Critical angle: this is angle of incidence that produces an angle of refraction equal to 90 degrees
If the angle of refraction is 90 degrees then light will travel along the surface of the two mediums.
Total internal reflection occurs when the angle of incidence is equal to or greater than the critical angle.
Total internal reflection occurs if:
1. Light is traveling more slowly in the first medium than the second medium. This means that the first medium has the largest index of refraction.
2. The angle of incidence is large enough that no refraction occurs in the second medium. The ray gets reflected back into the first medium which occurs if the angle of incidence is equal to or greater than the critical angle.
A good example of total internal reflection is a diamond. Diamond has the highest index of refraction, but can increase the amount of total internal reflection by being cut properly making diamond sparkle.
Fibre optic cables transmit light as it travels through cables by bouncing from side to side. The critical angle inside the cable is very small, so the bouncing light will always have an angle of incidence greater than the critical angle and thus stay within the cable.
Triangular prisms can be oriented so light hits them at angles greater than their critical angles. The result is all the light is completely internally reflected and thus it has no absorption of light.
Retro-reflectors are simple optical devices that returns any light hitting it straight back towards the direction which it came from. An example of a retro-reflector is the corner of a glass cube. Retro-reflectors are used on many road signs to make them more noticeable.