The main point of this simulation is to show that light rays leaving an object travel in straight lines. The simulation has two light bulbs that can be turned on or off independently. The bulbs can be dragged around the screen to change their positions. The light from the bulbs either passes through a mask with a hole in it, or it is blocked by an object, as it travels to a screen on the right of the simulation. Investigate how the patterns of light and shadows change as you move the bulbs, the mask or object, and/or the screen, and as you change the size of the mask or object.
For more info: http://weelookang.blogspot.sg/2013/08/refraction-model.html