. A converging lens of focal length f 1 is placed in front of and coaxially with a convex mirror of focal length f 2 . Their separation is d and whole arrangement is in air. A parallel beam of lig

Question

. A converging lens of focal length f 1 ​ is placed in front of and coaxially with a convex mirror of focal length f 2 ​ . Their separation is d and whole arrangement is in air. A parallel beam of light incident on the lens returns as a parallel beam from the arrangement.

TestHub · Accepted Answer

The beam returns parallel only if the mirror sends the rays back along their own paths.

(i) Rays converge at the pole of the mirror: $d = f_{1}$ (B correct). (ii) Rays strike the mirror normally, i.e. head toward its centre of curvature: $d + 2 f_{2} = f_{1} \Rightarrow d = f_{1} - 2 f_{2}$ (D correct). Since the rays retrace their paths, the emergent parallel beam has the same diameter as the incident beam (A correct). $d = f_{2}$ satisfies no retracing condition. Hence (A), (B), (D).

Physics - Geometrical Optics Question with Solution | TestHub

Options:(select one or more)

Doubts & Discussion