Question

Which is the relationship between photon energy and frequency?

Answer 1

Photon has no rest mass but when photon travels at the speed of light, it gains momentum.
The energy of the photon is E= pc, c = speed of the photon and p= its momentum.

∴ E = pc
   but p = h/λ (This comes from De-broglie's theory)

∴ E = hc/λ


frequency = ν = c/λ
∴E = hν

Answer 2

The energy of a photon is directly proportional to its frequency.
The proportionality constant is Planck's constant.