Bohr Hydrogen Atom (1913)
(not to scale!)  

n = 1 lowest energy level; higher levels more closely spaced

each energy level specific electron energy

bright line emission of photon:

electron jumps down, from upper to lower unoccupied level

e.g. n = 3 to n = 2, the Hydrogen a famous red line

Absorption (dark) line:

photon kicks electron up from lower to higher unoccupied level photon is annihilated