We went over some problems with lenses, converging and diverging.
Then spent some time on problems with different indices of refraction. I would usually think of air to water and the behavior if you get mixed up.
Used Snell’s Law and another equation with the velocity of light, c, and the index of refraction.
The index of refraction is about 1 in air and is 1 in a vacuum. It doesn’t (in the context of this class) go below 1.