This chapter is largely descriptive. It only makes use of rather simple mathematics: algebra and trigonometry (including the exponential function).

The last chapter was about sources of terahertz radiation. The next chapter will be about sensors of terahertz radiation. In this chapter we will look at what happens between the source and the sensor. We will look at how terahertz radiation is manipulated: bent, focussed, collimated. We will use optics as a general term for manipulating light and optical elements as a general term for specific devices that manipulate light.

Following the progression of an electromagnetic field through a collection of optical elements can be carried out at various levels of sophistication.

1. Geometrical optics. At the simplest level, light rays can be traced through the optical system using the approximation that they travel in straight lines and that the effects of wave properties, namely diffraction, are ignored. The assumption that wave properties are neglected means this is referred to as geometrical optics or ray optics. The assumption that the optical elements, in particular lenses, are thin, and that the angle of divergence of light from the optical axis is small – in other words, that only paraxial rays are considered – means this is referred to as Gaussian optics, in memory of the one who developed this method. Associated with Gaussian optics are fundamental geometrical formulas you might have met in previous studies that go by names such as the thin lens formula, the lens makers’ formula, the mirror formula, the focal ratio or the f -number, formulas for magnifiers, microscopes and telescopes.

2. […]