# Perimeter of Image of a Square

A concave mirror of focal length =10 cm is placed 15 cm from a square. The square lie on the principal axis i.e it's one side coincides with the principal axis. What is the perimeter of the image? How to find it?

My Approach: I found image distance as -30 cm using mirror formula taking object distance as -15 cm and focal length as -10 cm. Then using this:

Magnification = Image Length/Object length = - Image distance/Object Distance

I found the side of image square as 0.02 cm. However the book states that the perimeter is 10 mm. Also the image is not a square. How?

-
 Not all vertices of your square are at the same distance from the ens, and not all of them are on the principal axis. Find theimage of each separately, and you should be on your way. – Jaime Dec 17 '12 at 14:52