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Q & A: Light affecting materials

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Most recent answer: 10/22/2007
Q:
I am wondering, since light has no mass but does have energy and momentum, then is there any physical result besides heating when photons collide with an object (something visible to our eyes, like a metal)?
- Dan (age 14)
Martin Meylin Middle School, Srasburg, PA, United States
A:
Yes, light can affect materials in a great many ways. The most trivial effect is that the photons can bounce off and you can see them. Most metals work in this way, as mirrors. Most materials absorb some colors of light and reflect others, to varying extents, so that they are colored. Some materials will even absorb light of one color and re-emit light of another color, an effect called fluorescense. Shine a "black" (ultraviolet) light on some fluorescent materials and the materials will appear to glow.

Some materials store up light energy and emit it later. Glow-in-the-dark stuff glows like this. Electrons are excited to higher levels of energy in the material and take sometimes minutes or hours to decay away.

Many materials just get hot when you shine light on them, and some will melt if you shine enough light on them (ice, wax,..). Some materials, like some inexpensive dyes, will "bleach" in sunlight -- they will lose some of their pigmentation and become more whitish.

Some special materials will even generate electricity when you shine light on them, like semiconductor solar cells. Some materials will emit electrons when hit with photons.

Plants even use the light energy via photosynthesis to manufacture foods the plant needs, at the same time generating oxygen and consuming carbon dioxide and water.

But maybe you are interested in the momentum that photons carry. This momentum is very small and it takes quite a lot of light and a very delicate apparatus to even be able to measure an effect of it.

Tom

(published on 10/22/2007)

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