Huygens' principle of secondary wavelets may be used to

Explain the use of wavefront to understand wave propagation.

Given below are two statements:
Statement-$I$: $A$ plane wave after passing through a prism remains a plane wave,but passing through a small pinhole may become a spherical wave.
Statement-$II$: The curvature of a spherical wave emerging from a slit will increase for increasing slit width.
In the light of the above statements,choose the correct answer from the options given below.

Explain the refraction of a plane wave by a thin prism.

Explain the wavefront and its types.

In the adjoining diagram, a wavefront $AB$ moving in air is incident on a plane glass surface $XY$. Its position $CD$ after refraction through a glass slab is shown along with the normals drawn at $A$ and $D$. The refractive index of glass with respect to air $(\mu = 1)$ will be equal to