An observer is standing at a point O, at a distance of 100 cm from a convex lens of focal length 50 cm. A plane mirror is placed behind the lens at a distance of 150 cm from lens. The magnitude of velocity of her own image as seen by the observer, at the moment when the mirror just starts moving with a velocity of 10 cm/s.

V→I=2V→m−V→0⇒   V→I=2V→m∴  velocity of image in the plane mirror will be 20 cm/s in the direction of motion of mirror.First refraction at lens, Image is formed at I1f=1v−1u​⇒150=1v−1−100​⇒v=100Plane Mirror will form image at I'This I' will behave as object for, 2nd refraction in lens.u=−200cm1f=1v−1u⇒150=1v−1−200⇒v=2003∴dVdt=v2u2dudt=200322002dudt=209cm/s