particle physics - Frequency of an electron

If frequency is defined as the cycles per time, then what is meant by "frequency of an electron"? If it refers to the rotation of electron around a nucleus, then which phenomenon is considered for a free electron i.e. an electron in a force field?

Is "frequency of an electron" an experimental quantity?

My teacher told me how to calculate the frequency of an electron. We started from finding energy of electron, then difference in energy, then we get this equation according to the Bohr radius of a hydrogen atom and

$$ f = \frac{z^2e^42\pi^2m}{h^3} \left(\frac{1}{n_1^2} - \frac{1}{n_2^2} \right)$$

Where:

• $z =$ atomic number


• $e =$ charge of proton



• $m =$ mass of electron


• $h =$ Planck constant


• $n =$ orbit number

From the last part of my equation, I am confused. Does the $n_1$ and $n_2$ show that that frequency will be the frequency of energy or electrons?