A concave lens produces an image 20 cm from the lens for an object placed 30 cm from the lens. The focal length is

The correct option is:A:-60 cm

Given Data:

Distance of image, $v=-20cm$ (-ve is because image is formed in front of mirror)

Distance of object, $u=-30cm$

Step 1: Finding focal length:

Using lens formula,

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

$\frac{1}{f}=\frac{1}{-20}-\frac{1}{-30}$

$\frac{1}{f}=\frac{-3+2}{60}=\frac{-1}{60}$

$f=\frac{60}{-1}=-60cm$

Focal length of lense is,

$=-60cm$