Talk:Interference (wave propagation)

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Old comments[edit]

I'd like to see information here regarding the phenomena of rainbow-like interference patters as created by a continuum light source repeatedly refracting in a thin film, such as soap bubbles or oil on water. If this isn't the right article for the discussion, where would be? --zandperl 01:07, 9 Mar 2004 (UTC)

I suggest that effort should be devoted to the subject of interference at the level of

words

two languages

memory and thinking

intent and expression

etc.

in other words in more lay areas of common experience of living

Apogr 13:43, 30 Aug 2004 (UTC)


How an interference pattern is produced[edit]

In the article it is stated:

Light beams that can produce interference patterns are called "coherent," and have all of their photons' phases aligned or having constant phase difference with each other.

This is incorrect. Incoherent light will produce an equally sharp interference pattern. White light will give an unsharp interference pattern, as the different wavelengths have different diffraction.
The famous Michelson-Morley experiment was conducted well before lasers were available. It must have tested the e~xperimentors ingenuity to obtain a lightsource that is both close to monochromous, and sufficiently bright to get a good reading.
According to quantum electrodynamics, each photon interferes only with itself. Laserlight is an exception to that: because of the cascading way they are produced laser photons are mutually connected in deeper ways than light from normal origin. --Cleon Teunissen | Talk 19:51, 22 Mar 2005 (UTC)

How do you explain this?[edit]

The text currently says:

When a single source interferes with itself, the principle of conservation of energy dictates that the energy "missing" from the darkened regions of an interference pattern where destructive interference has taken place will be found in the brightened portions where constructive interference has taken place.

Who says that there has to be a "brightened portion where constructive interference takes place"? Look at the diagram of out-of-phase reflection. If the two surfaces involved are the same distance apart at all points, then if there is destructive interference at one point there must be destructive interference at all points. It the same thing, essentially, as the noise cancellation headphones people use on airplanes. You don't cut sound to nothing at 440 hertz and get blasted by sound at 660 hertz or any other frequency. If two teams are contesting in tug of war and they are evenly matched, the flag tied to the midpoint of the rope doesn't go anywhere, but that does not mean that the energies expended by the two teams have evaporated. They are just balanced. P0M 08:07, 11 January 2006 (UTC)

Noise cancelation equipment has indeed the property that for every zone with cancelation there must be another with mutual reinforcement. To my knowledge: a noise cancelation headphone for a helicopter pilot would actually increase the noise level in the cockpit as a whole, the technique is to set up the device in such a way that right at the point of the pilot's eardrum there is a zone of cancellation. --Cleonis | Talk 19:06, 11 January 2006 (UTC)
I won't argue the point with you. Give me a relevant citation from a real physics textbook. P0M 01:16, 12 January 2006 (UTC)
And check out Feynman's QED, chapter 1. P0M 06:46, 12 January 2006 (UTC)
I suggest that you provide a description of a device in which self-interference of oscillations from a single source is elicited, with only destructive interference. Here is an example of a device that doesn't meet that requirement: a trumpet with a single mouthpiece, but with two tubes. The length of the two tubes is such that the soundwaves that are emitted from the cones are exactly in counter-phase. Let the two cones face each other. Then very little of the acoustic energy will come out, but inside the tubes the two will reinforce each other.
This is not something that can be covered by a single textbook citation. It is similar to the principle of conservation of energy. Strictly speaking we have no proof that the principle of conservation of energy is universally valid. What we have is that despite looking hard for it, no exceptions to conservation of energy are known. That in itself is strong evidence. --Cleonis | Talk 11:32, 12 January 2006 (UTC)
I can do that, but it's not exactly to the point. You are actually supporting my position, albeit indirectly. If you produce interference as you suggest with the two trumpets, or if you produce Newton's rings with an optical device, or if you produce an interference pattern by making a double-slit device, you will get constructive interference alternated with destructive interference. There is no question about that.
If you make a sound cancellation device of the sort that I've seen described, you generate additional sound with an electronic device that is out of phase with the ambient sound. So in the cockpit of the airplane, or wherever we're talking about, there is more energy being put into sound than there was before. (And it's getting marginally hotter in there too because the additional electronic circuitry being run is warming up.)
All of that is irrelevant to the false statement: "the energy 'missing' from the darkened regions of an interference pattern where destructive interference has taken place will be found in the brightened portions". There is no demon in the double-slit (or other interference-demonstration) device that moves energy from the dark fringes to the bright fringes. Photons do not deviate in pairs from their original course toward the dark fringes and flock to the bright fringes. If it is a double-slit device and you merge the slits you will not find that the places that had bright fringes have now dimmed down by half because half their energy has moved back to where it belongs in the areas that were previously dark.
Think of it in terms of two teams of workers. Each team is divided into two squads. Each squad can either push north or south on their pole, and each of the two poles is attached to a rigid harness on a mule that doesn't particularly want to move.
Team one has its collective head together. They want their mule to go north and both squads push their poles to the north. The mule moves, kicking and braying to the north.
>>>>>>>>>> /NN^>>>>>>>>>>.

