What is the shape of the wavefront of light diverging from a point source?

You have learnt in the text how Huygens' principle leads to the laws of reflection and refraction. Use the same principle to deduce directly that a point object placed in front of a plane mirror produces a virtual image whose distance from the mirror is equal to the object distance from the mirror.

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

For light diverging from a point source,

Mention the main difficulty in establishing the wave theory of light.

$A$ 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: