Despite a lot of attention in classical antiquity and in the early Renaissance, the rainbow was not understood until the work of Theodoric of Freiberg in 1307. Not knowing about Theodoric's work, Rene Descartes repeated it in 1637.

Christiaan Huygens was the first to provide a correct derivation of Snell's Law. His ideas of wave propagation are particularly valuable in understanding the phenomena of diffraction, to be discussed later in the course.

Total internal reflection occurs when light travels from a medium of a given index of refraction to a medium with a smaller index of refraction. There will then be some incident angle for which the refracted ray angle is 90 degrees, meaning there is no refracted ray for this incident angle, or any larger incident angle. The critical angle is obviously given by θc = sin-1[nout/nin].