70. Photoelectric Effect

A very simple demonstration of the photoelectric effect is performed with a zinc plate as the electrode of an electroscope. An ultraviolet lamp covered with glass is arranged to shine on the plate. The plate is charged negative with an electrophorus, and the electroscope needle diverges indicating the charge. The blue light of the lamp will not knock out electrons from zinc, but if the glass (opaque to UV) is removed from the lamp, the needle quickly falls as electrons are kicked away from the plate. The zinc plate must be cleaned with steel wool within an hour or so of the demonstration to remove the oxide.

A variation of this experiment has a spiral electrode with a positive voltage in front of the zinc plate with a sensitive current meter to measure the small current of the photoelectrons through the air.

The photoelectric effect is also done as experiment 4 in the 8E lab. The stopping voltage is measured as a function of wavelength (color) of the exciting light, and Planck's constant determined from the slope of the line.