A screen is placed 90 cm from an object

cbse
ray-optics

#1

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.


#2

Given distance between screen and object a = 90cm
Distance between two locations of the lens, d= 20 cm
Using the displacement formula,
f = ${{a}^{2}}$ - ${{d}^{2}}$ / 4a = ${{90}^{2}}$- ${{20}^{2}}$/4x90
f = 7700/360 = 21.4 cm