Question

A simple microscope consists of a concave lens of power $-10D$ and a convex lens of power $+20D$ in contact. If the image is formed at infinity, then the magnifying power is $\frac{x}{2}$ , the value of $x$ is : (Take the least distance of distinct vision as $D=25$ cm)

Answer 1

The lens power of combined lens is
$P=P_1+P_1=-10+20=10D$
Focal length of combination
$F=\frac{1}{P}=\frac{100}{10}cm=10cm$
For the final image at infinity,the magnifying power of simple microscope is given by
$M=\frac{D}{f}=\frac{25}{10}=2.5=\frac{x}{2}$
$\therefore x=5$