A ray of light passes through the centre of curvature of a concave mirror and strikes the mirror. At what angle is the ray reflected?

Analyze the question: A ray passes through the centre of curvature (C) of a concave mirror and hits the mirror. We need the angle at which it is reflected.

Concept: The line from the centre of curvature to any point on the mirror is a radius, so it is the normal to the mirror surface at that point. A ray travelling along the normal strikes the mirror with an angle of incidence of 0°.

Law of reflection: Angle of incidence = angle of reflection.

Solution:

1. The ray passes through C, so it travels along the radius, which is the normal to the surface.
2. Angle of incidence, i = 0° (measured from the normal).
3. By the law of reflection, angle of reflection, r = i = 0°.
4. The ray retraces its own path back through C.

Note on option A (180°): The reflected ray does reverse its direction, so it turns through 180° relative to its original direction of travel. But the question asks for the angle of reflection, which by definition is measured from the normal. That angle is 0°, so C is the correct choice.

Exam tip: Always measure angles of incidence and reflection from the normal, not from the mirror surface. Remember: "Ray through C returns along the same path, so i = r = 0°."