Einstein proved his theory by proving that the Planck's constant he derived based on his experiments on the photoelectric effect exactly matched the constant 6.6260755 x 10-34 (Planck's constant) that German physicist Max Planck (1858 to 1947) obtained in 1900 through his research on electromagnetic waves.

This too pointed to an intimate relationship between the properties and oscillation frequency of light as a wave and the properties and momentum (energy) of light as a particle, or in other words, the dual nature of light as both a particle and a wave.