Photoelectric Effect

Einstein suspected that Planck's equation describing the observed blackbody radiation intensities was telling us something about light itself, not just about the matter that was emitting the light. That light comes in bursts of energy (photons), minimum E = hf = hc/l where h is Planck's constant, 6.36 x 10^-34 J s and f is the frequency in Hz. So he figured that a photon could collide with an electron in a chunk of metal and kick it out of there.

Using wave theory, you would think that the speed of the electrons kicked out would be a function of intensity, not frequency, so this was a radical idea. Einstein was way ahead of experimenters' ability to test his theory, but eventually he was found to be right.

It takes some energy to get an electron out of a metal (the work function, W_f). So the maximum kinetic energy an electron could acquire is hc/l - W_f. To test this, you shine light on a metal in a vacuum, with a power supply connected to the metal and to a nearby plate to collect electrons. A current is detected under the right conditions (small enough l ). Then if the collecting plate is made a little negative, current ceases. The minimum voltage for this is called the stopping potential, V_s.

Find the theoretical values of V_s and l _max, and you have found what can be easily tested experimentally.

Show that V_s = (hc/l - W_f)/e, where e is the magnitude of the electron charge, and
l _max = hc/W_f. In the V_s equation if you use electron volts of energy then e = 1 electron, and hc = 1240 eV nm.

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