Sudha finds out that the sharp image

Sudha finds out that the sharp image of the window pane of her science laboratory is formed at a distance of 15 cm from the lens. She now tries to focus the building visible to her outside the window instead of the window pane without disturbing the lens. In which direction will she move the screen to obtain a sharp image of the building ? What is the approximate focal length of this lens ?

As the image is real, therefore the lens use is convex lens. The distance of the real image formed by a convex lens from the lens decreases as the object distance from the lens increases. Hence, the screen has to be moved towards the lens to obtain the sharp image of the building. Approximate focal length of the lens = 15 cm as the rays of light coming the window pane, are considered to come from infinity. These rays of light are focussed by the convex lens at its focus (i.e., on the screen).