Question

A photon of energy E travelling in the +x direction collides elastically with an electron of mass m moving in the opposite direction. After the collision, the photon travels back along the –x direction with the same energy E. 

(a) Use the conservation of energy and momentum to demonstrate that the initial and final electron momenta are equal and opposite and of magnitude E/c. 

(b) Hence show that the electron speed is given by v/c = (1 + (m_c²/E)²)^−1/2

Answer 1

(a) As the collision is elastic the total initial energy = total final energy.