No links please Mock Test Papers for H.S. & Competitive Examinations The numerical value of the focal length of a thin convergent lens is f and an object is at a distance u from the lens; a plane mirror is placed behind the lens perpendicular to the axis at a distance f from the lens. The image of the object after reflection at the mirror and a second refraction through the lens is at a distance v in front of the lens. Prove that u + v = 2f