From Wikipedia, the free encyclopedia
Jump to: navigation, search
WikiProject Physics (Rated C-class, High-importance)
WikiProject icon This article is within the scope of WikiProject Physics, a collaborative effort to improve the coverage of Physics on Wikipedia. If you would like to participate, please visit the project page, where you can join the discussion and see a list of open tasks.
C-Class article C  This article has been rated as C-Class on the project's quality scale.
 High  This article has been rated as High-importance on the project's importance scale.

Colored Shadows[edit]

I can be a shadow! Shadows are also colored in the absence of white light. The only time when a shadow is exclusively shades of gray is under a single color of light. Illuminate a room with a red light, and the shadows are exclusively gray. Illuminate the shadows with a white light, and the shadows are green. Where both lights are blocked, or in other words where the shadows intersect, they are gray. Away from the intersection, where the red light is blocked the shadows are green and where the white light is blocked the shadows are red. In other words, light colors shadows or brightens them, according to the complementary color of the light blocked to cast the shadow. In the case of white and red lights, the complement of white is red; with white and green lights, the complement of white is green. In the absence of white light, colored lights blocked by an opaque surface cast shadows in the colors complementary to the lights blocked. For green light, red shadows; blue, orange; yellow, purple; intermediate light, intermediate shadows. In the absence of multiple light sources, colored lights illuminate spaces where other lights are not blocked. The red shadow cast by blocking white light in the above example, is not a shadow with the white light off, but it is illuminated in red. oneismany 15:40, 12 November 2005 (UTC)

Now consider the following thought experiment: In the absence of light that we can see, there is yet invisible radiation analagous to visible light; for example, infrared light. What color is a shadow cast by blocking infrared light? What color is a shadow illuminated by infrared light? We cannot see it, but conceptually it is there. Cosmically, it is provable that invisible EM radiation preceded visible light everywhere. So, darkness is not simply dark, it is illuminated by invisible light. So, shadows precedes light. So, shadows contain information about the light blocked to cast the shadow, in addition to the shape of the object blocking the light. oneismany 15:40, 12 November 2005 (UTC)

Maybe my definition of light is incorrect, but I define light as something we can see, and therfore is within some frequnces limits. So infra red and ultraviolette, I dont think we can call light, if I could, then shouldnt all Em frequences be light? I dont define my microwave as a light source. "So, darkness is not simply dark, it is illuminated by invisible light." I mean this statement is incorrect, becouse in my opinion there is no such thing as invisible light. Light is the definition of EM we can see.

Sorry for my bad english.

Perhaps, but infrared (for example) behaves like ordinary light in that if you have a device for viewing infrared, you can notice a spot on a wall where you shine an infrared light. Video cameras with infrared sensitivity often come with a small infrared flashlight, which I believe, works like a normal flashlight. When you shine such a light at people's eyes, the light is reflected much like the "red-eye effect" for normal light. oneismany 08:39, 25 January 2006 (UTC)

Shadow of my Former Self[edit]

Hey, what is the origin of the phrase "I am a shadow of my former self"? Does anyone know? I think it'd be an interesting thing to add to the article.

FTL dispute[edit]

Correction to “shadows contain no information and can exceed the speed of light”: I propose that shadows contain information, namely the outline of the object casting the shadow. Furthermore, I propose that shadows propagate at exactly the speed of light, and wish to back it up with the following thought experiment:

Let us say that a huge plate appeared between the earth and the sun, 3 light-minutes from us, completely eclipsing the sun. It would take 3 minutes before we would see the plate, and thus three minutes before the sun would disappear behind the plate. If shadow traveled faster than light, the Earth would be in shadow before then. The result would be a ‘night-time’ Earth, with a ‘day-time’ sun in the sky.

However, I do not know the evidence backing the original claim, and am hesitant to make this change without general concent.

