Photoelectric effect

Hertz set up an electrical circuit that oscillated between two metal balls separated by an air gap, sending a spark across the gap whenever potential reached a peak. Hertz noted that when UV light shone on the negative terminal, the balls sparked more readily. This was the first observation of the photoelectric effect.
 Heinrich Rudolph Hertz Germany 1887 Photoelectric effect

Hertz set up an electrical circuit that oscillated between two metal balls separated by an air gap, sending a spark across the gap whenever potential reached a peak. Hertz noted that when UV light shone on the negative terminal, the balls sparked more readily. This was the first observation of the photoelectric effect.