(3-18)  We know, indeed, that the mass of a photon is zero and that its velocity is, obviously, the speed of light (c).  Now, your task is to find an expression for the momentum of a photon, to find out whether it is, as Nefertiti claims, not zero.  

①  Your first thought may be, let’s try the relativistic momentum equation (equation III-1). Give it a go.

②  You should have found, annoyingly, that your first approach just refused to answer the question. Now let’s try a different way. Courtesy of Uncle Albert and Max Planck, you already know an expression for the energy of a photon:

                    E  =  hν  =  hc/λ      

(where ν is the photon frequency, λ is its wavelength and h is Planck’s constant)

We have in this very chapter derived another expression that should give the total energy of a particle: the energy-momentum relation:

E  =  √(m2c4  +  p2c2)   

See if you can combine these two expressions to deduce an expression for the momentum of a photon.