Photoconductivity is an optical and electrical phenomenon in which a material becomes more electrically conductive due to the absorption of electromagnetic radiation (visible light, ultraviolet radiation, infrared radiation etc).
In 1873, the English electrical engineer Willoughby Smith discovered that the conductivity of the selenium rods he was using in a circuit for an experiment increased significantly when exposed to visible light. Here is what he observed in his experiment:
Here is a painting of Willoughby Smith:
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