Question

A concave lens of focal length is 20 cm from an image at distance of 10cm from lens what is distance of the object from lens

Answer 1

HEY MATE HERE IS YOUR ANSWER:-

Given f = - 20 cm, (sign convention) , v = - 10 cm

(∴ image formed by concave lens is virtual)

By using lens formula,  

$-\frac{1}{U} +\frac{1}{V}=\frac{1}{F}$

$-\frac{1}{U} =\frac{1}{F} +\frac{1}{V}$

$-\frac{1}{U} =-\frac{1}{20}+ \frac{1}{10}$

or u = - 20  

Thus the object is placed at 20 cm from the concave lens