The distance between object and its two times magnified real image as produced by a convex lens is 45 cm. The focal length of the lens used i s ______ cm.

Question

G1 Physics · Accepted Answer

Magnification: v/u = -2 Therefore, v = -2u ...(1) Distance between object and image: v - u = 45 ...(2) Using (1): -2u - u = 45 u = -15 cm Therefore, v = 30 cm Lens formula: 1/f = 1/v - 1/u = 1/30 - 1/(-15) = 1/30 + 2/30 = 3/30 = 1/10 Therefore, f = 10 cm