Question

A concave mirror produces an image of a long vertical pin, placed 40 cm from the mirror, at the position of the object. Find the focal length of the mirror.

Answer 1

Given `u=upsilon=" 40 cm"`
`(1)/(f)=(1)/(upsilon)+(1)/(u)`
`(1)/(f)=(1)/(40)+(1)/(40)`
`(1)/(f)=(2)/(40)`
f = 20 cm.