A screen is placed 90 cm from an object. The image of the object on the screen is formed by a convex lens at two different locations separated by 20 cm. Determine the focal length of the lens.

Explanation

a) Distance between object and image
`D=90 cm = u+upsilon`
Distance between two positions of lens `d=20=u=upsilon`
`u=55cm and upsilon = 35cm`
For lens formula,
`(1)/(f)=(1)/(55)+(1)/(35)=(18)/(385)`
`f=(385)/(18)=21.4`