The Photoelectric Effect demonstrates that the energy of a photoelectron depends on the wavelength of the incoming radiation and not on its intensity. While monochromatic radiation irradiates the photocathode, a potential is applied opposing the energy of the emitted electrons. The voltage required to just stop the current flow is proportional to the energy of the photoelectrons. Plotting this voltage as a function of the reciprocal of the wavelength gives a straight line plot. Students can use the graph's slope to calculate Planck's constant.