Question

A convex lens of focal length 15 cm is kept in front of a concave lens of focal length 30 cm. What is the power of the two lens system?

• A. - 0.033 D
• B. + 0.033 D
• C. - 0.6 D
• D. + 0.6 D

Answer 1

The correct option is B

+ 0.033 D

Let $P$ be the power of lens,
$f$ be the focal length of lens
$f_1$ - focal length of a convex lens, $P_1$ is the power corresponding to this lens
$f_2$ - focal length of a concave lens, $P_2$ is the power corresponding to this lens
$P_{total}$ - Power of combination of lens
As per the sign conventions,
$f_1=+15cm$
$f_1=-30cm$
($P_{total}$) = $P_1$ + $P_2$

= $\frac{1}{f_1}$ + $\frac{1}{f_2}$ $[AsP=\frac{1}{f\left(inm\right)}]$
= $\frac{1}{0.15}$ - $\frac{1}{0.30}$
= $\frac{2-1}{0.30}$
= $\frac{1}{0.30}$
$P_{total}$ = + 3.33 D