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, . 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.