Light absorption and color confuses me. Electrons absorb photon energy at certain frequencies, and the light that's not absorbed is reflected and that gives the color, but doesn't the energy get emitted again with the same frequency? So, the emitted and reflected photons would add together to give back the original color of the light rather than the object. Why doesn't that mean everything is the same color?
#physics #light #color
@2ck The light may not get emitted with the same frequency. If it did, there would be no greenhouse effect. CO2 leads to global warming because sunlight absorbed in the visible can be shifted to the infrared, which is blocked on the way out by the atmosphere. (It’s more complicated, but that’s the basic mechanism.)
@leephillips OK. I think the part that tripped me up is that I read that electrons of individual atoms will emit photons with the same energy as they absorb; however, for the most part, we don't have free atoms. Molecules OTOH can do other things with absorbed energy....which has to do with bond energy, I think? Am I getting warmer?
@2ck The emitted photon is the same energy as the absorbed one only for single-photon absorption. An atom can absorb multiple photons, and then emit photons of any energy up to the sum of all the energies in the absorbed photons. So the emission spectrum is actually continuous. For molecules, there are various vibrational modes as well, and energy in those can be fed into translational energy, raising the temperature of the material.
@leephillips That helps. I hadn't thought of absorbing multiple photons - makes sense.
I'm trying to relate the "mechanics" of absorption and emission of photons to Maxwell's equations. I started with one of Feynman's lectures, but need some "guardrails" to avoid confusing or unrelated topics since I'm working backwards for things I don't know or forgot from high school and college. Maybe I just need to retake undergrad-level physics and chemistry 🙄
@2ck Well, Maxwell’s equations don’t explain the quantized nature of radiation absorption and emission, and stuff like that. That’s why quantum mechanics was invented. First big step was by Einstein in 1905, with his paper on the photoelectric effect (that he got the Nobel prize for).
@leephillips I'm not trying to "explain" the subatomic with Maxwell... I'm just trying to understand how they relate. Like, I read that the transmission thru a medium is meditated by some process of absorption or "vibration" of dipoles in the medium: I want to understand how that transmission works and how it explains refraction, which phenomenon Feynman's lecture notes show me can be derived by modeling plane wave interaction at a boundary between two mediums using Maxwell's equations and some conservation laws. I think that's doable😅
@2ck The bounce/reflection changes the frequency depending on material, different frequencies are perceived as different colors by the receptors in the eye. The absorbed photons turn into heat, the ones which gives partial energy to an atom will be reflected at a new angle. The average frequency of all photons reaching your eye determines color.