Wave Optics – Notes

Huygen’s Principle
The locus of all points having the same phase of oscillation is called a wavefront.
An outward normal drawn at any point on the wavefront represents the direction of the wave at that point.
The speed with which a wavefront travels is the speed of the wave.
A wavefront is a surface of constant phase.
According to the Huygens’ principle
Each point of a wavefront is a source of a secondary disturbance and generates spherical secondary wavelets, which spread out in all directions with the speed of the wave in that medium.
If a tangential surface common to all these secondary spherical wavefront is drawn, then it gives the new position of the wavefront at a later instant. This surface envelops all the secondary wavelets.

Reflection of a Plane Wave
A plane wavefront refracted through a convex lens becomes a converging spherical wavefront.
When a plane wavefront is refracted through a concave lens, we get a diverging spherical wavefront.
If a plane wave is incident on a concave mirror, upon reflection it becomes a converging spherical wave.
The change in the frequency of light due to the relative motion of the source and the observer is called the Doppler Effect.