Wave-particle Duality

Einstein’s study of the photoelectric effect showed that light is composed of photons which explains why Planck had to assume that energy is quantized in deriving the formula for blackbody spectra.

The assertion that light is essentially a particle went against the then accepted ideas about light. The question of whether light is a particle or a wave had been going on for centuries. Newton believed that it is a particle, and he worked out a theory of refraction – the bending of light when it passes, for example, from air to glass – on the basis of light speeding up when it enters the glass. (We now know, however, that as a wave it slows down.) The wave theory became dominant with the demonstration of interference effects by Young and the explanation of electromagnetic waves by Maxwell. In explaining the photoelectric effect, Einstein was saying that the particle picture must be revived. The explanation that, somehow, light can exhibit both wave and particle properties is referred to as the wave–particle duality. This concept is the foundation of what is referred to as the quantum revolution, since it was such a radical departure from previous theories.

This was the beginnings of the quantum revolution.