When we were doing the eye, most kids had noticed that the image cast on the retina was upside down. Also they knew that when you use a magnifying glass close up, the image is right side up, but when you hold it up and look at something far away, it is upside down again. Time to do some simple optics. The same shop light I used before was set up on the other end . First I mounted the big lens on the box, to show what effect we were after.
All incoming light gets bent towards one point, the focal point.
When you run your finger through the light, the shadow on the far side of the lens pivots through the focal point. The same thing can be shown using a little laser pointer. To explain how this happens, I replaced the lens with the prism . You can see the light being bent down. The lens from the magnifying glass is fat enough to show that you could imagine it being made up of prism-like shapes.