Using hydrogen in his experiments Bohr showed that an electron spins around the atom nuclei at a specific/discrete radii (orbital levels). Electrons at every orbital level spin at a discrete angular momentum and energy. When the electron moves one orbital level down, it releases a photon of energy specific to the jump (that is, of a particular spectrum that is unique to the atom). When the electron goes up an orbital level, energy (of a specific spectrum) is absorbed. This emission and absorption spectrum varies across atoms and is calculated as (delta)E = hf where;
E = energy
h = Planck's constant
f = frequency