Problem 7

  Radiation of a certain wavelength causes electrons with a maximum kinetic energy of 0.68 eV to be ejected from a metal whose work function is 2.75 eV. What will be the maximum kinetic energy (in eV) with which this same radiation ejects electrons from another metal whose work function is 2.17 eV?

SOLUTION: We can determine the energy of the incident light by using the photoelectric effect equation (which we can easily derive from energy conservation) applied to the first metal.

hf = KE_max + W_o = 0.68 eV + 2.75 eV = 3.43 eV

Now, in the second case, we use the same energy of light, so we plug the new work function into the photoelectric effect equation to get

KE_max = hf - W_o = 3.43 eV - 2.17 eV = 1.26 eV .