# Image size in concave mirror

by roam
Tags: concave, image, mirror, size
 P: 916 1. The problem statement, all variables and given/known data An object 23.0 mm high is 31.0 cm away from a concave spherical mirror with a radius of curvature of 54.0 cm. What is the absolute size of the image? 2. Relevant equations Maybe these are relevant Mirror equation in terms of focal length: $$\frac{1}{p}+\frac{1}{q}=\frac{1}{f}$$ Magnification of image: $$M=\frac{h'}{h}=-\frac{q}{p}$$ 3. The attempt at a solution I have already calculated the distance of the image from the mirror to be 209 cm. But how do I calculate the size of the image? Is there a special formula or do I need to use some kind of trigonometry? I had no luck using the trying the above equations... The correct answer must be 155.
Thank you! I got the right answer using the ratio $$\frac{h'}{h}=-\frac{q}{p}$$, but I had to exclude the minus '-' sign. Is that because the resulting image is upright (not inverted)?