Bohr makes the hypothesis that electrons can exist only at certain ‘special’ distances from the nucleus, that is, only on certain particular orbits, the scale of which is determined by Planck’s constant h. And that electrons can ‘leap’ between one orbit with the permitted energy to another. These are the famous ‘quantum leaps’. The frequency at which the electron moves on these orbits determines the frequency of the emitted light and, since only certain orbits are allowed, it follows that only certain frequencies are emitted.
Frequency is when electron jumps from one to another in certain orbit
