In the given scenario, when a short linear object lies along the axis of a concave mirror, the size of the image is approximately equal to the length of the object, b.
When a short linear object of length b lies along the axis of a concave mirror of focal length f at a distance u from the pole of the mirror, the size of the image can be approximated using the magnification formula:
m = -v/u
where:
m is the magnification,
v is the image distance, and
u is the object distance.
In this case, the object is short, so we can assume that the entire object is within the focal length of the mirror. This means that the image formed will be virtual, erect, and magnified.
Since the object is short, we can assume that the image size is approximately equal to the object size, which is b.
Therefore, the size of the image is approximately equal to the length of the object, b.