The Photoelectric Effect

The photoelectric effect is the emission of electrons from a metal surface when light of sufficiently high frequency shines on it. It was one of the most important experiments in the history of physics because its results could not be explained by the classical wave theory of light. Einstein's explanation — that light consists of discrete packets of energy called photons — earned him the Nobel Prize and laid the foundation for quantum physics.

---

The Observations

When ultraviolet light shines on a zinc plate connected to a negatively charged electroscope, the electroscope discharges — electrons are being emitted from the zinc surface. These emitted electrons are called photoelectrons.

Careful experiments revealed several observations that were deeply puzzling from the standpoint of classical wave theory:

Observation 1: Threshold frequency. For each metal, there is a minimum frequency of light (the threshold frequency, f₀) below which no electrons are emitted, no matter how intense the light. For zinc, visible light (even very bright visible light) cannot cause emission. Only ultraviolet light (above the threshold frequency) works.