A screen is placed 90cm from an object. The image of the object on the screen is formed by a convex lens at two different positions separated by 20 cm. Calculate the focal length of the lens.
Given: |v| + |u|= 90cm .....(i)
|v| - |u| = 20cm ......(ii)
From equation (ii) , we get,
|u| = 35 cm
|v |= 55 cm
we know that,