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.