Explain why, a ray of light passing through the centre of curvature of a concave mirror gets reflected back along the same path.

A ray of light passing through the centre of curvature of a concave mirror is reflected back along the same path because it

strikes the concave mirror perpendicularly (at right angles) to its surface due to which the angle of incidence and angle of reflection

both are 0º.

Explanation

The reason for a ray of light passing through the centre of curvature of a concave mirror gets reflected back along the same path because when the ray of light passes through the centre of curvature and strikes the curved reflecting surface making an angle 90o with the tangent at the point it strikes.

At this point of contact, the radius of curvature (here, the incident light acts as a radius of curvature) is perpendicular to the tangent making the incident ray and the reflected ray coincide with each other. 

This all happens because, the ray is coming from the centre of the sphere of which this curved mirror is the part, and in the case of a circle normal at a point always passes through the centre of the circle. 

Therefore the angle of incidence = 0o, and also the angle of reflection = 0o, (according to the law of reflection) $\angle i=\angle r=0$.

Thus ray of light retraces its path.

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Updated on: 10-Oct-2022

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