Question

A concave lens of focal length 15cm forms an image 10 cm from the lens . How far is the object placed from the lens? Draw the ray diagram.

Answer 1

Given: $f=-15$

$v=-10$

From mirror formula, $\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$

$\frac{1}{-10}-\frac{1}{u}=\frac{1}{-15}$

$\frac{1}{u}=\frac{1}{15}-\frac{1}{10}$

$u=-30cm$