$A$ wave front is the locus of all the points where the particles of the medium vibrate with the same:

Assertion $A$: For light diverging from a point source,the intensity at the wavefront does not depend on the distance.
Reason $R$: In a diverging beam of light from a point source,a spherical wavefront is observed.

Which one of the following phenomena is not explained by Huygen's construction of wavefront?

$A$ 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:

What is the shape of the wavefront in each of the following cases:
$(a)$ Light diverging from a point source.
$(b)$ Light emerging out of a convex lens when a point source is placed at its focus.
$(c)$ The portion of the wavefront of light from a distant star intercepted by the Earth.

Is Huygens' principle valid for longitudinal sound waves?