Question

A convex lens of focal length 20 cm placed in contact with a plane mirror acts as a

• 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

Answer 1

Answer: (A)

convex mirror of focal length 10 cm

Answer 2

$\frac{1}{F}=\frac{1}{f_{1}}+\frac{1}{f_{m}}+\frac{1}{f_{1}}$ $=\frac{2}{f_{1}}+\frac{1}{f_{m}}$ $=\frac{2}{-20}+\frac{1}{\infty}=-\frac{1}{10}$ $F=-10 \,cm$ This combination will behave like a convex mirror of focal length of $10 \,cm$.