Lenses and mirrors

The position of the image formed by a lens (or spherical mirror) is described by the lens equation:

$\frac{1}{f}=\frac{1}{x}+\frac{1}{y}$

where $f$ is the focal length, $x$ — the object distance, $y$ — the image distance. The magnification is $p=\dfrac{y}{x}$.

The sign of the result tells you the type of image: $y>0$ → a real image (inverted, on the other side), $y<0$ → a virtual one (upright, on the same side). A converging lens (and a concave mirror) has $f>0$, a diverging one (and convex) — $f<0$.