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

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?

Answer 1

Without the mirror, di = 1/(1/f-1/do) = 100 cm. With the mirror at 75 cm, the converging rays are reflected back to form the image at 50 cm.