(3-15) Energy of electron at rest = energy of positron at rest = mc2

           =  9.11 x 10-31  x  (3.00 x 108)2  =   8.20 x 10-14 J

So each of the photons produced in the mutual annihilation should have this energy.

photon energy  E  = hc/ λ   where λ is the wavelength, h is Planck’s constant and c is the speed of light. So we would predict:

           λ = hc/E   =   6.626 x 10-34 x 3.00 x 108 / 8.20 x 10-14     =   2.42 x 10-12 m

which accords with what is actually observed:  so it works!