Question

A convex lens forms a real image on a screen placed at a distance 60 cm from the object. When the lens is shifted towards the screen by 20 cm, another image of the object is formed on the screen. The focal length of the lens is: А)45 cm B)40 73 cm С)30 cm D)12 cm​

Answer 1

Explanation:

in optics towards the direction of light is taken as positive against direction negative

so v is positive and u is negative

so by lens formula

1/v - 1/u =1/f

transforms to

1/v + 1/u =1/f

by signconvention