I suppose you have to think of sideways movement of the plate, causing a faster movement of the shadow on Earth.--Patrick 04:57, 16 July 2005 (UTC)

After doing a bit of research, I edited and explained this phenomenom a bit better. Shadows do contain information (such as the outline of the object casting the shadow) but moving the shadow around does not transmit information.-- 13:56, 18 July 2005 (UTC)

I'm sorry, but I have to insist that the "faster than light" section be removed. A shadow isn't even a "thing", per se, but rather an absence of light, and is defined not by what it "is", but rather by the light around it. I like that eclipsing plate analogy. Having the plate "appear" in front of the sun would block the light from propagating to the Earth. The light that was en route to Earth would continue as such, and after it has reached its destination (3 minutes later, in the analogy?) then the shadow would "appear". It's the same analogy if the sun were to "black out", we wouldn't see it until eight and a half minutes (the actual distance in light-minutes) later. "I suppose you have to think of sideways moving of the plate, causing a daster movement of the shadow on the Earth" ??? What are you talking about? Are you talking about finding the absolute velocity, taking into account components? It's the same as before. Any disturbance in and EM-field, which is what light is, propagates at the speed of light. Period. I'm taking out the "faster than light?" section.

Section removed without discussion[edit]

In December, the section titled 'Faster than light?' was removed without discussion. I am restoring it here, with small modifications. If anyone wishes to debate its value, please do so before removing it. Oneismany

What is the purpose of this section? A shadow can move slower or faster than the object which casts it. There's nothing interesting about that. And it isn't a speculation that a shadow can move faster than light, it is just a simple fact. It is obviously in accordance with general relativity, because there's no mass or energy moving with a shadow. And a laser is not an object moving at lightspeed. Maybe a laser light, but a light usually doesn't cast shadows... I think that the section should be removed, or at least reformulated. Bertik 17:48, 14 May 2006 (UTC)
This section seems to exist purely to advertise the linked-to website. I'm inclined to re-remove it, or at least reformulate it in a way that doesn't rely on a single thought experiment in an external source. -- nae'blis 14:53, 7 August 2006 (UTC)
The conclusion is incorrect. The shadow itself NEVER moves faster than c. Projecting a shadow over a distance faster than C is not moving anything faster than light, nor is it transmitting anything faster than light. There is NO communication or relation between the ends of the shadow, only between the light that is casting the shadow and the object that it is reflected (or the lack thereof) upon, which never moves faster than c. This is like sending two courier pigeons in opposite directions and saying that your pigeons are moving twice as fast. falsedef 00:45, 12 March 2007 (UTC)

Here's a simple mock up of shadow propagation.

a shadow does not move faster than light

A shadow's projection may appear to move faster than the speed of light, however the shadow actually moves at the speed of light. The image shows a light source being blocked by an interference object, with each frame representing 1 year. The left side shows a 3d perspective, while the right side shows an overhead view of the shadow's movement. The shadow's movement down the middle is at the speed of light, therefore the shadow (absence of light) moves upwards a light year every frame. When the shadow is finally projected upon the back surface, the shadow's projected width is larger than 1 light year, making the shadow appear to have moved (grow) faster than the speed of light. However, the shadow's true movement is not actually along the back surface, it is outward from the interference object.

This diagram should only be used for helping understand shadow propagation, and does not realistically portray other physics. falsedef 02:20, 12 March 2007 (UTC)

