What is Total Internal Reflection?

We are sure you must have encountered images of a water layer on the road while driving on a hot sunny day and how it vanishes when your car reaches closer. This mirage is a perfect example of an integral topic of the physics syllabus called Total Internal Reflection (TIR). TIR is a phenomenon of deviation of light due to a change in its medium. If you are a school student or someone preparing for SAT subject test, then you can’t afford to miss this blog on Total Internal Reflection!

What is Total Internal Reflection (TIR)?

Whenever a ray of light travels from an optically denser medium to an optically rarer medium, the phenomenon of Total Internal Reflection (TIR) takes place. In the given media like glass, water etc. the ray of light continues to get reflected without getting refracted. 

In the above-mentioned picture, an incident ray of light is travelling from the medium of water to that of air. The ray of light will refract at the common point of the two media which is separating them. As the ray is travelling from a medium of a higher refractive index to a medium with a lower refractive index, the ray of light will move away from the normal. At a certain angle of incidence, the incident ray of light gets refracted in such a way that it passes parallelly along the surface of the water. This very angle of incident is called the critical angle. TIR only takes place when the angle of refraction is 90 degrees. 

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Conditions for Total International Reflection

Majorly, there are two conditions upon which the phenomenon of total internal reflection is based. Even minor changes in these conditions may not provide us with the desired result, hence, it is vital that these conditions are satisfied in order to receive actual results. Let us take a detailed look at the conditions of TIR:

The ray of light travels from denser medium to rarer medium 

TIR will only take place when the light is travelling from an optically denser medium to an optically rarer medium. For example- The phenomenon will take place when the light is travelling from the water towards the air, it will not happen for vice versa. Similarly, Total internal reflection will take place in the case of light travelling from water to air but not from the light moving from water to glass, as the Refractive Index of glass is greater than water. 

The angle of incidence is always greater than the critical angle 

For TIR to place, the angle of incidence must be larger than the critical angle, but what exact value makes the incident angle larger? The critical angle for the water-air media is 48.6 degrees, hence the incident angle for this set-up must be greater than this value. The real value of the critical angle is different for each set of media, hence, the largeness of the angle of the incidence depends on the type of media you are using. 

Formula for Total Internal Reflection

TIR is an essential concept from the examination point of view whether scholastic or competitive exams. The students can be asked to solve a variety of numerical problems using the formula of total internal reflection along with a combination of other formulas. Tabulated below are the important formulas of this topic:

Quantity	Formula
Total Internal Reflection
Critical Angle

In the formulae, 

• r is the angle of refraction 
• i is the angle of incidence 
• n1is the refractive index in medium 1
• n2is the refractive index in medium 2
• θ is the critical angle    

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Examples of Total International Reflection 

Till now, you must be thinking of TIR as a completely scientific phenomenon with critical application in scientific experiments, but total internal reflection can be seen in our daily lives. Refer to the below-mentioned examples of it-

• Mirage or optical illusions like water spilt on a road or an oasis in a desert is nothing but a naturally occurring example of TIR
• Those lustrous diamonds entice us all. Have you ever thought about what makes them sparkle? It is the phenomenon of TIR. Once a ray of light enters a diamond, it gets captured inside a undergoes TIR on various faces of the diamond 
• Optical Fibre is a thin and long wire of glass which has a thin coating of a material with a low refractive index 

FAQs

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