Book 1.11.3: Create a figure to illustrate the "total internal/external reflection" text better.

[hollasch]

See @armansito's comments in #1425. We need one or more figures to back up the text.

[hollasch]

Note that for consistency, we use Limnu (https://limnu.com/) to generate figures. Images should be 800 pixels wide.

[hollasch]

Ref #900