Question

A simple microscope consists of a concave lens of power 10D and a convex lens of power 15 D. Its magnification at near point is

(least distance of distinct vision, D $=$ 25 cm)

• A. 2.25
• B. 5
• C. 10
• D. 20

Answer 1

The correct option is A 2.25

Focal length of concave lens :
$f=-\frac{100}{10}$$=-10cm$
Focal length of convex lens $=\frac{100}{15}$$=6.67cm$
Using $\frac{1}{f}=\frac{1}{f_1}+\frac{1}{f_2}$$\Rightarrow$ $\frac{1}{f}=\frac{1}{10}+\frac{1}{6.67}$
$f=\frac{6.67\times 10}{10-6.67}$
$=20cm$
$\therefore m=1+\frac{D}{f}$
$=1+25/20$
$=2.25$