There is a common misconception that when half of a lens is covered that half of the image disappears. This problem can be helped by two methods. First, the simulation should be capable of drawing extra rays to show that there are multiple rays going through the lens. This would be a useful option for the thin lens applet. Second, a means for "blocking" part of the lens by an object should show the image becoming fainter, but still there. This would be another useful enhancement.