Photons may be generated by the transition of an electron from one energy level in an atom or molecule to a lower energy level. Photons may be absorbed as they cause an electron to be raised from a lower energy level to a higher energy level (in an atom or molecule).
The photon itself does not undergo a transition of energy: it either exists (with an energy defined by its wavelength), or it doesn't exist (it was destroyed!). You could say that the emitting or absorbing atom/molecule/etc. undergoes a change, or transition, in energy. But "transition" is usually used as a name for the process of jumping in energy.
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