# Total internal reflection

Definition of total internal reflection

Total internal reflection is the complete reflection of a light ray travelling from a dense medium to a less dense medium, for example, the total reflection of light travelling from glass (dense medium) to air (less dense medium) (Raymond, Chris & Jerry, 2011).

Avadhanulu (2012) stated that when light travels from one medium to another it changes speed and is refracted. If the light rays are travelling for a less dense material to a dense medium they are refracted towards the normal and if they are travelling from a dense to less dense medium they are refracted away from the normal. For total internal reflection to occur the light must travel from a dense medium to a less dense medium (e.g. glass to air or water to air).  As the angle of incidence increases so does the angle of refraction. When the angle of incidence reaches a value known as the critical angle the refracted rays travel along the surface of the medium or in other words are refracted to an angle of 90°. The critical angle for the angle of incidence in glass is 42°.

The illustration of the behaviour of light travelling from a dense medium to a less dense medium according to Bowley (2009) is as shown below. 1. For angle of incidence less than the critical angle:When the angle of incidence of the light ray leaving the glass is less than the critical angle, the light ray speeds up on leaving the glass and is refracted away from the normal.
2. For angle of incidence equal to the critical angle: When the angle of incidence of the light ray reaches the critical angle (42°) the angle of refraction is 90°. The refracted ray travels along the surface of the denser medium in this case the glass.
3. For angle of incidence greater than the critical angle:When the angle of incidence of the light ray is greater than the critical angle then no refraction takes place. Instead, all the light is reflected back into the denser material in this case the glass. This is called total internal reflection.

Requirements for total internal reflection

Total internal reflection is the phenomenon that involves the reflection of all the incident light off the boundary. Axelrod (2011) stated that total internal reflection only takes place when both of the following two conditions are met:

• the light is in the more dense medium and approaching the less dense medium.
• the angle of incidence is greater than the so-called critical angle.

The above conditions mean that total internal reflection will not take place unless the incident light is travelling within the more optically dense medium towards the less optically dense medium. Total internal reflection will happen for light travelling from water towards air, but it will not happen for light travelling from air towards water. Total internal reflection would happen for light travelling from water towards air, but it will not happen for light travelling from water towards glass.

Total internal reflection occurs because the angle of refraction reaches a 90-degree angle before the angle of incidence reaches a 90-degree angle. The only way for the angle of refraction to be greater than the angle of incidence is for light to bend away from the normal. Since light only bends away from the normal when passing from a more dense medium into a less dense medium, then this would be a necessary condition for total internal reflection (Massa, 2010).

Conclusion

Total internal reflection is an optical condition that shows the behaviour of light as a result of the light travelling from more dense medium to a less dense medium at an angle greater than the critical angle.

References

Avadhanulu, M. (2012). A textbook of engineering physics for B.E., B.Sc. (Engg.). New York: Sage Inc

Axelrod, D. (2011). Cell-substrate contacts illuminated by total internal reflection fluorescence. The Journal of Cell Biology. 89 (1): 141–145.

Bowley, R. (2009). Total Internal Reflection. Nottingham: Brady Haran

Hui, R. &O’Sullivan, M. (2009). Fiber optic measurement techniques. London: Academic Press.

Raymond, S.,Chris V.,Jerry, S. (2011). College physics. California: Appleton.

Massa, N. (2010). Fiber optic telecommunication. International Conference on Education and Training in Optics and Photonics, 15: 20.

Vasan, S. (2004).Basicsof photonics and optics. Edinburgh: Crest Books.