Question

The focal length of a planoconvex lens is $20$cm. The plane face of it is silvered. An illumination object is placed at a distance of $60$cm from the lens of its axis on the convex side. Then the distance of the image is?

Answer 1

we know that $\frac{1}{f_m}=\frac{1}{v}-\frac{1}{u}$

$f_m=\frac{f}{2}=20/2$

$u=-60cm$

$\frac{1}{10}=\frac{1}{v}+\frac{1}{60}$

or

$\frac{1}{10}=\frac{1}{v}-\frac{1}{60}$

$\frac{(6-1)}{60}=\frac{1}{12}$

v is 12cm