What happens when an atom emits a photon of light?

An electron transitions from a higher energy orbital to a lower energy orbital.  Therefore the energy carried away by the photon is very specific ... only one wavelength.  E = frequency * c
wavelength = 2*pi/frequency

There are many different energy transitions that could occur, from any higher orbital to any lower orbital.  There is a spectrum of discrete energies (wavelengths) that can be emitted from a given atom.  These characteristic frequencies can be used to identify the atom ... that's how they know the composition of stars.

The atom loses energy.  Usually this is a result of an electron dropping to a lower energy state.  

PartB: an atom can emit light in certain wavelengths governed by the type of atom, how energetic the electron was, and how many levels it dropped.   Different types of atoms can emit photons in different, but discreet, wavelengths.   This is the source of 'emission lines' in spectroscopy.  Scientists using this instrument can tell you what kind (or kinds) of atoms emitted the light, and what kind (or kinds) of atoms were between the light source and the observer.  The second bit of data comes from absorption lines.