According to Huygens principle, the phase difference between oscillations of any two particles on the same wave front is . . . . . . rad.

Explain the wavefront and its types.

When a point source of light is placed at the focus of a convex lens,light emerges from the lens. What is the shape of the wavefront of the emerging light?

The idea of secondary wavelets for the propagation of a wave was first given by

Explain the refraction of a plane wave from a denser to a rarer medium using Huygens' principle.

$A$ plane wavefront of width $x$ is incident on an air-water interface, and the corresponding refracted wavefront has a width $y$. The refractive index of air with respect to water in terms of distances $w$ and $z$ is: