1. The most important instruction on the right is the first sentence “The light source can either be the red dot in medium 1, or the purple dot in medium 2.” That means that you can trace light rays both ways (from medium 1 into medium 2 OR the other way) by simply clicking the button to change the direction.
2. Also, observe that when light travels from one medium into another it usually happens that some of the light is reflected and some is refracted. However, under certain circumstances, all of the light is reflected (total internal reflection).
Question: What are the conditions which cause light to undergo total internal reflection?