Demonstrate what Einstein theorized about back in 1905, and later confirmed-that the energy of an electron ionized by a photon of light depends only on the wavelength of light, and not its intensity. Irradiate the photocathode with a monochromatic light source and a current will be produced when the electrons are "knocked" free of the cathode metal. Use the built-in amplifier to apply an opposing voltage that just stops the flow of electrical current. This stopping voltage is proportional to the energy of the emitted electrons. Plotting the stopping voltage against the reciprocal of the wavelength of light generates a straight line that can be used to calculate Planck's constant. Repeat the demonstration with a higher intensity of light. Students will see first-hand that the brightness of the light has no effect on the energy of the emitted electrons. An important result that paved the way to discoveries in modern physics and a better understanding of the universe. Includes three filters to provide the appropriate monochromatic light sources from an ordinary incandescent or fluorescent lightbulb. A digital voltmeter is required to measure the stopping voltage, but not included.