Question

A concave lens of $20cm$ focal length forms an image $15cm$ from the lens. Calculate the object distance.

Answer 1

Given: Focal length, $f=-20cm$ Image distance, $v=-15cm$ (concave mirror)

To find: Object distance, $u$

From lens formula,

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

$\frac{1}{-15}-\frac{1}{u}=\frac{-1}{20}$

$\frac{1}{20}-\frac{1}{15}=\frac{1}{u}$

$u=-60cm$