[b]Total Internal Reflection[/b] occurs when light bend so much away from the [b]normal[/b], when it [b]refracts[/b], that it ends up coming back and instead reflects off of the surface, back into the material.
The light effectively gets trapped inside of the material.
[b]In glass, Total Internal Reflection occurs at 42[degree][/b].
The point where it reflects internally (42[degree] for glass), is called the critical point.
The light which goes towards the surface to exit the material is the [b]incidence ray[/b], the one that gets totally internally reflected.
To work out the angle for Total Internal Reflection:
[code]
Critical Angle =
[img src='https://res.cloudinary.com/deylrqt2d/image/upload/v1487981850/refractive_index_equations_2_azd5zg.svg']
[/code]
[b]Note that this is calculating the critical angle when light goes from a more dense substance into a less dense one, the other way round will not work.[/b]
Another way to look at it is to say that Total Internal Reflection occurs when the light refracts away from the normal, until it is 90[degree] or more.
(Also note that before this critical angle, light is both refracted, and partially reflected, causing multiple [b]ghost[/b] images).
[h3]Examples of Total Internal Reflection[/h3]
[img style='width:100vw' src='https://res.cloudinary.com/deylrqt2d/image/upload/v1487982058/6479bfbbce38595157f4ff1cd0111190edacd806_cgcrdc.gif']
[img src='https://res.cloudinary.com/deylrqt2d/image/upload/v1487982058/refr-180dev-prism_dbegf6.jpg']