The other team is stuck in tug-of-war mode, and one squad tries to move south while the other squad tries to move north. The mule stands quietly grazing in the middle.

<<<<<<<<<< /NN^>>>>>>>>>>
Like a book sitting on the table, you don't necessarily realize that there is force acting on the second mule. However, if the harness breaks, or if one of the attachments that holds a pole to the harness breaks, then you will realize that the forces haven't moved over to cooperate moving the mules of other teams. No work is being done because forces are balanced, but the forces haven't gone away, and the teams will get hungry and tired because they are expending energy just to stay where they are. P0M 17:14, 12 January 2006 (UTC)
What if two light beams of the same wavelength are joined together in such a way that one is exactly 180 degrees out of phase with respect with the other. Wouldn't they exactly cancel each other out so that there would be no energy left in the beam? It would seem that in this case there would be a dark region created but no brightened region, and the law of conservation of energy as stated above would not be a valid description of where the missing energy has gone. What if the missing energy is actually going into creating dark matter, or dark energy? --Carter43 05:16, 21 September 2007 (UTC)
I'm not sure that you can combine beams to cancel everywhere. With half-silvered mirrors you also create an accompanying beam (coming off at another angle) where you have constructive interference. --Michael C. Price talk 07:30, 21 September 2007 (UTC)

Indeed, there is no demon in the double slit[edit]

I copy and paste from above

There is no demon in the double-slit (or other interference-demonstration) device that moves energy from the dark fringes to the bright fringes. P0M 17:14, 12 January 2006 (UTC)

You are quite correct.
The current formulation in the article goes:

When a single source interferes with itself, the principle of conservation of energy dictates that the energy "missing" from the darkened regions of an interference pattern where destructive interference has taken place will be found in the brightened portions where constructive interference has taken place.

I think I agree with you in criticising the suggestion of direction of causality. The principle of conservation of energy is something that we see around us, but it is in itself not a causative agent. It is plain wrong to suggest that conservation of energy is in a position to dictate anything, so that should be fixed. --Cleonis | Talk 20:14, 13 January 2006 (UTC)

Destructive interference merge[edit]

If anyone is interested, the former article lacking verifiable sources is available here Alan.ca 04:39, 30 October 2007 (UTC)

Energy?[edit]

I think this is a common confusion: could someone explain where the energy "goes" in destructive interference? If two photons are headed toward the same point, but destructively interfere, where does the energy go? 155.212.242.34 20:57, 30 October 2007 (UTC) hi it is —Preceding unsigned comment added by 203.197.200.20 (talk) 05:18, 11 April 2009 (UTC)

Also known as[edit]

Interference is also in a sport something or someone who comes in the game that shouldn't be in the game. —Preceding unsigned comment added by 68.225.67.225 (talk) 16:48, 15 January 2008 (UTC)

Move to Interference (Physics)[edit]

Since there are so many other meanings of Interference, I suggest that the old Interference article should be moved to Interference (physics), and that Interference should point at the Interference (disambiguation) article.

If this is okay, all articles pointing at Interference should be moved to Interference (physics). Mange01 (talk) 09:26, 6 May 2008 (UTC)

That's okay, but what sonofabitch moved it to interference (wave propagation) without discussion, and then deleted all the non-wave stuff.?--Michael C. Price talk 12:23, 3 November 2009 (UTC)

Examples[edit]

Cited from the text. This is an important topic which requires more explanations, graphics and cites.

"A conceptually simple case of interference is a small (compared to wavelength) source – say, a small array of regularly spaced small sources (see diffraction grating).

Consider the case of a flat boundary (say, between two media with different densities or simply a flat mirror), onto which the plane wave is incident at some angle. In this case of continuous distribution of sources, constructive interference will only be in specular direction – the direction at which angle with the normal is exactly the same as the angle of incidence. Thus, this results in the law of reflection which is simply the result of constructive interference of a plane wave on a plane surface." —Preceding unsigned comment added by 60.53.158.253 (talk) 15:54, 4 June 2009 (UTC)


Are those action figures in the picture from the sea? It looks like a scuba-diving gnome. It seems like it would be simpler to just put a picture of an oil slick. —Preceding unsigned comment added by 128.146.34.30 (talk) 18:20, 12 August 2009 (UTC)

What about polarization?[edit]

The article states that

“Interference usually refers to the interaction of waves that are correlated or coherent with each other, either because they come from the same source or because they have the same or nearly the same frequency.
Two non-monochromatic waves are only fully coherent with each other if they both have exactly the same range of wavelengths and the same phase differences at each of the constituent wavelengths.”

but it seems to me that, for example, two waves with perpendicular polarizations would not interfere even if they have
a. same frequency
b. constant phase difference
ie. they are coherent. So why is there no mention of the polarization state, is it a glitch or is it not relevant? --DagnirA (talk) 18:38, 29 November 2009 (UTC)

To the resident rocket scientist[edit]

I marvel at your mathematical skill set...I genuinely do. But lob us cretins a bone and toss in some English. Nobody likes a showoff. Grin a bit - even Einstein had a goofy mug to the camera on occasion...

