Introduction

Optics deals with the propagation of light through transparent media, and its interaction with mirrors, lenses, slits, etc. Optical effects can be divided into two broad classes. Firstly, those which can be explained without reference to the fact that light is fundamentally a wave phenomenon, and, secondly, those which can only be explained on the basis that light is a wave phenomenon. Let us, for the moment, consider the former class of effects. It might seem somewhat surprising that any optical effects at all can be accounted for without reference to waves. After all, as we saw in Sect. 11, light really is a wave phenomenon. It turns out, however, that wave effects are only crucially important when the wavelength of the wave is either comparable to, or much larger than, the size of the objects with which it interacts (see Sect. 14). When the wavelength of the wave becomes much smaller than the size of the objects with which it interacts then the interactions can be accounted for in a very simple geometric manner, as explained in this section. Since the wavelength of visible light is only of order a micron, it is very easy to find situations in which its wavelength is very much smaller than the size of the objects with which it interacts. Thus, ``wave-less'' optics, which is usually called geometric optics, has a very wide range of applications.

In geometric optics, light is treated as a set of rays, emanating from a source, which propagate through transparent media according to a set of three simple laws. The first law is the law of rectilinear propagation, which states that light rays propagating through a homogeneous transparent medium do so in straight-lines. The second law is the law of reflection, which governs the interaction of light rays with conducting surfaces (e.g., metallic mirrors). The third law is the law of refraction, which governs the behaviour of light rays as they traverse a sharp boundary between two different transparent media (e.g., air and glass).