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[sorry, this way below the level of this forum -- flames are most welcome]

When a photon is absorbed by a piece of matter that does not reflect it -- where does the photon "go"? Eg, one shines light at a black object -- clearly the photon arrives at the object and then, well, vanishes. Or does it?

Thanks very much.

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Don't worry about being "way below the level of this forum" - we don't have a level ;-) Just because most of the questions are more advanced doesn't mean we don't welcome well-thought-out elementary questions. (Also it's not a forum, it's a Q&A site, but that's beside the point) –  David Z Jun 20 '11 at 2:13

2 Answers 2

You could think of a photon in this case as simply a little bundle of energy. That energy is absorbed by the material, and the photon ceases to exist. An analogy would be waves crashing on a shore -- the wave carries energy, which is absorbed by the shore. Where did the wave go?

Of course this is a vastly simplified answer. A photon is the quanta of energy in the electromagnetic field, while ocean waves are not quantized at all. The important thing to understand is that a photon is not a material object like something you can hold in your hand, and there should be no reason to expect that it should continue to exist after an interaction.

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One way or another, the photon is absorbed by the material. If the photon has a high enough energy it could raise the energy level of an electron or even liberate one completely from an atom. This is the basis of the photo-electric effect that Einstein explained (and got the Noble prize for if I recall).

A low energy photon can excite a phonon, that is, a vibration in the solid. These increase the heat of the solid and with enough of them, you could get a detectable increase in temperature.

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