Momentum of a photon

According to the theory of relativity, the momentum of a beam of light is given by p = E/c (see homework problem 10 on page 39). Apply this to find the momentum of a single photon in terms of its frequency, and in terms of its wavelength.

Combining the equations p = E/c and E = hf , we find

To reexpress this in terms of wavelength, we use c = fl:

The second form turns out to be simpler.