All of this discussion is straightforward and agrees with classical physics. It suggests that light and other electromagnetic radiation are waves.

It was very disturbing, therefore, when phenomena were discovered (around 1900) that clearly indicated that light was made up of particles. One such phenomenon involved the photoelectric effect.

It was known that if one shines a beam of light on a clean surface of a metal of low electronegativity, electrons would be knocked from the metal. This is familiar to us today from the use of cesium metal in photocells, electric eyes, and vidicon TV cameras (right).

The electrons that are ejected from the metal by light in these devices are used to trigger a signal or to record an image.