It was assumed that the more intense the beam of light was, the greater its energy. Hence it was deeply disturbing in 1900 to discover that a light beam of greater intensity did not make the electrons move faster after emission; it only caused more of them to be emitted (see right).

Light behaved as if it consisted of separate particles all with the same energy, with each ejected electron being the result of a collision between one particle of light and one electron in the metal.

Greater intensity of light meant only that more light particles were hitting the metal per second, not that there was more energy per particle.