Q. A concave lens of focal length 20 cm placed in contact with a plane mirror acts as a convex mirror of focal length
A) 10 cm
B) 40 cm
C) 60 cm
D) 20 cm
Explanation
(a) For lens, mirror combination, combined focal length is given by
$\frac{1}{F} = \frac{2}{f} + \frac{1}{f_{m}}$
Here, $f_{m} = \ \infty$
$\therefore\frac{1}{F} = \frac{2}{f}$
Or $F = \frac{f}{2} = \frac{20}{2} = 10cm$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.