Question

A concave 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

By using $\frac { 1 } { F } = \frac { 2 } { f _ { l } } + \frac { 1 } { f _ { m } }$

Since $f _ { m } = \infty$ ⇒ $F = \frac { f _ { l } } { 2 } = \frac { 20 } { 2 } = 10 \mathrm {~cm}$

(After silvering concave lens behave as convex mirror)