Huygens' principle

The first person to explain how wave theory can also account for the laws of geometric optics was Christiaan Huygens in 1670. At the time, of course, nobody took the slightest notice of him. His work was later rediscovered after the eventual triumph of wave theory.

Huygens had a very important insight into the nature of wave propagation which is nowadays called Huygens' principle. When applied to the propagation of light waves, this principle states that:

Every point on a wave-front may be considered a source of secondary spherical wavelets which spread out in the forward direction at the speed of light. The new wave-front is the tangential surface to all of these secondary wavelets.

According to Huygens' principle, a plane light wave propagates though free space at the speed of light, $c$. The light rays associated with this wave-front propagate in straight-lines, as shown in Fig. 85. It is also fairly straightforward to account for the laws of reflection and refraction using Huygens' principle.

Figure 85: Huygen's principle.