Answer
See answer.
Work Step by Step
Assume the hydrogen atom starts at rest. When it emits a photon, the photon carries some momentum.
By the principle of the conservation of momentum, the H atom recoils in the opposite direction, with equal and opposite momentum. Therefore the atom also has some kinetic energy.
Energy is conserved, so there is less available energy for the photon. The photon energy will be slightly less than the prediction of equation 27–10.
