(3-18)   ①  We have our relativistic momentum definition,

Now, for a photon, m = 0 and v = c.  Inserting these values into our definition, we get:

Since 0/0 does not have a defined value, this does not give us an answer as to the momentum of a photon.

②  Since a photon has zero mass, the energy-momentum relation simplifies nicely:

                          E  =  √(m2c4  +  p2c2) =   √(0 x c4  +  p2c2) =  √(p2c2) =  pc

 We can now easily equate this energy with that defined by Planck and Einstein:

                          E  =  hc/λ    =  pc      

From which it follows that the momentum of a photon, p  =  h / λ.