The next theory was provided by the brilliant Scottish physicist James Clerk Maxwell (1831 to 1879). In 1864, he predicted the existence of electromagnetic waves, the existence of which had not been confirmed before that time, and out of his prediction came the concept of light being a wave, or more specifically, a type of electromagnetic wave.

Until that time, the magnetic field produced by magnets and electric currents and the electric field generated between two parallel metal plates connected to a charged capacitor were considered to be unrelated to one another. Maxwell changed this thinking when, in 1861, he presented Maxwell's equations: four equations for electromagnetic theory that shows magnetic fields and electric fields are inextricably linked.

This led to the introduction of the concept of electromagnetic waves other than visible light into light research, which had previously focused only on visible light.