Question

Two plane wavefronts of light, one incident on a thin convex lens and another on the refracting face of a thin prism. After refraction at them, the emerging wave fronts respectively become
A. plane wave front and plane wave front.
B. plane wave front and spherical wave front.
C. spherical wave front and plane wave front.
D. spherical wave front and spherical wave front.
E. elliptical wave front and spherical wave front.

Answer 1

If a small part of the spherical either cylindrical wavefront tends to be plane while the point source either linear light source is at such a wide wavelength, this very wavefront is called a plane wavefront. A component of the surface of the spherical wave is named as the spherical wave front.

Stepwise solution:
Given,
Two plane wavefronts of light, one incident on a thin convex lens and another on the refracting face of a thin prism.
We have to find out what happens to the emerging wave fronts after refraction.

As the wave passes the prism, the outer parts first pass into the air as well as first speed up. This means as first the outer parts pass out more easily and then the curvature of the wave is further improved such that the light converges more intensely.

Therefore, the emerging wave front will be a spherical wave front.

It is the bottom half of the wave which reaches the glass first when the wave enters the prism, and so this portion of the wave is slowed down immediately. This suggests that the upper component shifts quickly and therefore the wavefront bends when it approaches the lens, so we get a slanted plane wavefront.

Therefore, the emerging wave front will be plane wave front.

Hence, option C is correct.

Note:
It is the core of the wave which reaches the glass first when the wave strikes the rim, so this section of the wave is slowed down (light waves travel slowly in glass than those in air). This indicates the 'catch up' of the major shift of the wave such that the curvature rises to create a converging beam.