Question

the focal length of convex lens be f. how does the size of the image placed infront of it changes as the object is brought progressively closer to the focus from a distance which is just greater than 2f​

Answer 1

Answer:

The distance 2f is special, because it means that the image is the same size as the object. As you move the object closer, the image recedes and gets bigger, See Greg Lehey's answer to What is the focal length of a converging lens when an object is placed 45 cm away from it and a real and very magnified image is formed?:

For example with a 450 mm lens, an object distance of 500 mm will give 4x magnification. At 480 mm it’s 15x, at 460 mm it’s 45x, at 451 mm it’s 450x

Explanation: