James Clerk Maxwell, living from 1831 to 1879, was one of the great mathematicians of his time. Shortly after being set on the project of electromagnetism by Faraday, he soon saw the connection between the approach of Faraday and that of the Germans (Reimann and Gauss) and (perhaps to Faraday's chagrin) showed that they were equivalent. In one of the most elegant theories of all time, he wrote down the equations that described electromagnetism and the propagation of electromagnetic waves. The equations are now known, of course, as Maxwell's Equations.
One of the first things he did with his equations was to calculate the velocity of the propagation of an electromagnetic wave. When he did so, he found that the velocity of such a wave was almost identical to the measured velocity for light. Based on this, he was the first to propose that light was actually an electromagnetic wave.