The section on propagation speed as it currently stands is self contradictory. I am adding my voice to this conversation in hopes that it will add some clarity to the discussion. I am a physicist - take that or leave it, I myself would leave it. The issue at hand here is not one of physics, the physics is absolutely clear-cut and leaves little room for argument if one is to accept any experimental work done in the last half-century. The issue is one of words and their definitions. To the point: what is a shadow and what does it mean when one says a shadow has moved. I think we should agree that without an object(or objects) onto which light is falling we cannot refer to a shadow as being extant. The shadow only exists as a non-illuminated subset of a partially illuminated object. When one sees the shadow of a bird in flight projected onto the ground, first in one spot, then in another, is it not common to say that the shadow of the bird has moved across the ground? What has really happened here? The only change is that the non-illuminated subset (bird shadow) of the illuminated object (ground) has changed. Regardless of what anyone might want(for the sake of their argument) the meaning of 'shadow' and 'shadow movement' to be, that is the way they are used by people who speak english on a day-to-day basis, whether they realize it or not. With these definitions in mind one would have to agree that it is possible for a shadow to move faster than light - unless you want to argue against some VERY simple geometry. But it is okay that it 'breaks the speed limit', because a shadow is not a THING, it is a subset of a thing whose extent is only the result of a logical partitioning that we as observers make - i.e. it exists only in our minds - only as a logical object, not a physical object. Of greater relevance, one cannot use the faster than light movement of a shadow to transmit information faster than light. If anyone seeks greater clarification on these points, write below. I will not edit the article. —Preceding unsigned comment added by (talk) 00:19, 7 December 2009 (UTC)

About my edit...[edit]

I made an edit saying that the stuff I removed was already in the disambiguation and it was not. Just saying. 00:19, 28 April 2006 (UTC)

Dimmest shadow[edit]

What's the minimum ambient light necessary to cast a shadow? Say a dark-adapted eye, white background, no other lights around and rapidly moving caster (to activate the rods)?

Venus (mag. -4) supposedly makes easily seen shadows. Sagittarian Milky Way 02:31, 17 January 2007 (UTC)

Einstien never stated...[edit]

Although popular internet rumor states otherwise, Einstien never stated, "There is no substance called darkness,dark is only the absence of light. It is the same with good and evil.Evil is the absence of god's holy light."


I am deleting it for the time being.

KinAlpha 21:16, 23 January 2007 (UTC)

Shadows and Substance[edit]

San Antonio, Texas (November 2006).

Shadows exhibit color, does this mean shadows have wavelengths or that shadows are just interruptions in light-waves? Light-waves have no weight, nor do shadows, so how is it meaningful to say shadows are insubstantial? Light is also insubstantial. Neutrinos were thought to have no mass, but now they are known to have mass. Could it be that light/shadow has mass but we cannot measure it because it is the basis of all other measurements? Perhaps under certain conditions light or shadow might exhibit masslike characteristics. Oneismany 22:37, 2 March 2007 (UTC)

Erm... shadows do not exhibit color; they are "just interruptions of light-waves." Much like how there isn't such a thing as "hot" or "cold" but just "temperature" of which hot and cold are measures, there is the level of light. When the light gets blocked, it leaves a shadow. That's all there is to it.
Also, light does have mass and is not insubstantial. To see why, imagine you fly up right next to the sun. The light frying your ship would prove quite substantial then. Remember Einstein's E=mc^2; a lot of energy is only a tiny amount of mass, so even though there's plenty of light energy hitting Earth, it's a very small amount of mass. But it is there. SnowFire 23:22, 8 March 2007 (UTC)
Sorry, but light does not have mass. Light has properties similar to a mass particle, but it does NOT have mass itself.falsedef 10:42, 12 March 2007 (UTC)
Well, according to Photon, they're massless but increase the invariant mass of the system. Fine, so photons are energy now, they can become mass later. I was imprecise, but the general point was light = energy = mass. SnowFire 01:10, 13 March 2007 (UTC)
No, what they mean is that if a room is illuminated by differently colored light sources then the partial shadows will be colored. Sagittarian Milky Way 07:57, 13 March 2007 (UTC)
A shadow can also be of an absent specific wavelength band. A color LCD projector emits a single white light, but projects a "colored" shadow projection. The shadow itself has no color (since it is the absence of color), but its projected surface might reflect or emit color. The outdoor pictures of silhouettes in the article are colored, due to ambient lighting, but have singular sources of lighting (the sun). falsedef 14:30, 13 March 2007 (UTC)
Right, that too. Like a red background will pretty much have a hard time not getting red shadows casted on it. Well, if there were *really* only one light source and the caster were *opaque* then the shadow would be black. It's only regular everyday things such as skylight, earthlight, light from the observer, the air, diffraction and the non-point sourceness of the sun that permit us to see anything inside shadows. There's still the case of the caster being non-opaque, or even a color filter. Sagittarian Milky Way 21:58, 13 March 2007 (UTC)
Yeah, diffraction is definitely something to consider with shadows. When talking about singular light sources, you'd also have to take into consideration things like penumbra shadows, which would block a grayscale rather than pure white. Shadows are also not just on the visible spectrum, the absorption spectrum is technically a shadow of elements. Things like sunblock and the ozone cast UV shadows. I don't know of any material that is truly opaque, except maybe around a blackhole. You can also red shift or blue shift a light source and shadow. There's a lot more to shadows than this article goes over; it could reach FA if enough info was added. falsedef 02:39, 14 March 2007 (UTC)

