Huygen's Wave Theory

In 1678, Christian Huygens proposed that light was a wave.
He suggested that all the points on a wavefront act as point sources of secondary spherical wavlets that spread out in the forward direction at the speed of the wave. The new wave front is the surface that is at a tangent to all the secondary wavelets.

This is known as Huygen's Principle.

Make sure you can carry out Huygen's geometric construction

Huygen could explain both reflection and refraction using his wave theory by considering the wavefronts.
He said that when light passed from a less optically dense to more optically dense medium, it slowed down