Question

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

Answer 1

Focal length of concave lens (OF1), f = −15 cm

Image distance, v = −10 cm

According to the lens formula,

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

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

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

= $\frac{-3+2}{-30}$

= $\frac{-1}{-30}$

u = -30 cm

The negative value of u indicates that the object is placed 30 cm in front of the lens. This is shown in the following ray diagram.