The same size images are formed by a convex lens when the object is placed at 20 cm or at 10 cm from the lens. The focal length of convex lens is

Question

The same size images are formed by a convex lens when the object is placed at 20 cm or at 10 cm from the lens. The focal length of convex lens is,

MARKS App · Accepted Answer

m = f u + f + m = f - 10 + f . . . . 1 - m = f - 20 + f . . . . . 2 1 / 2 - 1 = f - 20 f - 10 10 - f = f - 20 30 = 2 f f = 15 cm