A convex lens forms a virtual image of an object which is 3 times the size of the object. If the distance between the object and its image is 20 cm , the focal length of the lens is :

Step-by-step solution

For a convex lens forming a virtual image, the image is erect and formed on the same side as the object. Given magnification m = +3 . We know that m = v u = 3 v = 3u . Since both the object and the virtual image are on the same side of the lens, and the image is magnified ( |v| > |u| ), the distance between them is given by: |v| - |u| = 20 -v - (-u) = 20 -3u + u = 20 -2u = 20 u = -10 cm Then, v = 3(-10) = -30 cm . Using the lens formula: 1 f = 1 v - 1 u 1 f = 1 -30 - 1 -10 1 f = -1 + 3 30 = 2 30 = 1 15 f = 15 cm An