As the image formed is real, therefore, the lens in 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 from 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).