# A short object of length L is placed along the principal axis of a concave mirror away from focus.

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A short object of length L is placed along the principal axis of a concave mirror away from focus. The object distance is u. If the mirror has a focal length f, what will be the length of the image? You may take L <<|v - f|.

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Let the two ends of the object be at distance u1= u – L/2 and u2 = u + L/2, respectively, so that |u1–u2|= L. Let the image of the two ends be formed at v1 and v2, so that the image length would be

the image of the two ends will

Since the object is short and kept away from focus, we have