What is the photoelectric effect? Describe the observations of Hertz, Hallwachs and Lenard.

1. 1 · Define the effect. Emission of electrons from a metal surface when light of suitable frequency falls on it.
2. 2 · Hertz. Ultraviolet light on the emitter plate made sparks jump more easily across the detector gap (1887).
3. 3 · Hallwachs. A negative zinc plate lost its charge in ultraviolet light; a neutral plate became positive. So negative particles were emitted.
4. 4 · Lenard. Ultraviolet light on the emitter in an evacuated tube made a current flow; the current stopped when the light stopped.
5. 5 · Threshold. No emission below a threshold frequency ν₀, however intense the light.
6. 6 · Real life. Photocells in burglar alarms use this effect.
7. 7 · Close in one line. So, light of high enough frequency can free electrons from metals.

Scoring tip: Name each scientist with his observation; one clear line each is enough.