What is Huygen's principle?

What is the importance of Huygens' principle?

State Huygen's principle of propagation of light and prove the laws (Snell's law) of refraction on its basis.

A plane wave front AB propagating from denser medium (1)into a rarer medium (2) is incident on the surface P_1P_2 separating the two media as shown in the figure. Using Huygen's principle, draw the secondary wavelets and obtain the refracted wave front in the diagram.

According to Huygen's principle, light is a form of

What are the two assumptions on which Huygens' principle is based? Explain Huygen's geometrical construction for the wavefronts.