When visible light, X rays, gamma rays, or other forms of electromagnetic radiation are

shined on certain kinds of matter, electrons are ejected. That phenomenon is known as the

photoelectric effect. The photoelectric effect was discovered by German physicist Heinrich

Hertz (1857–1894) in 1887. You can imagine the effect as follows: Suppose that a metal plate

is attached by two wires to a galvanometer. (A galvanometer is an instrument for measuring

the flow of electric current.) If light of the correct color is shined on the metal plate, the

galvanometer may register a current. That reading indicates that electrons have been ejected

from the metal plate. Those electrons then flow through the external wires and the

galvanometer, providing the observed reading.

Photoelectric theory

The photoelectric effect is important in history because it caused scientists to think about light

and other forms of electromagnetic radiation in a different way. The peculiar thing about the

photoelectric effect is the relationship between the intensity of the light shined on a piece of

metal and the amount of electric current produced.