Lens simulations

https://phet.colorado.edu/sims/geometric-optics/geometric-optics_en.html

https://graphics.stanford.edu/courses/cs178-10/applets/thinlens.html

http://www.physicsclassroom.com/Physics-Interactives/Refraction-and-Lenses/Optics-Bench/Optics-Bench-Refraction-Interactive

https://simbucket.com/lensesandmirrors/