Take the 2-minute tour ×  When an electron emits a photon from
changing energy levels, the frequency of the photon depends on
the difference between the energy levels.  But if someone is
moving with respect to the atom, the frequency will be
apparently red shifted or blue shifted.  Does this mean that the
energy levels of the orbits of the atom look to have different
values if you are moving with respect to them? But, the apparent
energy difference can be different depending on whether the
photon is emitted towards you or away from you.  What's going
on? Aren't energy levels of orbits supposed to be a fixed value
given the kind of atom?  share|improve this question Good
question, but is the photon really emitted in a given direction?
I'm sure a hydrogen transition doesn't emit a plane wave photon
(but I'm not sure what it does emit...maybe s-wave, at least for
some transitions?) –  twistor59 Jan 18 '13 at 14:02 Broadening
of spectral lines occurs due to the Doppler effect (as well as a
number of other effects