Ok, had to be said. every time i see calculus equations on wikipedia... and not just an errant one in an article... the entire bleeding thing... i think of the recursivel small fraction of a percent of readers who a) read it and b) wonder if they're reading Martian.

I'm not telling you to stop... maybe just try a few targeted electroshock treatments. Either to stimulate that 'other' lobe (cuz i'm betting it looks like a raisen next to the State Fare's blue ribbon cauliflower) or as a Pavlovian response. Your choice.

Chew of this - i don't get a page consisting solely of sheet music when i wiki my favorite songs. Sheet music is great... not all of us can read it. Hope you caught the deeper correlation I just tried to make.

Ok, my gawk at the geek (gypsy circus connotation, please!) diatribe has run its course. Hopefully somebody chuckles... and the savant editor what makes all them thar purdy scratches on the page don' get his panties in a twist. Some of us need to take off bits of clothing to make small change.

Or, to quote black adder... 2 beans plus two beans equals... "some beans" —Preceding unsigned comment added by 208.59.150.114 (talk) 12:52, 2 December 2010 (UTC)

Merge[edit]

I propose that we merge Interference (optics) into this article. User Epzcaw has prepared a draft of the merged article. I propose that we replace the existing text with Epzcaw's version. It will need some copyediting afterward; that is best done in main article space rather than on the existing user space page.--Srleffler (talk) 05:33, 10 November 2011 (UTC)

Merge now doneEpzcaw (talk) 17:02, 10 November 2011 (UTC)

DIFFERENCE BETWEEN CONSERVATIVE AND NON CONSERVATIVE FORCES[edit]

In physics, it’s important to know the difference between conservative and nonconservative forces. The work a conservative force does on an object is path-independent; the actual path taken by the object makes no difference. Fifty meters up in the air has the same gravitational potential energy whether you get there by taking the steps or by hopping on a Ferris wheel. That’s different from the force of friction, which dissipates kinetic energy as heat. When friction is involved, the path you take matters — a longer path will dissipate more kinetic energy than a short one. For that reason, friction is a nonconservative force.

For example, suppose you and some buddies arrive at Mt. Newton, a majestic peak that rises h meters into the air. You can take two ways up — the quick way or the scenic route. Your friends drive up the quick route, and you drive up the scenic way, taking time out to have a picnic and to solve a few physics problems. They greet you at the top by saying, “Guess what — our potential energy compared to before is mgh greater.”

“Mine, too,” you say, looking out over the view. You pull out this equation:

image0.png This equation basically states that the actual path you take when going vertically from hi to hf doesn’t matter. All that matters is your beginning height compared to your ending height. Because the path taken by the object against gravity doesn’t matter, gravity is a conservative force.

Here’s another way of looking at conservative and nonconservative forces. Say you’re vacationing in the Alps and your hotel is at the top of Mt. Newton. You spend the whole day driving around — down to a lake one minute, to the top of a higher peak the next. At the end of the day, you end up back at the same location: your hotel on top of Mt. Newton.

What’s the change in your gravitational potential energy? In other words, how much net work did gravity perform on you during the day? Gravity is a conservative force, so the change in your gravitational potential energy is 0. Because you’ve experienced no net change in your gravitational potential energy, gravity did no net work on you during the day.

The road exerted a normal force on your car as you drove around, but that force was always perpendicular to the road (meaning no force parallel to your motion), so it didn’t do any work, either.

Conservative forces are easier to work with in physics because they don’t “leak” energy as you move around a path — if you end up in the same place, you have the same amount of energy. If you have to deal with nonconservative forces such as friction, including air friction, the situation is different. If you’re dragging something over a field carpeted with sandpaper, for example, the force of friction does different amounts of work on you depending on your path. A path that’s twice as long will involve twice as much work to overcome friction.

What’s really not being conserved around a track with friction is the total potential and kinetic energy, which taken together is mechanical energy. When friction is involved, the loss in mechanical energy goes into heat energy. You can say that the total amount of energy doesn’t change if you include that heat energy. However, the heat energy dissipates into the environment quickly, so it isn’t recoverable or convertible. For that and other reasons, physicists often work in terms of mechanical energy