Light inside a dense medium reaches a boundary with a medium of lower optical density. Total internal reflection occurs for which angle of incidence?

1. The critical angle is the angle of incidence for which the refracted ray runs along the interface. 2. Below the critical angle some light still crosses the boundary and refracts away. 3. Total internal reflection needs light travelling towards a medium of lower optical density. 4. It then needs an angle of incidence greater than the critical angle for that medium. 5. All the light is then reflected back into the first medium and no refraction takes place. 6. A ray sent straight along the normal has an angle of incidence of zero and simply passes through, which is why those options fail. _Source: Siyavula Physical Sciences Grade 11 (Everything Science, CC BY 4.0), Chapter 5: Geometrical optics_