Momentum of a photon

Total energy of a photon: E = hf

From Einstein's equation, E = mc²    

The momentum, p, = mc      ð  E = mcc = pc    ð p = E/c   = hf/c

We know that c = fl

so p = hf/c =  hf/fl = h/l

The momentum of a photon is given by plank's constant divided by the wavelength

Force exerted by a photon