Absorbing Photons

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For electrons in some atom to absorb a photon having some amount of energy, must this energy be (exactly) equal to the difference between the two energy levels?, can't it be absorbed when its energy is more than that difference?

Because in the case of (exact-equality), it would be hard for an electron in a hydrogen atom to absorb photons emitted by different atoms, since the differences of their energy levels can hardly (or even impossibly) match that of hydrogen's.

Consequently, hydrogen receives only from hydrogen..?!

ThanQ U
 
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Short answer: No

Actually, the energy does not have to match exactly. One speaks of broadening of spectral lines, meaning that there is a finite range of frequencies that "match". Broadening comes from several sources.

First of all, there is natural linewidth: Since the excited atomic state has a finite lifetime, the energies do not have to match exactly (roughly speaking, there is a Heisenberg uncertainty principle trading off energy against time, although this explanation is a bit simplistic).
You also have doppler broadening: Because of the Doppler effect, the exact frequency matching your atomic transition depends on the relative motion between the atom and the light source. Finally you have collisional broadening coming from the scattering between atoms.
http://en.wikipedia.org/wiki/Spectral_line" [Broken] has a short explanation of this as well, however with poor references. Textbooks on quantum optics (like Loudon's classic "The quantum theory of light) explains this on an advanced level.
 
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Bill_K

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Consequently, hydrogen receives only from hydrogen..?!
What typically excites an atom is black body radiation, which has a continuous spectrum. Thus you observe discrete absorption lines in the solar specturm.
 

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