# The energy of a photon?

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## Main Question or Discussion Point

please look at he underligned formula. is it a axiom we have to ake for granted ?

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Drakkith
Staff Emeritus
No, we can and have measured this as far as I know.

I think i got it
E=h.v
-De broglie : p=h/(the wace longitude:L)
hence:
E=h.c/L=p.c

jtbell
Mentor
For relativistic particles, the general relationship between energy, momentum and mass is $$E = \sqrt {(pc)^2 + (mc^2)^2}$$ where m is the invariant mass (often called the "rest mass"). For photons, m = 0 which leads to E = pc.