Figure, shows wave fronts in still water, moving in the direction of the arrow towards the interface $PQ$ between a shallow region and $a$ deep(denser) region. Which of the lines shown may represent one of the wave fronts in the deep region?

What is visible light ? Write various view on it. 

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 also along with the normals drawn at $A$ and $D$. The refractive index of glass with respect to air ($\mu = 1$) will be equal to

Consider a point at the focal point of a convergent lens. Another convergent lens of short focal length is placed on the other side. What is the nature of the wavefronts emerging from the final image ?

A plane wavefront travelling in a straight line in vacuum encounters a medium of refractive index $\mu$. At $P,$ the shape of the wavefront is :