Discovery of the photoelectric effect

Heinrich Hertz came across photoelectric emission in 1887 while producing electromagnetic waves with a spark discharge. The sparks across his detector loop grew stronger when ultraviolet light from an arc lamp fell on the emitter plate.

The light helps free charged particles, which we now know to be electrons, to leave the surface: electrons near the surface take in enough energy from the light to overcome the attraction of the positive ions.

Wilhelm Hallwachs and Philipp Lenard studied the effect in detail between 1886 and 1902. Lenard shone ultraviolet light on one of two plates sealed in an evacuated tube: a current flowed in the circuit, and it stopped the moment the light was cut off.

Hallwachs, in 1888, joined a zinc plate to an electroscope. A negatively charged plate lost its charge under ultraviolet light; an uncharged plate became positive; a positively charged plate became more positive. So the light was driving negative particles out of the zinc.

Below a certain minimum frequency of light, the threshold frequency, no electrons came out at all. The threshold depends on the material of the emitter.

Zinc, cadmium and magnesium need short-wavelength ultraviolet light before they give off electrons. The alkali metals (lithium, sodium, potassium, caesium, rubidium) give them off even in visible light.

Once the electron was known, the emitted particles were called photoelectrons and the whole phenomenon the photoelectric effect.