# A short object of length L is placed along

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|

Solution:

The mirror formula is

1/v + 1/u = 1/f

u is the object distance

v is the image distance

du = |u1 – u2| = L

Differentiating on the both sides we get,

dv/v2 = -du/u2

v/u = f/u-f

du = L, therefore, image length is f2/(u-f)2L