Science question!

[Næt.]

Okay, so, if - say - a submarine was made out of something that absorbs all visible light, would it still cast a shadow?

[Tanwen]

would there be a void - a sort of negative shadow?????

[Aimeendfire]

if it absorbed all visible light it would be invisible..right

[Tanwen]

It would be invisible to the eye but it would still be a solid...now my brain cell has begun to hurt!!!!!

[Næt.]

So it would still leave a shadow, because the light isn't going through it...

Also, why does the lense on my glasses leave a shadow?

[ratatosk_lemur]

Whether an object reflects all light--a perfect mirror--or absorbs all light--a perfect blackbody--or absorbs some light and reflects the rest--a normal opaque object--won't affect its shadow at all. An object's shadow is essentially a void--a place light from some source can't reach because something in its path stops it. Whether than object reflects or absorbs the light is completely irrelevant.

As for why glasses or other lenses cast shadows, this has to do with two things. First, they aren't perfectly transparent--as you notice when wearing them, you can see some reflections of their surfaces. The same is true of ordinary glass windows and things. Second, glasses lenses are designed to bend light--refract it--which means that light doesn't go straight through them, and light that would go straight through if there wasn't a lens there is bent to the side so it hits somewhere else. Thus a shadow.

[ratatosk_lemur]

if it absorbed all visible light it would be invisible..right

Not necessarily. You couldn't see it directly as a light emitter or by reflection. However, if it was between you and a light source, you would still see its silhouette.

[Kelly]

If it absorbed all visible light, it would be black. And since the light does not go through it, then certainly it would cast a shadow.

And as mentioned about "black bodies"*, it would emit photons. If the temperature of the object were normal air/water temperatures, the peak wavelength would be in the infrared.

But since it is black, it would be visible against the background, rather than invisible.

*black body: see: http://en.wikipedia.org/wiki/Black_body

In physics, a black body is an object that absorbs all light that falls on it. No electromagnetic radiation passes through it and none is reflected. Because no light is reflected or transmitted, the object appears black when it is cold.

If the black body is hot, these properties make it an ideal source of thermal radiation. If a perfect black body at a certain temperature is surrounded by other objects in thermal equilibrium at the same temperature, it will on average emit exactly as much as it absorbs, at every wavelength. Since the absorption is easy to understand—every ray that hits the body is absorbed—the emission is just as easy to understand.

A black body at temperature T emits exactly the same wavelengths and intensities which would be present in an environment at equilibrium at temperature T, and which would be absorbed by the body. Since the radiation in such an environment has a spectrum that depends only on temperature, the temperature of the object is directly related to the wavelengths of the light that it emits. At room temperature, black bodies emit infrared light, but as the temperature increases past a few hundred degrees Celsius, black bodies start to emit at visible wavelengths, from red, through orange, yellow, and white before ending up at blue, beyond which the emission includes increasing amounts of ultraviolet.

[Tanwen]

Knew we could rely on you for a sensible answer Kells...

[Næt.]

Ugh. Science has to spoil a perfectly good story idea doesn't it?

Well...fuck it, I'm gonna write it anyway.

[Forensic]

Why not just allow it to be 'transparent' or somesuch redirect of the light so it appears invisible?

[Næt.]

Because it needs the shadow! The shadow was the thing! And I need a certain amount of possibly-plausible pseudo-scientific bullshittery.

[ratatosk_lemur]

Okay, so, if - say - a submarine was made out of something that absorbs all visible light, would it still cast a shadow?

Also, come to think of it, a submarine is unlikely to cast a shadow unless it's in very shallow water / very near the bottom, simply because of water being quite good at making light diffuse so that unless the ground was very close to it, it would be lit by light going around the sub.

[luigifred]

No, it would not cast a shadow. Think of a black hole in outer space--something that really DOES absorb all light (among other things.) It lacks a shadow, too.

[Næt.]

No, it would not cast a shadow. Think of a black hole in outer space--something that really DOES absorb all light (among other things.) It lacks a shadow, too.

How do you know it lacks a shadow? There's sod all for it to cast one on to. And even it there was it would be sucked in.

[ratatosk_lemur]

No, it would not cast a shadow. Think of a black hole in outer space--something that really DOES absorb all light (among other things.) It lacks a shadow, too.

