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Total Internal Reflection

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Introduction to Total Internal Reflection

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Total internal reflection definition falls under the list of the most significant concepts of physics. You can get to know some chapters like Optics. This topic can be easier to understand thoroughly. When you get a clarity-based basic concept, this will be easier for you. 

In this article, students can go through the chapters that have simple definition total internal reflection, refraction, and their applications in your day-to-day life.

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Generally, the entire phenomenon occurs only when light passes from one optically denser medium to another optically rarer medium. Total internal reflection explanation is considered as one of the special cases of refraction that happens during the incident of light on the incident surface at an angle more than the critical angle (ic).


What is Total Internal Reflection?

Total internal reflection happens at a time when a light ray that travels from a denser to a rarer medium. The ray is incident at an angle of incidence that is greater than the critical angle. After that, the light rays are reflected in another denser medium. It is the same medium before reflection. The entire process is known as Total internal reflection.

When light travels between two surfaces made of transparent materials, then it gets refracted. This is also called as the bending of light. Light or rays that come towards or drive away from the normal have interfered. This happens due to a change of one medium to another medium. This gives the simple definition of total internal reflection.


When Does Total Internal Reflection Take Place?

Take the example of two lights that incident at certain points from an optically denser medium to an optically rarer medium. 

The bending of light, i.e. from the normal happens under the phenomena called refraction of light. This is a special condition where the refracted angle is more than the incident angle.

The above statement explains that the increase in the angle of incident results in the increment of the angle of refraction.

A point still exists where the angle of refraction becomes perpendicular. When this happens, the refracted ray will become parallel to the interface. 

The incident ray angle of the denser medium corresponding with the refracted ray angle of the rarer medium is 90°. This is called the Total internal reflection critical angle(ic).

That time when the ray is incident on the surface at an angle greater than the critical angle, the ray comes back to the same medium. The entire procedure of returning a light ray away from the denser medium is known as Total Internal Reflection.

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Explain Total Internal Reflection of Light

We have two major conditions that help us to decide the phenomenon of total internal reflection. (TIR) is based upon. The minor change in the two conditions may not give the appropriate result. 

Total internal reflection has 2 necessary conditions such as:

(i) The light incident upon the interface of two different media should choose to travel from a denser medium to another rarer medium.

(ii) The greater angle of incidence is necessary than the critical angle for these two media.


The Important Formula That Stands Total Internal Reflection

  • Total internal reflection

n1 / n2 = sin r / sin i

  • Total internal reflection Critical angle

sin = n2 / n1

The list of the names of the symbols that are used in the total internal reflection formula and critical angle:

  • r = angle of refraction

  • I = angle of incidence

  • n1 = refractive index in medium 1

  • n2 = refractive index in medium 2

  • θ = critical angle


Practical Applications of Total Internal Reflection

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  1. Optical Fibre

Total internal reflection method is used in optical fibre. The inner part of the fibre lies inside the core of the higher refractive index. All of these fibres are surrounded by another layer of glass.  They lie just beneath the lower refractive index. A plastic jacket is there to surround the fibres.

Back to back total internal reflection occurs when the light from one end of the core travels toward cladding, and the light propagates through it. Optical fibres usage is quite popular among decorative table labs. They have huge applications in the medical field for endoscopy.


  1. Mirage

Mirage is also known as another name called optical illusion of water. You must have seen this during the summer. The appearance of mirages is quite higher in the deserts on a hot summer day. 

Mirage occurs when a ray of light falls on earth by travelling from the top of a tree or sky; it gradually deviates away from the normal. We know that total internal reflection exists when the angle of incidence becomes greater than the critical angle. So this generates mirage in hot deserts or any open spaces during the hot summer.


FAQ (Frequently Asked Questions)

1. If an optical fibre is having a refractive index of n1 = 2 enclosed by another glass of refractive index n2. Calculate the value of other glass’s refractive index such that the critical angle between the two cladding is 60°.

Data given, 

  • Critical angle, θ = 60°

  • Refractive index, n1 = 2

  • Refractive index n2 =?

To calculate n2, the formula is Sin θ = n2/n1 

Sin 60°= n2 / 2 

or, n2 =2 × sin 60°

or, n2= 1.732


2. What is the value of the refractive index of a glass which has the critical angle of 45°?

Data given, 

  • Critical angle θ = 45°

  • Refractive index of the medium, μ = ?

According to formula, 

μ = 1 / sin θ 

μ = 1 / sin 450

μ= 1 / 0.5

µ = 2 

3. Define refractive index in simple terms.

Refractive index is the measurement of the curving of a ray that passes from one medium to another medium.

We can find the value of the refractive index with the formula, n = c / v

4. What is the reason behind the reflection of light?

Reflection of light is also another form of electromagnetic radiation. This happens when the incident rays face a surface or a boundary that consumes the energy of the radiation and returns the waves away from the incidence surface.