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A convex lens is placed in contact with ...

A convex lens is placed in contact with a plane mirror. An axial point object at a distance of `20 cm` from this combination, has its image coinciding with itself. What is the focal length of the convex lens ?

Text Solution

Verified by Experts

The focal length of the lens = 20 cm
Explanation: As the image of this combination coincides with the object itself, the rays from the object, after refraction from the lens should fall normally on the plane mirror, so that they retrace their path. So the rays from the point object after refraction from the lens must form parallel beam. For clarity, mirror has been placed at a small distance from the lens.
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