A black hole is a confusing analogy, because it has strong enough gravity that it warps space-time significantly and you have to use general relativity to predict what is going on. I'm not convinced that it is accurate to say that one doesn't cast a shadow. (What is your basis for claiming this?) In any case, though, it does do things like bend light around it, so that it can be detected without being seen. And a submarine or other object on Earth that absorbed all light (or all visible light) without having an absurdly high density and mass (and so without noticeable relativistic effects) wouldn't have gravitational lensing, so there is no way for light to be bent around it--it would block out light just like something that reflects some or all of the light that hits it. That area in which light is blocked is a shadow by definition.

[Kallan]

anything that was perfectly transparent would not cast a shadow, otherwise it would not be transparent. Now it might be possible to absorb some but not all lightl, but this would, to some degree, make it opaque and visible.

Oh Wait!!! I know the answer!!! Non-euclidean geometry!

[metalgirl2045]

If it absorbs all visible light, it would be perfectly black and cast a shadow. Shadows are where light can't get through. The water would diffuse the light, but submarines are quite big so there would be some considerable darkening underneath.

If you want it to be invisible but cast a shadow in a far-fetched but not totally impossible sci-fi story, how about it can bend light around it (this has actually been done for very small things, find out how they did it) back to front and left to right (and vice versa) but nobody bothered with top to bottom as well? I'm not sure if the known methods of beding light around and object can be choosy about the dircetion, possibly not come to think of it.

Another idea, what about the whole thing is covered in tiny cameras and fibre obtic cables which relay the light from the other side of every point on the sub, but they hadn't bothered with covering the bottom?

[Næt.]

If it was covered in tiny cameras and what-not blah blah blah it would still cast a shadow, because the light isn't getting through, right?

[Kallan]

except the cameras would also have to be screens projecting all the light as if it weren't there. Meaning the underside of the sub would be projecting an exact replica of the light that was hitting the top.

an object cant have light pass through it and still have a shadow. That's not science, that's a mathematic reality.

Shadow = absence of light due to obstruction of the light's direct path

Obstruction of light path = absorption and/or reflection

absorption/reflection = visible pressence.

Therefore Shadow = visible.

Unless of course you use non-euclidean geometry.

[metalgirl2045]

That's why I said the light isn't being routed to the bottom, if there was no light coming out of one side that one would be able to cast a shadow.

[SorryNotSorry]

Ugh. Science has to spoil a perfectly good story idea doesn't it?

Well...fuck it, I'm gonna write it anyway.

Why the disappointment? In all of my fiction, only the villains have the power to break the laws of physics.

[Shockwave]

I was reading an article a few days ago (but unfortunately can't find it now) about scientists having made 2D objects invisible and now be very close to doing it to 3D objects. The technology works by bending the light rays around the object rather than it absorbing the light. If the technology isn't 100% effective there would be some distortion and it could cast a very faint shadow.

[Gatto]

I was reading an article a few days ago (but unfortunately can't find it now) about scientists having made 2D objects invisible and now be very close to doing it to 3D objects. The technology works by bending the light rays around the object rather than it absorbing the light. If the technology isn't 100% effective there would be some distortion and it could cast a very faint shadow.

Making a 2 dimensional object invisible is easy: just turn it sideways. :D

[mad_scientist]

If it absorbs all visible light, it appears totally black.

It would cast a shadow. A shadow has nothing to do with the light that hits the object, it's because the object stops the light hitting the background. A totally transparent object, which would neither absorb nor deflect light, would not cast a shadow, and nor would the light-bending military technology that Shockwave is talking about if it was flawless. (Of course, it won't be flawless, but nevertheless they're talking about using it to hide tanks. Yeah, invisible tanks.)

[LonePiper]

Another idea, what about the whole thing is covered in tiny cameras and fibre obtic cables which relay the light from the other side of every point on the sub, but they hadn't bothered with covering the bottom?

If it was covered in tiny cameras and what-not blah blah blah it would still cast a shadow, because the light isn't getting through, right?

Actually, certain deep-sea fish do something similar - they have colour changing pigments on their underbellies and simply match it with whatever miniscule light there is to avoid predators. I think something like this would be a much more elegant solution than a massive array of cameras and optic cables. You just have to find some kind of paint or other material which can change colour according to its surroundings. (Shouldn't be too hard if we're talking about sci-fi. Just make something up! It certainly won't be too far fetched, as I'm pretty sure similar things exist) In an underwater environment devoid of features it would be near-invisibe, and even where there are features it would probably be difficult to spot unless you were looking right at it.

[Guest disjointed]

Surely it would be a reverse shadow ?

As the light hits the light absorbing submarine the light would stop so you would be able to see the light going in and then no more that surely would create a sillouhette.

assuming of course you could swim at submarine depths to see it

[metalgirl2045]

It would have a silhouette, but it would also have a shadow.