(a) Calculate the momentum of a visible photon that has a wavelength of 500 nm.

(b) Find the velocity of an electron having the same momentum. 

(c) What is the energy of the electron, and how does it compare with the energy of the photon?

Strategy

Finding the photon momentum is a straightforward application of its definition: p = h/λ . If we find the photon momentum is small, then we can assume that an electron with the same momentum will be nonrelativistic, making it easy to find its velocity and kinetic energy from the classical formulas.