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 ?

Explanation

Sudha should move the screen towards the lens to obtain a sharp image of the building because window pane was lying beyond focus 2F or centre of curvature and convex lens forms its image, on the other side between F and 2F and when she tries to focus the building visible to her outside the window (i.e., at distant object at infinity), then lens forms the image of building at a distance of focal length.
The approximate focal length of this lens is 15 cm.