An accidental discovery

The photoelectric effect was first discovered, quite by accident, in 1887, by Heinrich Hertz (1857-1894), who was working on the production of radio waves by means of an electrical spark discharge between two metal spheres. He noticed that when he illuminated the spheres with ultraviolet light, the sparks which came across were bigger and brighter. At the time he did not realise that this was due to electrons liberated from the metal by the light. A year later Wilhelm Hallwachs (1859-1922) confirmed the phenomenon for various other metallic surfaces.

There are 'grains' in the light beam

The fact that the photoelectric effect works at all is already strong evidence for the particle-like nature of light. Bits of matter thrown out look more like the result of sandblasting! Particularly convincing is the fact that there is no measurable delay between exposure to light and the appearance of photo-electrons. Light of intensity as low as a few microwatts per square metre produces an immediate, detectable photoelectric current. The effect is practically instantaneous (the delay is less than 10-9s).

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