Overview

The Snell-Descartes law of refraction introduces the index of refraction, n, very early in the development of Optics where, as a matter of fact, it is considered as a normalized value of the speed of propagation, V, of light in a transparent material. The normalization being made with respect to the speed of propagation, c, of electromagnetic waves in a vacuum.

In this chapter, we analyze the physical mechanisms, which indicate that V is different from c, as well as the law of variation of the index of refraction versus the frequency of the electromagnetic waves. First of all, we would like to introduce some important notions that a physicist should bear in mind when he starts to study the index of refraction:

• Principle of Relativity: No information can be transmitted faster than the speed of light in a vacuum; more specifically, the electric and magnetic fields that are produced at time t, by a moving electrical charge, cannot be perceived earlier than ( t + r/c) at a distance r from the charge.

• Vacuum is the only medium to show no dispersion: radio waves, as well as light waves or ?-rays, propagate exactly at the same speed c.

• The phase velocity of a sine wave in a given material depends on the frequency, the variation being more important in the vicinity of the bands of absorption of the considered material.

• Although it's not generally the...