For light diverging from a finite point source,

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

Which of the following is correct for light diverging from a point source?

Write the limitation of Huygen's principle.

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.

The following figure represents a wave front $AB$ which passes from air to another transparent medium and produces a new wave front $CD$ after refraction. The refractive index of the medium is $:$ ($PQ$ is the boundary between air and the medium)