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 location separated by `20 cm`. Determine the focal length of the lens.

Explanation

Distance between the image (screen) and the object, D=90 cm
Distance between two locations of the convex lens, d=20cm
Focal length of the lens=f
Focal length is related to d and D as:
`f=(D^(2)-d^(2))/(4D)`
`=((90)^(2)-(20)^(2))/(4 xx 90) = 770/36 = 21.39cm`
Therefore, the focal length of the convex lens is 21.39 cm.