This is a very "old skool" experiment that has appeared in the syllabus again. I found it in our store. The idea is that there is a round glass jar to represent your eyeball and that it is filled with a fluorescent dye called Fluorescein so that you can see the path of the light rays inside your eyeball.
If you are lucky enough to have normal sight, the lens makes the rays come together as a focussed point on your retina. You can see this happening down the middle of the picture below.
I am short-sighted, either because my lens is too strong or that my eyeball is too long. As a result, the light focuses in the middle of my eyeball and then spreads out again as shown below. I wear concave lenses to push the focal point back onto the retina.
If you are long sighted, the rays don't even come together in your eyeball. You need to wear convex lenses to bring the focal point in onto your retina.