Definition of Shadow[edit]

What is the basis for the definition of shadow to begin the article? I think most dictionaries define the word as the projection of darkness onto a surface. This is the first definition for the Encarta defintion. I think that at the very least it should be noted that shadow can be defined as either the projection, or the region of blocked light since both definitions are used in common parlance. This has a direct implication to the argument of whether shadows can move faster then light. If you define the shadow as the area of projected darkness, then it can move faster then light, but with the current definition on this site the shadow cannot move faster then light.

Psychological meanings of shadows[edit]

Victor I. Stoichita, Professor of the History of Modern and Contemporary Art at the University of Fribourg, Switzerland, is the author of A Short History of the Shadow (Reaktion, 1997)

He is interviewed in Cabinet Magazine here I any of you all want to check it out and cull some information, it's probably a good resource.

attraction between 2 shadows[edit]

i have many times noticed this phenomenon but really dont know the reason behind it. actually what happens is whenever two dark shadows come very near(in most of the cases penumbra is almost absent even for sunlight) they seem to be joined even if there is no joiming of two objects( particulaly i hv observed this when i bring my head near to some other person ) it looks like as if it have got srtreched towards other shadow. and usually this is observed during morning in presence of sunlight.not in the afternoons may be this is because morning sunlight has got more intensity at that particular time of day.while in artificial lights it cant be seen as those lights dont havr capability to form comparatively dark shadow region(umbra) much of penumbra can be seen in those lights..

if anybody could answer to this phenomenoon why this happens please sent it to —Preceding unsigned comment added by (talk) 04:59, 4 October 2007 (UTC)

I would think it has to do with the angle of the sun in the morning/ evening. Midday shadows won't stretch as much as the sun is overhead. The shadows aren't 'attracted', merely stretched. //dan —Preceding unsigned comment added by (talk) 08:27, 13 June 2008 (UTC)

Shadow at night/inside a body[edit]

The article does not explain negative and undefined shadows in trigonometry. These refer the shadows at night and shaows within a body. Anwar (talk) 10:41, 30 June 2008 (UTC)

Variation with time[edit]

I gutted the "Time effects" section for the reasons given in Talk:Sunrise#Shadow. Spiel496 (talk) 13:46, 7 August 2008 (UTC)

Page won't load[edit]

Any ideas why the Shadow article will not load as of this timestamp? I tried editing some things to see if I could make it work, but in neither Firefox nor IE will it load. I can only see the page by manually typing in the URL for editing. Strange doings!
GodhevalT C W 19:13, 20 October 2008 (UTC)

Inverse proportion[edit]

Is a shadow an example of inverse proportion? —Preceding unsigned comment added by (talk) 08:30, 28 October 2010 (UTC)

Mythological connotations[edit]

Should the word "theory" be replaced with "idea", as "theory" has a definite scientific definition and may not be appropriate here — Preceding unsigned comment added by (talk) 11:25, 16 September 2011 (UTC)

Baaaad Example[edit]

Why using an active light source (street lamp) as an example? — Preceding unsigned comment added by (talk) 17:26, 17 November 2012 (UTC)