Question

Question 26
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?

Answer 1

1. Sudha has to move the screen towards the lens, as for a convex lens, the image distance increases from the lens with the decreasing object distance and vice-versa.
2. Thus, as the object moves away from the lens, the image moves closer to the lens.
3. The approximate focal length of this lens is 15 cm because the image is being formed at 15 cm from the lens.
4. And as per the definition, when the object is at infinity its image is formed at the focus of the lens so the image distance for this case is equal to the focal length of the lens.