# Expression for the (relativistic) mass of the photon [closed]

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?

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## closed as too localized by Qmechanic♦, Manishearth♦, Waffle's Crazy Peanut, Emilio Pisanty, Sklivvz♦Dec 26 '12 at 13:08

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It seems that in my book there has been a misprint, as the mass got rendered as $h\nu c^{-3}$ instead of $h\nu c^{-2}$. In that case I see how they deduced that. – andreas.vitikan Aug 13 '12 at 9:59

We could equate Planck's quantum theory (wave nature) which gives the expression for energy of a wave of frequency $\nu$, ($E=h\nu$) with Einstein's mass-energy equivalence (particle nature) which gives relativistic energy for photon ($E=mc^2$)
$$mc^2=h\nu$$