And God said ∇ • E = ρ/ε0 ∇ • B = 0 ∇ × E = −∂B/∂t ∇ × B = μ0ε0 ∂E/∂t And there was light. It is not possible to attend a physics convention without seeing at least one of these inscribed on a wearer's T-shirt. However, Maxwell earned it. That electricity in a wire produced a magnet was observed by Hans Christian Ørsted. That moving a magnet and a wire with respect to each other could produce current in the wire was observed by Michael Faraday. These discoveries led to the invention of the electric motor and generator. Maxwell's stroke of genius was to generalize the ideas: oscillating magnetic fields produce oscillating electric fields, which in turn produce oscillating magnetic fields and so on—in empty space. The combination of these waves produced light. Thus, Maxwell showed that light was an electromagnetic wave. Among other things, this led to Einstein's theory of relativity.