# 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?

To get the sharp image of building, Sudha should move the screen towards the lens. It is because the window pane was lying beyond the centre of curvature and its image is formed between F and 2F. The building is further away thus it can be assumed that the light coming from the building is parallel to the principal axis of the lens and the convex lens converge these light rays at focus F. So image will be formed at a distance of focal length. The approximate focal length will be slightly less than 15 cm.

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