Total Internal Reflection

Introduction:

Whenever a light ray strikes any object three phenomena can happen – Reflection, Refraction, and Observations. We know that whenever a wavefront or a light ray changes its direction of propagation after striking at the boundary which separates two mediums is known as reflection.

The reflection happens within the medium in which the light ray is traveling or we can say that incident ray, normal, and reflected ray all lie in the same plane. While refraction is defined as the bending of the light rays when it passes from one medium to another medium. In refraction speed of the light rays also change when it enters a different medium.

In refraction, the light rays bend towards normal when they passed from a rare to a denser medium while it bends away from the normal when it passes from a denser to a rare medium. Observation of light is a phenomenon in which the object observes the light. Even if reflection or refraction is happening some light rays always get observed by the objects.

Total Internal Reflection:

So now we come to our question. What is total internal reflection? To understand the total internal reflection, we will take an example of a light ray passing from water (denser medium) to air (rare medium).

When the light rays pass from water to air two things happen – refraction and partial reflection. Now we will discuss the phenomenon one by one. In this case, partial reflection is happening because some numbers of rays are going through refraction so only a few amounts of rays are left to reflect. This is why it is called a partial reflection.

The reflection happens within the water because according to the law of reflection all three – incident ray, normal, and reflected ray has to be in the same plane. So, let’s say the angle of incidence is Ө1 and the angle of reflection is Ө2. Both will be equal according to the law of reflection.

Now the rays which are getting refracted bend away from the normal when it enters the air from water since it is passing from a denser medium to a rarer medium.

Since refracted ray is bending away from the normal angle of refraction which is let’s say Ө3 will be greater than the incident angle Ө1. Now say if we keep increasing the incident angle then the angle of refraction will also increase. But there will come a value of incident angle at which the angle of refraction will be ninety degrees. That means at some value of incident angle the refracted ray will make ninety degrees angle with the normal and will pass along or parallel to the surface of the water.

So, let’s assume Ө4 is the value of the incident angle at which the angle of refraction is ninety degrees. That means at Ө4 angle of incident Ө3 which is refraction angle will be ninety degrees. If we keep increasing the incident angle beyond Ө4 then refraction will not occur only reflection will occur and that’s what we call Total Internal Reflection and the Ө4 will be called the Critical Angle.

So now in simple words, we can say that total internal reflection is a phenomenon that occurs when a light ray pass from the denser medium to a rarer medium and the angle of incidence is greater than the critical angle.

NOTE – If the angle of incidence is less than or equals to the critical angle when a ray is passing from a denser to a rare medium then refraction will occur with partial reflection. There will be no total Internal reflection. So, the critical angle is very important.

Critical Angle:

It is the value of an incident angle at which the refracted ray makes a ninety-degree angle with normal or the refraction angle is ninety degrees.

Critical angle, Ө = Sin Ө = n1/n2

Ө = Critical angle
n1 = Refractive index of 2nd medium
n2 = Refractive index of 1st medium

The Condition For Occurring Of Total Internal Reflection:

• The angle of incidence should be greater than the critical angle.
• The total internal reflection only occurs when the light rays pass from a denser to a rare medium.
• The total internal reflection won’t occur if light rays will pass from the rare medium to a denser medium because the light rays will bend towards the normal, so it won’t make a ninety-degree angle with it. So basically, ninety degrees of refraction angle is not possible when rays pass from a rare to a denser medium. That’s why critical angle can never be achieved in this case.

Total Internal Reflection Examples:

• Shining Of A Diamond.
• Mirage Formation.
• Optical Fibres Which Use For The Communication.
• Endoscopy In Which Optical Fibres Are Used To Examine The Internal Parts Of An Organ.