# Absorbing all the Light

Q:
Is there a substance that absorbs 100% of all the light that hits it? and Is there anything that bends light besides gravitiy?
- Lucas Wolford (age 19)
Winter Part, Florida
A:
The answer is "well, sorta" and requires some elaboration. Light which falls on the event horizon of a black hole will certainly not be seen again. Is a black hole a "substance?" I’m not sure. Also, black holes radiate light (and other particles) via a fascinating process called Hawking radiation. One might say that light is absorbed and then re-radiated (at different frequencies). Does this count as "absorbed?"

More practically, you can make stuff which absorbs nearly all light which shines on it. Black paint on a flat surface doesn’t do a perfect job, reflecting maybe a few percent of the light that shines on it. Some black surfaces are very shiny (think of polished black leather shoes). Fortunately you can do much better than this if you really want to absorb some light. You can make a box, painted black on the inside, entirely closed except for a small hole in one of the faces. Light shined in the hole will bounce will be mostly absorbed on the opposite face, and some will bounce off. Most of the reflected light will hit other faces of the box, and only a tiny amount will come back out the hole. Making the hole smaller or the box bigger makes the hole "blacker".

A variant on this approach is to paint black the inside of a cone with a small opening angle. Light coming in along the axis of the cone hits the side wall, and most gets absorbed. The remainder gets reflected at a small angle, and must be reflected many times before it changed direction to come out the other side, reducing the amount of reflection.

A variant on the cone approach is to coat a surface with black paint that has lots of little pits and holes in it -- each one acts like the cone mentioned above. Some applications, like cameras and telescopes, need surfaces painted very black to reduce the amount of light contamination that can blur images. Here’s a of such a new kind of coating.

Note: Just because an object absorbs all the light that hits it doesn’t mean that it’s black. Something that’s hot will glow, even if it absorbs all the incoming light. Heat up the box or the cone above, and they will radiate energy in the form of light. The sun is most likely a very good absorber of light that hits it, but it would be rather hard to tell how much gets reflected because of all of the other light that gets emitted (although there are tricks of course which can be used to separate a signal from all the background -- this sounds like a good experiment!).

As for bending light: You can change the direction light goes in with gravitational fields, but it is easier to do it with lenses and mirrors.

Tom

(published on 10/22/2007)

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