Question

A ray incident on a spherical mirror retraces its path after reflection. The point on the mirror from where the Ray retraces its path is_______
(a) pole
(c) centre of curvature
(b) focus
(d) between focus and pole​

Answer 1

If a ray is incident normally on a point on the spherical reflecting surface, the angle of incidence, i=0. So, the angle of refraction, r=0. Therefore, the ray retraces its own path after reflection.

Answer 2

The point on the mirror from where the Ray retraces its path is (a) pole.

What is pole?

• Pole is a point that lies on the aperture of the spherical mirror.

What is centre of curvature?

• Centre of curvature is the centre point of the spherical mirror from which the aperture is cut out.

What is Focus?

• Focus is a point on principle axis. Focal length is half of the centre of curvature.

From the problem statement:

• A line passing through focus, centre of curvature and pole is perpendicular to the spherical mirror.
• So, the incident ray makes angle 0 with the normal and so the reflected ray also makes 0 degree angle and retraces its path.
• Since, pole lies on the mirror, so the point on mirror where ray retraces its path is pole.