The Photoelectric Effect

Light acting as a stream of particles.

Shine light on a clean metal surface and electrons may be ejected, but only if the light's frequency is above a threshold for that metal. Below it, even very bright light releases nothing. This puzzled physicists who thought of light only as a wave.

Einstein explained it by treating light as photons, each carrying energy proportional to frequency. One photon frees one electron, and whatever energy remains after paying the work function becomes the electron's kinetic energy. Brighter light means more photons and more electrons, not faster ones.

• Threshold frequency depends on the metal.
• Intensity changes the number of electrons, not their maximum energy.
• The effect is nearly instant.

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