Answer: when moving to a lower orbit
Explanation:
In Bohr's model of the atom, electrons orbit around the nucleus in some atomic shells. Each shell is characterized by a different fixed energy: the shell closest to the nucleus has the lowest energy, and the farther the shell from the nucleus, the greater the energy.
Therefore, electrons orbiting on a fixed shell have always the same energy. When they jump through different shells, however, they change energy, releasing or absorbing photons with energy equal to the difference in energy between the two shells.
In particular, when an electron moves to a lower orbit, its energy decreases, so (since energy must be conserved) it emits a photon with energy equal to the difference in energy between the two shells.
