That change in speed causes a redirection, or bend, of the wave, a process known in physics as refraction. You might remember this concept from high school science classes. Well that’s only half the story. Think of the glass as a converging lens, which is one that is curved on all sides. As light passes through it, the curved shape forces the light to bend from every direction. The dense medium of water significantly enhances this bending. The refracting light waves converge at what’s called the focal point, then cross each other.