Light is a wave, but when its wavelength — a few hundred nanometres — is much smaller than the size of the objects it meets (mirrors, lenses, windows), its wave nature becomes irrelevant. In this regime light can simply be treated as a bundle of straight rays. This is the viewpoint of geometrical optics, born with Euclid and refined over the centuries by Snell, Descartes, Huygens and Newton.
The underlying idea is powerful precisely because it is simple: forget the crests and troughs of the wave and follow only the direction along which the light energy propagates. Every phenomenon in this chapter — shadows, reflection, refraction, mirrors and lenses — is built by tracing rays and seeing where they end up.
Principle — The light ray
In a homogeneous medium, light travels in a straight line at constant speed. When it changes medium (or the medium is not homogeneous), rays can change direction and speed, but remain straight within each medium.
This condition — straight-line travel within each homogeneous medium — is what makes rays drawable: a ruler and pencil are enough to predict the path of light, until it meets a surface that deflects it.
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