Question

For a plane convex lens $(µ = 1.5)$ has radius of curvature 10 cm. It is silvered on its plane surface. Find focal length after silvering:

• A. $10\ cm$
• B. $20\ Cm$
• C. $15 \ cm$
• D. $25 \ cm$

Answer 1

Answer: (A)

$10\ cm$

Answer 2

Equivalent power of combination

$P_{eq}=2P_L+P_M$

$P_{eq}$ =$2(µ–1)(\frac 1R)+0$

The required focal length

$f=–\frac {1}{P_{eq}}$

$f =–\frac {R}{2(μ-1)}$

$f =–\frac {10}{2(1.5-1)}$

$f =–10\ cm$

So, the correct option is (A): $10\ cm$