A double size real image is produced by a convex lens when an object is placed 18 cm away from it. To produce a triple size real image, where should the object be placed?

Step-by-step solution

For a convex lens, the magnification m in terms of focal length f and object distance u is given by m = f f + u . For a real image, the magnification is negative. In the first case, a double size real image is formed when the object is placed 18 cm away. m₁ = -2 and u₁ = -18 cm . -2 = f f - 18 -2f + 36 = f 3f = 36 f = 12 cm In the second case, a triple size real image is required. m₂ = -3 -3 = 12 12 + u₂ -36 - 3u₂ = 12 -3u₂ = 48 u₂ = -16 cm The object should be placed 16 cm away from the lens.