Electron Absorbing a Photon I'm trying to understand the absorption spectrum in terms of what happens when an electron absorbs a photon. If we shine white light through a sample and use a prism to disperse the light, we would see black lines corresponding to the wavelengths absorbed by the electron. However, if the specific wavelength is absorbed, wouldn't it be released once it comes down anyways? Why then do we see dark lines?
My assumption is that when the electron falls back down to a lower energy level, the photon is scattered in all directions so the intensity of the light for that wavelength is reduced in comparison to the other wavelengths we can see (i.e the rest of the spectrum).
 A: First we need to clarify:


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*when an photon interacts with an atom, three things can happen:


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*elastic scattering, the photon keeps all its energy, and changes angle

*inelastic scattering, the photon gives part of its energy to the atom, and changes angle

*absorption, the photon gives all its energy to the atom, sending the valence electron to a higher energy level


*what you are wrong about is that you think the bound electrons are absorbing and emitting photons

*it is the atom that does it, and it, the nucleus and electron system has many available energy levels according to QM

*the energy of the photon has to be exactly the same as the difference between the energy levels of the valence electron

*the probability of the atom absorbing the photon is very high if the energy level is exactly the same.

*the probability is very small if the energy level of the photon is not the same

*that is why you see spectra

*the angle of elastic scattering (reflection) is the same as absorption for glass, and the opposite for mirrors (though in the case of glass, most of the photons get elastically scattered, and only few get absorbed, because elastic scattering is the only way to keep a mirror image, and the photons that get absorbed, will heat up the glass)
A: First, an electron cannot absorb a photon, it can only scatter it. But your question stands when discussing absorption by atoms.
You are quite right, that if your "sample" is in equilibrium, that for every atomic transition that absorbs a photon, there must be a downward transition that produces a photon.
There are basically two photon production processes - spontaneous and stimulated emission.
The situation where you see absorption lines is caused by spontaneous emission, because the photons are emitted in all directions, not just in the direction of the original beam.
The net effect is to remove photons at the characteristic wavelength from the beam and thus produce an "absorption line".
Interestingly, if stimulated emission dominates, then the emitted photons would be in the same direction, at the same wavelength, as the absorbed photons and there would be no absorption lines. This is rarely the case at visible wavelengths, but often the case at radio wavelengths.
