The quickest route

In geometrical optics we are not particularly interested in the nature of light, but we want to know where it goes and what route it chooses when there are many possible routes.

Everybody knows that, all else being equal, light travels from A to B by the most direct route, i.e. in a straight line. But sometimes light may undergo reflection at one or more points along its journey, or it may pass through different media, such as glass or water. Depending on the angle at which it enters a new medium, it will be bent (refracted). The light no longer travels in a straight line from A to B, but chooses the quickest possible route. This is known as Fermat's principle of least time. All rules of geometrical optics on angles of reflection and refraction follow from that principle.

In this chapter we deal with reflection. The phenomenon of refraction, which is the change of direction when light enters a different medium, will be considered in Chapter 3.

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