Wavefront: Locus of all points vibrating in the same phase at a given instant.

Types of wavefronts:

Huygens' Principle (1678):

1. Every point on a primary wavefront acts as a new secondary source of spherical wavelets.
2. The new (secondary) wavefront is the forward tangential envelope of all these secondary wavelets.

Derivation of Snell's Law using Huygens' Principle: When a plane wavefront passes from medium 1 (speed $v_1$) to medium 2 (speed $v_2 < v_1$, denser):

• One end of the wavefront enters medium 2 first and slows down.
• The wavefront tilts → direction of propagation changes.
• Geometric construction gives: $\dfrac{\sin\theta_1}{\sin\theta_2} = \dfrac{v_1}{v_2} = \dfrac{n_2}{n_1}$

This is Snell's law, derived purely from wave principles.