Geometrical optics is the study of image formation. The function of lens and mirror systems is to collect radiation from the object and produce a geometrically similar distribution of flux, an image, across the image plane.

We consider first the electromagnetic spectrum, then proceed to the principles of image formation. We then consider issues affecting image quality and conclude this chapter by surveying the optical properties of materials.

Light is a sinusoidal electromagnetic wave that travels either in a material medium or in a vacuum. We describe electromagnetic (EM) radiation using wavelength, defined as the distance over which a sinusoidal waveform repeats itself. We will consider many optical phenomena that depend on wavelength. Radiation classified according to wavelength falls into one of the categories listed in Table 1.1. The divisions between the various categories are approximate and based on differences in source and detector technologies, the response of the human eye, and atmospheric transmission. Optical radiation is only a small portion of the entire EM spectrum, which extends from radio waves at long wavelength to x rays at short wavelength. Electromagnetic radiation to which our eyes respond has wavelengths between approximately 0.4 μm (violet) and 0.7 μm (red). Invisible radiation includes ultraviolet (UV) radiation, which has wavelengths shorter than violet, and infrared (IR) radiation, which has wavelengths longer than red.