Photoelectric Effect: Hertz's Observations

What Hertz was actually doing. In 1887, Heinrich Hertz was not investigating the photoelectric effect at all -- he was carrying out the historic experiment that first produced and detected electromagnetic waves in the laboratory, directly confirming Maxwell's theoretical prediction of their existence. His apparatus used a spark gap at a transmitter (an induction coil driving two metal spheres separated by a small air gap, across which a spark could jump) and a similar, separate spark gap at a receiver placed some distance away, arranged so that the electromagnetic wave radiated by the transmitter's spark would induce a small enough voltage across the receiver's gap to make a faint spark jump there too, confirming the wave had actually travelled across the intervening space.

The chance observation. While refining this setup, Hertz noticed something that had nothing directly to do with the wave he was trying to detect: the receiver's spark jumped more easily, and across a slightly wider gap, when the receiver gap was exposed to the ultraviolet light coming from the transmitter's own spark. Shielding the receiver gap from this ultraviolet light (for instance, with a sheet of ordinary glass, which absorbs ultraviolet strongly) made the receiver spark noticeably harder to produce, even though the electromagnetic wave itself was completely unaffected by the glass. …