Chromatic Aberration
Khoa from Vietnam is writing his EE on Chromatic aberration and the achromatic doublet. Actually making an achromatic doublet is rather difficult so he is focussing on building a simulation. You can simulate refraction in Algodoo but you can't vary the difference between the refractive index of a material for red and blue light so attempts failed. Instead he is using GeoGebra.
I am a bit of a GeoGebra nut so can't help but try for myself. I let Khoa work out how to do it himself first then reveal my solution. The simulation is based on the lens makers equation and the thin lens formula. A better approach would be to apply Snell's law at each boundary, this is a bit laborious so I think I'll leave Khoa to do this one himself, if it's any good I'll steal it and put it on my GeoGebra account ;-)