Question

A point source of light is placed at the focus of a concave mirror. Consider only paraxial rays. The shapes of the wavefronts of incident and reflected lights respectively are: 

• A. spherical spherical
• B. spherical, planar
• C. spherical, cylindrical
• D. planar, spherical

Answer 1

The Correct Option is B

To solve the problem, we need to determine the shape of wavefronts for light incident on and reflected from a concave mirror, when a point source is placed at its focus.

1. Understanding Wavefronts from a Point Source:
A point source placed at the focus of a concave mirror emits spherical wavefronts. As the light rays travel toward the mirror, they are in the form of diverging spherical wavefronts.

2. Behavior at the Mirror (Reflection):
For a concave mirror, if the point source is exactly at the focus, the reflected rays become parallel to the principal axis (paraxial approximation). This results in planar (plane) wavefronts after reflection.

3. Conclusion:
- Incident wavefronts = spherical (from point source)
- Reflected wavefronts = planar (after reflection from mirror)

Final Answer:
The shapes of the wavefronts of incident and reflected lights respectively are spherical and planar.

Answer 2

The correct option is: (B): spherical, planar.

When a point source of light is positioned at the focus of a concave mirror, paraxial rays that are nearly parallel to the principal axis are considered. In this scenario:

Incident Light Wavefronts: The incident light rays, originating from the point source at the focus, diverge after reflection from the mirror. Due to the geometry of a concave mirror, the wavefronts of the incident light are spherical. This is because the light rays converge toward the focal point before striking the mirror.

Reflected Light Wavefronts: The reflected light rays follow the principle of reflection, where the angle of incidence equals the angle of reflection. Since paraxial rays are considered, the angles are small. As a result, the reflected light rays are parallel or nearly parallel to the principal axis. Consequently, the wavefronts of the reflected light are planar or nearly planar.

In summary, the incident light wavefronts are spherical, and the reflected light wavefronts are planar for a point source of light placed at the focus of a concave mirror.