The Photoelectric Effect

The Photoelectric Effect shows that Planck's hypothesis, used to fit the Black Body data, is actually correct for EM radiation. Einstein went further and proposed, in 1905, that light was made up of particles with energy related to the frequency of the light, $E=h\nu$. (He got his Nobel prize for the Photoelectric effect, not for Special or General Relativity.)

When light strikes a polished (metal) surface electrons are ejected.

Measurements were made of the maximum electron energy versus light frequency and light intensity. Classical physics predicted that the electron energy should increase with intensity, as the electric field increases. This is not observed. The electron energy is independent of intensity and depends linearly on the light frequency, as seen the the figure above. The kinetic energy of the electrons is given by Planck's constant times the light frequency minus a work function $W$ which depends on the material.

$$\bgroup\color{black}{1\over 2}mv^2=h\nu-W\egroup$$

This equation just expresses conservation of energy with $h\nu$ being the photon energy and $W$ the binding energy of electrons in the solid. Data from the Photoelectric effect strongly supported the hypothesis that light is composed of particles (photons).