Show that p = E/c for the photon considered in the Example 29.5.

Strategy
We will take the energy found in Example 29.5, divide it by the speed of light, and see if the same momentum is obtained as before.

Data from Example 29.5

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

Find the velocity of an electron having the same momentum. 

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.