Total Internal Refraction

Critical angle

The critical angle is the angle of incident in an optically denser medium for which the angle or refraction is 90o.

The critical angle is the maximum angle of the angle of incident

Critical angle and Refractive Index

Total Internal Reflection

Total internal reflection is the reflection of light at the boundary of 2 medium where the angle if incident exceeds the critical angle of the medium.

Requirements for Total Internal Reflection

1. The light ray must propagate from an optically denser medium to an optically less dense medium.
2. The angle of incident must exceed the critical angle.

Mirage

(The lake you see at the picture is not real. It is just a mirage )

1. In a very hot day (normally at desert), the air near the ground is much hotter than the air at higher altitude.
2. Cold air is (optically) denser than hot air.
3. When light ray travels from the upper part of the air to the lower part, it will be refracted away form the normal, and consequently experiences total internal reflection.
4. As a result, a water like image appear on the surface of the earth, which is known as mirage.

Rainbow

(This photo is created by Eric Rolph. The original file can be found in the English Wikipedia)

(Shared under the GNU Free Documentation License. Original file can be found in Wikipedia)

As we can see in the diagram above, light ray experiences total internal reflection in a water droplet, forming rainbow in the sky.

Optical Fibre

1. An optical fibre is a long thin strand of glass surrounded by glass cladding with low refractive index.
2. Rays of light entering one end of the fibre experience repeated total internal reflection until they emerge at the other end.

Prism Binocular

Total internal reflection happen in the prisms of a binocular.

Raised Pavement Marker (Reflector of Road)

Diamond

(This image is shared under GNU Free Documentation License. The original file can be dound in Wikipedia)

1. Diamonds have very high refractive index, and hence very small critical angle.
2. Any light rays that enter a diamond at an angle greater than its critical angle will undergo total internal reflection.
3. Multiple internal reflections always happen in a diamond, making it sparkle all the time.