 Total internal reflection - Slow to fast

When light travels from a media such as glass to air total internal reflection can occur.

remember: Fast to slow, towards normal line

Slow to fast, away from normal line

Glass to air is slow to fast therefore: If theta\ \o\u\t=90^circ they call this the critical angle.

The initial angle theta\ \i\n occurs when light travels along the boundary between two media.

The critical angle only occurs when traveling from a slow medium to a fast medium. As the angle of incidence is increased the angle of refraction increases.  At a certain angle (theta\ \i\n) the light will refract along the boundary. This is the critical angle for glass to air. It's worth knowing that (theta\ \i\n) is about 42 degrees but it varies according to the material.

 Water to air ~~50^circ Glass to air ~~42^circ Diamond to air ~~24^circ

Past this point "total internal reflection" occurs. It is not just a bit of it gets reflected but all of the light ray gets reflected. This is why they call it total internal reflection.

Total internal reflection occurs when the light ray going in at angle (theta\ \i\n) is greater than the critical angle.

Total internal reflection theta\ \i\n\> critical angle