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 wavefronts respectively become

• A. Plane wavefront and plane wavefront
• B. Plane wavefront and spherical wavefront
• C. Spherical wavefront and plane wavefront
• D. Elliptical wavefront and spherical wavefront

Answer 1

The correct option is C

Spherical wavefront and plane wavefront

The explanation for the correct option:

The wavefront of the first wave

1. Since the wave incidents on a convex lens, the center of the wave first slows down, as the speed of light in glass is slower than that of in the air.
2. Due to this the outer section of the wave start catching up, resulting in a converging beam.
3. Again, when the wave leaves the lens, the outer part of the wave starts accelerating, resulting in an increase in the curvature.
4. Thus the wavefront produced will be spherical.

The wavefront of the second wave

1. When the wave incidents on the refracting face of a thin prism, the lower portion of the wave first enters the prism and slows down.
2. Hence, the upper portion of the wave, moving faster, when enters the prism, and the wavefront bends, resulting in a slanting plane wavefront.
3. Thus the wavefront produced in this case will be a plane.

The diagram for the wavefront can be drawn as follows:

Therefore, option (C) is correct.