Total internal reflection

Introduction

Internal reflection is a part of optics that explains the concept of travelling of light. When a light beam or ray travels from an optically denser medium to a rarer medium or less optically denser medium, internal reflection occurs. In such a phenomenon, some portion of the light gets reflected back into the same medium and another portion of the incident light gets refracted into the different medium. When the angle of incidence made by the incident ray is greater than the critical angle, the phenomenon is known as total internal reflection. When a light incident on a refracted surface at an angle and refracted at 90 angle then incident angle is defined as critical angle. Read further to find more about total internal reflection and its applications.

Definition Of Total Internal Reflection

In short terms, total internal reflection is defined as the light ray that travels from an optically denser medium to an optically rarer medium. When the travel medium changes, the ray gets bent away from the normal. This, in turn, makes the angle of refraction more than the angle of incidence. Let’s take an example to discuss total internal reflection in detail. Consider water and air as the two mediums in which the refraction of light takes place.

1. When the light travels from water (high refractive index) to air (low refractive index), the light ray gets refracted at the junction point. During the refraction, the light ray bends away from the normal, increasing the angle of refraction.
2. At a certain angle of incidence, the incident light ray passes straight along the water surface. This specific angle of incidence is termed as the critical angle, denoted by ic. In such a case, the angle of refraction is always 90°.
3. The third case is when the critical angle is less than the angle of incidence. In such a case, the light ray that is made to strike the surface reflects back into the same medium. And, this phenomenon is known as total internal reflection.

Formula Of Total Internal Reflection

Total internal reflection is the ratio of the refractive index of medium 1 to the refractive index of medium 2. In other terms, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is known as total internal reflection. Total internal reflection =n₁ / n₂ = sin(i) / sin(r). Where r is the angle of refraction; i is the angle of incidence; n₁is the refractive index of the first medium; and n₂ is the refractive index of the second medium. Here is the formula of critical angle in the sine form:- sin Ө = n₂ / n₁ Critical angle in the formula is defined as:- the sine of critical angle is equal to the ratio of the refractive index of the second medium to the refractive index of the first medium. θcritical or θ= sin-1(n₂/n₁)

Important Conditions For Total Internal Reflection

For total internal reflection to take place, the light must satisfy the below-mentioned two conditions.

1.     The beam of incident light should travel from a more dense medium to a less dense medium.
2.     The critical angle should always be less than the angle of incidence (made by the incident ray).

If the incident beam of light satisfies the two conditions, it will lead to total internal reflection.

Total internal reflection and its applications.

Now that you’ve got an idea of what total internal reflection is, it’s time to discuss the most important applications of total internal reflection.

1.     Diamond 

Diamond is popular because of its amazing shine and its glamorous look. The shine of the diamond is due to the phenomenon of total internal reflection. The reason is due to the lower diamond and air interface. Because of the lower diamond and air interface, the beam of lights entering into the diamond remains inside the diamond and suffers multiple internal reflections. The cut of the diamond is made in such a way that the light that once entered the diamond remains in it and comes out from the top after suffering multiple reflections.

2.     Mirage 

Mirage is an optical illusion concept that is based on total internal reflection. You can observe this optical phenomenon during the hot summer days when total internal reflection occurs. According to this phenomenon, the air that is present near the ground gets hotter as compared to the other place above it. Hot air has a small refractive index and is less dense. Alternatively, the cold air has a large refractive index and is denser. Thus, with the increase in height, the optical density also increases and vice versa. When a beam of light passes through a longer object, the light bends away from the normal and undergoes total internal reflection. When you walk or drive on hot sunny days, you will find the light coming to your eyes is coming from below ground. This results in the inverted image of tall objects in the water, and the phenomenon is known as Mirage.

3.     Optical fibre 

Another important application of total internal reflection is the concept of optical fibre. During such a phenomenon, when a beam of light strikes the cladding, the concept of total internal reflection takes place. In this case, the formed angle is always greater than the critical angle. Let’s explain the concept of optical fibre in detail! Optical fibres consist of core and cladding and are fabricated with high-quality composite glass fibre. The refractive index of the material of cladding is lower than the refractive index of the material of the core. When the beam of light moves from one end of optical fibre to another fibre at a certain angle, it undergoes recurrent total internal reflections. This reflection takes place in the whole fibre and then the light comes out from the other end. This concept is used to transmit and receive electrical signals. Then, with the help of a transducer, these signals are then converted to light. In addition to that, these optical fibres are used in the light pipe. This helps doctors and other healthcare professionals use this phenomenon to examine internal organs of the human body.

Conclusion

Total internal reflection or internal reflection is the bending of light when it passes from one medium to another. This effect is also known as bending of the light. During this phenomenon, the light either drives towards or away from the normal, giving rise to the total internal reflection.