I started learning a bit ahead from an old physics book, and they were discussing the photoelectric effect and after that Planck's hypotheses and energy quantas.

The book said that the mass of a microscopic oscillator (what is that?) is not continuous, but discrete and the difference between states is an energy quanta:

$ \varepsilon = h\nu = E_k - E_i $ And since $ E = mc^2 $ then the mass of the photon is

$ m = \frac{h\nu}{c^3} $

How did they deduce that?

I started learning a bit ahead from an old physics book, and they were discussing the photoelectric effect and after that Planck's hypotheses and energy quantas.

The book said that the mass of a microscopic oscillator (what is that?) is not continuous, but discrete and the difference between states is an energy quanta:

$ \varepsilon = h\nu = E_k - E_i $

And since $ E = mc^2 $ then the (relativistic) mass of the photon is

$ m = \frac{h\nu}{c^2} $

How did they deduce that?