This simulation allows you to play with the thin lens formula:
$$\frac{1}{f}=\frac{1}{d_O}+\frac{1}{d_I}\label{elens}$$
where $f$ is the focal length, $d_O$ is the object distance to the lens, and
$d_I$ is the image
distance to the lens. Note that you can drag the lenses around by clicking inside
the lens area, and you can drag the object around by clicking on the arrow (and you can
change the object height as well). You can also change the focal length of the lens by
clicking on the focal point (labelled) and dragging it around, and you can raise and lower
the baseline (the horizontal line through the center of the canvas) via clicking and
dragging.