in the laboratory frame, a photon of energy 8.0 x 10^3 eV and a free electron of energy 2.0 x 10^3 eV (consider what is meant by 'energy' here; is it kinetic energy? total energy? can it be total energy?) are moving directly towards each other and collide head on.
a) in the reference frame where the electron is at rest initially, what is the energy of the photon?
b) after the collision, in the reference frame where the electron was at rest initially, the photon moves in a direction opposite to its initial direction. Find the energy of the photon and the the energy of the electron after the collision in this reference frame.