Please have a look at a newest entry in the NPN WIKI for a physical demonstration I just added to help those wondering how light travels through and converges in a lens. There’s also a link to a video demonstrating how to make a lens out of water. It’s pretty cool and finally, something physical instead of just a theoretical drawing.
This could be helpful for mentors working with beginner photographers as well.
Please let me know if I made any mistakes or if it needs work.