When you take a narrow bunch of rays you obtain a focus. If not you can see another interesting thing - a caustic. The applet gives a great explanation of the figures made by light on the bottom of shiny (e.g. metal) mugs owing to reflection and outside a glass of water owing to refraction. I dream about such an applet generating caustics in 3D for lenses and bottles when a bunch of sun rays falls at different angles on the objects.