Scientists' enthusiasm for the wave theory of light was temporarily dampened, however, by the discovery of the photoelectric effect toward the end of the 19th century. A typical apparatus for investigating this effect is shown in a simplified schematic. After all, waves can carry energy, presumably enough energy to cause electrons to flow. The problem resides in the time factor. Calculations based on wave theory indicate that it would take about a year for the average electron on the metal plate to receive enough energy for it to migrate to the other electrode.
Experiments indicate, however, that electron flow begins immediately as light impinges on the plate.
In 1905, showed how an idea that had recently been proposed by could be used to explain the photoelectric effect . From Planck's theory comes the notion that light travels as a stream of little bundles of energy known as quanta. Though the quantum theory solved the photo-voltaic mystery, many physicists were incredulous that light could propagate as a beam of discrete packets of energy instead of like a wave rippling outward from a source. For example, radio waves are strictly wave-like, whereas cosmic rays behave like particles. With light, certain of its characteristics are best described by wave theory while others require a quantum approach.