A) convex mirror of focal length 10 cm
B) concave mirror of focal length 40 cm
C) concave mirror of focal length 60 cm
D) concave mirror of focal length 10 cm
Correct Answer: A
Solution :
When two lenses are kept in contact, the combined focal length of the combination is \[\frac{1}{F}=\frac{2}{f}+\frac{1}{{{F}_{m}}}\] For a plane mirror, \[{{f}_{m}}=\infty \] \[\therefore \] \[F=\frac{f}{2}=\frac{20}{2}=10\,cm\] Hence, arrangement behaves as a convex mirror of focal length 10 cm.
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