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Q.
A straight line joining the object point and image point is always perpendicular to the mirror
Ray Optics and Optical Instruments
Solution:
Any line joining the object point and the image point always passes through the centre of curvature of the mirror. This, is because of the law, that any ray passing through the centre of curvature of the mirror will retrace its path back.
Thus, any ray passing through the centre of curvature will be perpendicular to the mirror irrespective of the type of the mirror used.