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A student obtained a sharp image of a burning candle, placed at the farther end of a laboratory table, on a screen using a concave mirror. For getting a better value of focal length of the mirror, what should the student do?(a) He should move the mirror away from the screen.(b) He should move the mirror slightly towards the screen.(c) He should move the mirror as well as the screen towards the newly selected object.(d) He should move only the screen towards the newly selected object.
(b) He should move the mirror slightly towards the screen.
To get the sharp image of a distant object (or, the object at infinity), the mirror should be adjusted so that the image will form at the focus $F$ or at the focal distance $f$ of the mirror. Because we know that when an object is at infinity or at a considerable distance, then the image forms at the focus, $F$ of the mirror. The nature of the image is real, inverted, and highly diminished.
Thus, a sharp image can be obtained, only when the screen will be at the distance of focal length $f$ of the mirror. Therefore, the mirror should be slightly moved towards the screen.
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