The Kerr Effect was the first electro-optic effect discovered. It is largely similar to photoelasticity, except it causes birefringence by placing a optically isotropic substance in an electric field. The direction of the birefringence cooresponds to the direction of the elecric field. It was discoved in 1875 by John Kerr, a Scotsman. Kerr was able to go beyond Brewster's photoelasticity, however, and realized that the index of refraction of the material changed and quantified it mathematically. The formula he derived:

(change in)n = (wavelength of light)*K*E²

n being the index of refraction, E being the electric field, and K being the Kerr Constant and varies with the material used. It is measured in electrostatic units, often (10^-7 cm * statvolt^-2).