Question

Calculate the focal length of a convex lens which produces a virtual image at a distance of 50 cm of an object placed 20 cm in front of it.

Answer 1

Given

Image distance, $v=50cm$

Object distance $u=20cm$

Since, the image is virtual it will be formed on the same side of the object.

From lens formula,

$\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$

$\frac{1}{f}=\frac{1}{-50}-\frac{1}{-20}$

$=\frac{3}{100}$

$f=\frac{100}{3}cm=$ focal length