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Question:

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 better value of focal length of the mirror, the subject teacher suggested him for focussing a well illuminated distant object. What should the student do?

Solution:

As the image distance is increased, the object distance would decrease and thus the mirror-screen distance should be decreased. Therefore, the student should move the mirror towards the screen.