Question

An object is placed 25 cm to the left of a lens of focal length 20 cm. 75 cm to the right of this lens is a plane mirror. Where does the final image form?

The answer is 50 cm to the left of the lens, but a step by step solution would be appreciated.

Answer 1

focal length of lens = f = 20 cm

object distance = p = 25 cm

image distance = q = ?

    1 / f = 1 / p + 1 / q

     1 / 20 = 1 / 25 + 1/ q

image distance = q = 100 cm

now distance from lens and mirror = 75 cm

now image of the lens act as a object of mirror

object distance of mirror = 100 - 75 = 25 cm it is right of plane mirror

for plane mirror = p1 = - q1

image is form 25 cm left of mirror ( image is virtual)

from lens it is 75 - 25 = 50 cm right of lens