technology. Zaereth (talk) 01:03, 13 March 2015 (UTC)

Hello fellow Wikipedians,

I have just added archive links to 2 external links on Mirror. Please take a moment to review my edit. If necessary, add {{cbignore}} after the link to keep me from modifying it. Alternatively, you can add {{nobots|deny=InternetArchiveBot}} to keep me off the page altogether. I made the following changes:

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Archived sources still need to be checked

Cheers. —cyberbot IITalk to my owner:Online 06:12, 27 August 2015 (UTC)

## Assessment comment

The comment(s) below were originally left at Talk:Mirror/Comments, and are posted here for posterity. Following several discussions in past years, these subpages are now deprecated. The comments may be irrelevant or outdated; if so, please feel free to remove this section.

 "A mirror is used for inspecting parts of one's body which are difficult or impossible to see directly, such as the face, neck or the whole body. This may be to checkphysical appearance (including clothing, make-up, hair, etc.) or to control applying make-up, shaving, cutting hair, fixing one's tie, etc." My God, did the author of this article think there would be people unfamiliar with the use of a mirror, but logging onto the Internet and using Wikipedia to discover they could see their face in one?

Last edited at 13:54, 4 November 2006 (UTC). Substituted at 00:11, 30 April 2016 (UTC)