[SOLVED] Focal length of a mirror 1. The problem statement, all variables and given/known data A 2.0-cm-tall object is placed in front of a mirror. A 1.0-cm-tall upright image is formed behind the mirror, 150 cm from the object. What is the focal length of the mirror? 2. Relevant equations 1/f=1/s+1/s' m=h'/h m=-s'/s 3. The attempt at a solution .5=-s'/s s+s' = 1.5m Combining those 2 equations, I get: s'=3 s=-1.5 1/f=(1/-1.5)+(1/3) f=3m When I calculate it the focal length, f, it comes out to 3m, which is obviously way too big.