Image formation on the mirror

We make use of the fact that optical paths are reversible. If we change the direction of the light and pretend that the eyes would send the light, we can find the answer right away. We just have to apply the laws of reflexion.

In the left picture we pretended as if the light was sent from the eyes of the observer. The light is reflected on the mirror and hits the body of the observer. This marks the lowest point that the observer can see of himself in the mirror.
Now, we just have to change the direction of the arrows from the left picture. In the right picture we adjusted these changes already. Therefore, we determined graphically how much the observer is able to see of himself in a small mirror.