Hey,

The scientist name Bohr basically workout few postulates in order to explain the movement of the negatively charged electron around the positively charged nucleus.

Within these postulates he also explained the relationship for the frequency of emitted photons in a transition.

According to his postulate, electron moves jumps from higher energy to lower energy state and in order of doing that they release energy in the form of photon whose energy is equal to difference between the energy of higher or lower states or orbits .

hv= Ei- Ef

Hope that helps.

Thank you