CBSE Class 12 Physics 2014 Delhi Set 1 Paper

Question : 6

Total: 30

A convex lens is placed in contact with a planer mirror. A point object at a distance of 20‌cm on the axis of this combination has its image coinciding with itself. What is the focal length of the lens?

Solution:

Object distance, =20‌cm.
Image distance is also =20‌cm.

As the image I of the object coincides with O, the rays refracted first from the lens and then refracted by the plane mirror must be retracing their path. It is the condition only when rays refracted by the convex lens fall normally on the mirror i.e., the refracted rays form a beam parallel to principal axis of the lens.
Hence, the object O must be at the focus of the convex lens.
f=CO=20‌cm.

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