In the previous chapter, we analyzed several optical systems using geometrical laws that govern the propagation of light rays. Such analysis adheres to the corpuscular theory of light, which is based on treating the propagation of light energy as light rays. These rays are characterized by two parameters: their position and slope with respect to an optic axis. It was demonstrated that geometrical optics provides a good approximation in analyzing imaging systems. Furthermore, geometrical optics have shown excellent results in designing optical systems under the assumption that the wavelength of light is extremely small, which is the case in the visible spectrum.