An atom can drop from an allowed energy E₁ to a lower allowed energy E₂ by emitting a photon with energy:
hf = E₁ - E₂ Only frequencies corresponding to differences between allowed energies can be emitted.

An atom can drop from an allowed energy E1 to a lower allowed energy E2 by emitting a photon with energy: hf = E1 - E2 Only frequencies corresponding to differences between allowed energies can be emitted.