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Question

# A student performs an experiment on finding the focal length of a convex lens by keeping a lighted candle on one end of laboratory table, a screen on its other end and the lens between them as shown in the figure. The positions of the three are adjusted to get a sharp image of the candle flame on the screen. If now the candle flame were to be replaced by a distant lamp on a far away electric pole, the student would be able to get a sharp image of this distant lamp on the screen by moving: (1) The screen in the direction of the lens or the lens in the direction of the screen (2) the screen in the direction of the lens or the lens away from the screen (3) the screen away from the lens or the lens in the direction of the screen (4) neither the screen nor the lens

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Solution

## A convex lens always forms the image of a nearby object away from its focus. So, if a student obtained the image of the candle on the screen, that means the screen was away from the focus somewhere. Now, if he needs to form an image of a distant lamp on the same screen then he needs to move the screen in the direction of the lens or the lens in the direction of the screen in order to place the screen at the focus of the lens, because a convex lens converges all the rays coming from a distant object on its focus. Hence, the correct answer is option 1.

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