Question

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|.

Answer 1

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