I’m confused about why we don’t see absorbed light. The way I understand it is if an atom absorbs a photon then the electrons move up to an energy level corresponding to the energy of the photon. The electrons move down and emit a photon with the same energy so the emitted light would be of the same or lower wavelength as the absorbed light? When hydrogen is excited the colour seen is a mixture of the emitted wavelengths which are the same as the wavelengths absorbed. So why is it that objects that absorb red light reflect blue light? Is red light still emitted but we don’t see it, or does this energy absorbed completely disappear with no emission? Is emission of photons from excited atoms different to how colour is seen from other objects? I’d really appreciate if someone could explain this to me
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1$\begingroup$ I think that this is similar to the question physics.stackexchange.com/questions/740940/…. Look at my answer there and see if it helps. $\endgroup$– nasuCommented Oct 2, 2023 at 12:28
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$\begingroup$ Thank you, I read your answer and get how absorption and emission isn’t the same as seeing color due to reflection of light. So is seeing color in a discharge tube of a particular element related to atomic spectra but not the same as color because of reflection of light at a surface? Also how would you explain seeing color in transition metals, is it the same as fluorescence where electrons excite and de-excite, or do they stay in the excited state and so there is only absorption and no emission? Is this also the case for seeing reflection of color? $\endgroup$– lemonmeringueCommented Oct 2, 2023 at 13:07
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$\begingroup$ Do you mean the colour of a solid piece of metal, like copper or gold? $\endgroup$– nasuCommented Oct 2, 2023 at 18:01
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$\begingroup$ The transition metal complexes, and also solid pieces of metal $\endgroup$– lemonmeringueCommented Oct 2, 2023 at 18:57
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$\begingroup$ There are different mechanisms depending on the system. There is no general mechanism. It may be light interaction with free electrons in metals or with defects in ionic crystals and so on. $\endgroup$– nasuCommented Oct 2, 2023 at 19:32
2 Answers
For free atoms, you are correct. An atom will absorb on resonance light and reemit it after some time. This is resonance fluorescence.
However, if you take a bunch of atoms, or molecules, and make a solid then some of the absorbed light will be converted to heat. The excited atoms are coupled to phonons of the material, so in this case the reemission doesn't happen.
The colour of solids is due to various processes. There is no unique explanation for all types of solids. For solid metals, light interacts with the free electrons in the conduction bands. This results in a very strong reflexion for all the frequencies in the visible band. So, most metals have no special colour. You could call them white or grey but due to the strong reflexion they look shiny or "metallic". There are a few exceptions, like copper or gold. They have free electrons too, as all metals, so they are shiny. But there is some absorption of light in the green-blue band so the reflected light is displaced towards red. This absorption is not due to free electrons but to the electrons in the ionic cores, electrons on the "d" levels who can be excited to the "s" levels which for these few metals are not fully occupied.
For salts and other ionic compounds, the absorption can be due on either transitions between electron bands or on localized absorption centers like impurities or defects levels. For example, the colour of rubies is due to impurities. A pure crystal of aluminum oxide is colorless. With chromium impurities is ruby red.
If you are interested in the colour of a specific compound you need to look up the optical properties of that compound.
