Question

sudha finds that the sharp image of the window pane of her laboratary is formed at a distance of 15 cm from the lens .she now tries to focus the building visible to her outside the window without disturbing the lens.in which direction will she move the screen to obtain a sharp image and what is the focal length of the lens?????????

Answer 1

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).