Question

One side of the bi-concave lens is silvered. Now it will behave like

• A. Convex mirror of $f=\frac{R}{2\mu }$
• B. Concave mirror of $f=\frac{R}{2\mu }$
• C. Convex mirror of focal length $=\frac{R}{4\mu -2}$
• D. Concave mirror of focal length $=\frac{R}{4\mu -2}$

Answer 1

The correct option is C Convex mirror of focal length $=\frac{R}{4\mu -2}$

Using the lens formula, for addition of powers of a lens,
$P_1=\frac{4(\mu -1)}{R}$

$P_2=\frac{2}{R}$

Hence, net P = $P_1+P_2$

$P=\frac{4(\mu -1)}{R}+\frac{2}{R}$

Or, $f=\frac{1}{P}$ = $\frac{R}{4\mu -2}$