photoelectric effect

The photoelectric effect is the ejection of electrons from a metal surface when light shines on it. Discovered in the late 1800s, it looks simple, but its details defied classical expectations so completely that explaining them earned Einstein his Nobel Prize. Shine the right light on a clean metal and electrons fly off; measure their energy and the surprises begin.

Classically, light is a wave whose energy depends on its brightness, so a bright enough beam of any colour should, given time, shake electrons loose with ever more energy. Experiment says otherwise. Below a certain threshold frequency — a certain colour — no electrons come off at all, however blinding the light. Above it, electrons appear instantly, and making the light brighter releases more of them but does not give each one more energy. Only raising the frequency raises an electron's energy.

These facts make no sense if light is a continuous wave, but they fall into place at once if light arrives as individual packets. Each packet, a photon, carries energy proportional to frequency and gives all of it to a single electron in one shot. A redder photon may simply not carry enough to free an electron, no matter how many of them rain down. The photoelectric effect is the cleanest early evidence that light is granular.