Who proved that light behaves as electromagnetic waves?

James Clerk Maxwell, who did it on paper. His equations unifying electricity and magnetism predicted waves travelling at a speed he could calculate from two constants measured in laboratory experiments with wires and magnets - and the number came out at the measured speed of light. He wrote that we can scarcely avoid the inference that light is an electromagnetic disturbance.

That is one of the great moments of theoretical prediction: light was shown to be electromagnetism using measurements that had nothing to do with light. It also implied a whole spectrum beyond the visible, which Hertz then generated and detected in the laboratory twenty years later, and which became radio within a decade of that.

Maxwell is chronically underrated outside physics, where he is generally ranked with Newton and Einstein. He also produced the first colour photograph, the kinetic theory of gases, the demon that connected thermodynamics to information, and the analysis showing Saturn's rings must be countless small particles. He died at 48 of the same abdominal cancer that had killed his mother at the same age. Einstein kept a photograph of him on his study wall.