Sudha finds out that the sharp image of the window pane of her science laboratory is formed at a distance of 15cm 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?
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solution Let us assume that the window pane is between $F_2$ and infinity from this lens and this is a convex lens. We know that when the object is between infinity and $F _2$, its inverted and real image is formed between 2 F and $2 F_2$. Now, the distant building is at infinity from the lens. Its image would be formed at 2 F . So, the screen needs to be moved towards the lens in order to get a sharp image. Its approximate focal length is 10 cm (less than image distance in earlier case).
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