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Einstein's Explanation of the Photoelectric Effect

By the late 1800's, it was generally believed that light was composed of electromagnetic waves. In 1887, the English physicist Thomas Young confirmed that general belief with his two-slit experiment.

The diagram below shows that light as an electromagnetic wave has a wavelength (the distance from crest to crest) and a frequency (the number of crests that pass a point in a given time). As you can see from the diagram below, radio waves have longer wavelengths and a lower frequencies than gamma waves, which have shorter wavelengths and higher frequencies.