Comment by IshKebab

I agree I think the motivation for refraction was the weakest point - they introduced waves, and even showed that the waves have to be continuous at the boundary... and then... back to rays!

It just needs a diagram showing the waves refracting, and the geometry of the wave fronts. (Pretty annoying that they never explained refraction like that in school either - Snell's law was just a given when it's actually really trivial to derive from first principles.)