During the years 1887 and 1888, German physicist Heinrich Rudolph Hertz conducted a series of experiments to evaluate James Clerk Maxwell's theory of electromagnetic waves. The experiments provided dramatic confirmation of Maxwell's theory. But, in an ironic by-product, they also produced the first bit of evidence that Maxwell's theory was insufficient to explain some electromagnetic phenomena.

Hertz found that when light was shined on a pair of metal spheres in the presence of an electric field, visible sparks jumped from one sphere to the other. A decade later, Joseph J. Thomson (discoverer of the electron) explained that the sparking occurred because electrons were emitted from the metal spheres. Hence the term photoelectric effect was invented to describe the release of electrons from a metal when light is shined on it.

Between 1902 and 1904, German-Hungarian physicist Philipp Lenard demonstrated the fact that the energy of electrons expelled from a metal...