Introduction

Since the earlier standards, light has been studied as a ray -- a straight-line path along which light energy travels through a uniform, homogeneous medium. Two laws were built entirely on this ray picture. The law of reflection: the incident ray, the reflected ray and the normal to the reflecting surface at the point of incidence all lie in one plane, and the angle of incidence equals the angle of reflection. The law of refraction (Snell's law): the incident ray, the refracted ray and the normal all lie in one plane, and $n_1\sin i = n_2\sin r$, where $n_1, n_2$ are the absolute refractive indices of the two media. These same ray laws were also used to understand reflection by spherical mirrors and refraction through prisms and lenses, where the position and nature (real/virtual) of an image depends on the object's position and the mirror's or lens's focal length.

But ray optics (also called geometrical optics, since it leans entirely on geometric constructions) is really only an APPROXIMATION -- a very good one whenever the objects, apertures and obstacles involved are much LARGER than the wavelength of light, but one that quietly breaks down otherwise. It cannot, on its own, explain WHY the laws of reflection and refraction hold in the first place, nor can it explain a family of phenomena -- interference, diffraction, and polarization -- that are only observed once objects or slits become comparable in size to light's own wavelength. Answering these questions requires treating light not as a ray but as a WAVE, and that is the subject of this chapter: wave optics.