Introduction

The classical description of radiation and optics provides two alternative approaches. In the first and more rigorous approach, radiation is viewed as waves of electric and magnetic fields propagating in space. In the second approach, radiation is modeled by thin rays traveling from a source to a target while neglecting all aspects of its wave nature. Rigorous considerations show that the second approach,geometrical optics, is merely a class within the broader picture described by the first approach, which is calledphysical optics orelectromagnetic theory. Electromagnetic theory is normally used to describe the propagation characteristics of electromagnetic waves. It is a very general theory that can depict most effects associated with the propagation of light. Many effects – such as the interference between several waves and diffraction – can be explainedonly by electromagnetic theory. Electromagnetic theory can also be used to design imaging and illuminating optical devices such as telescopes, microscopes, projectors, and mirrors. However, many of the wave characteristics of radiation are irrelevant for the successful design of these devices; only higher-order corrections require electromagnetic wave considerations. Therefore, in applications where the wave nature of radiation can be neglected, the alternative description of radiation and optics – geometrical optics – can be used. Although the information generated by geometrical optics is less detailed than results of electromagnetic theory, it is far less complex and yet provides a remarkable prediction of the performance of imaging and projecting optical devices.