Question

A concave-convex glass (index $=1.5)$ lens has radii of curvatures 60 cm and 40 cm respectively. Its convex surface is silvered and it is placed on a horizontal table with concave surface up. The concave is then filled with a liquid of index 2.0. The combination is like a

• A. concave mirror
• B. convex mirror
• C. flat mirror
• D. convex lens

Answer 1

Answer: (A)

concave mirror

effective focal length be $f$

$\frac{1}{f}=\frac{1}{f_{concavesurface}}+\frac{1}{f_{mirror}}$

$\frac{1}{f}=\frac{{\mu }_2-{\mu }_1}{R_1}+\frac{-2}{R_2}$

$\frac{1}{f}=\frac{-0.5}{60}+\frac{-1}{20}$

$f<0$ and it reflects the light hence it acts as concave mirror

option $A$ is correct