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

A screen is placed at a distance of 90 cm from an object. The image of

1.	A screen is placed at a distance of 90 cm from an object. The image of this object is formed on the screen by a convex lens at two different location separated by 20cm. The focal length of the lens is
Answer» SOLUTION :By the displacement method `f=(d^2-x^2)/(4D)= f=(90^2-20^2)/( 4 TIMES 90)= 21.4 CM`