Introduction: polarized light

James Clerk Maxwell (1831–1879) discovered the first unified field theory when in 1865 he showed, drawing on Faraday's experiments, the Biot–Savart law and other results, that interacting electric and magnetic fields and forces could be described by the same set of equations. Oliver Heaviside (1850–1925) reduced the twenty quaternion equations of Maxwell's theory to the four vector equations that ordinary mortals could understand. Maxwell showed that a wave equation could be deduced and that the consequent electromagnetic field propagates in a direction perpendicular to its electric and magnetic field vectors, and at a velocity which could be calculated from two quantities (permittivity and susceptibility) measurable in separate laboratory experiments. The resultant computed velocity was very close indeed to the experimentally measured speed of light. It was an easy conjecture then, that light was electromagnetic radiation. It was a further logical extension to predict radio waves, later confirmed by David Hughes' and Heinrich Hertz's spark-gap experiments.

The mutually perpendicular electric and magnetic field vectors oscillate in amplitude at about 5 × 1014 Hz for red light. If the direction of propagation is the positive z-direction, then the electric and magnetic vectors lie in the x−y plane. The light is said to be plane polarized if the angle the electric vector makes with the x-axis is constant in time.