A Quick Guide To Refractive Index

Light is fascinating to observe and understand. When light passes through a medium, several phenomena take place. One of them is Refraction. Refraction refers to a change in the direction of light when it travels from one medium to another. Refraction is dependent on the refractive index. 

The Refractive index can be defined as a ratio of the speed of light in two different mediums. The refractive index shows the velocity with which a light wave can travel through a medium. A dimensionless number represents how much light will bend in a medium. 

What is the Refraction of light, and what are the laws of refraction? 

One cannot speak about the refractive index without discussing refraction. Refraction can be defined as a change in the path or direction of a light wave when it travels from one medium to another. It causes a change in the direction and reduces the velocity of light, which in simple terms, is called refraction. Refraction of light or how much the velocity of the light will change depends on the refractive index of the medium, the temperature, and pressure.

 The two laws of refraction are:

• The three components of refraction-the refracted ray, the incident ray, and the normal ray all lie in the same plane.
• The second law of refraction, also called Snell’s law, states that the ratio of the sine of the angle formed by the incidence ray and the ratio of the sine of the angles formed by the refracted ray is constant. This constant is called the refractive index(RI)

Sin(i)/Sin(r)= Refractive index (RI)

How is the refractive index calculated? 

The Refractive index can be defined scientifically as the fraction (n) by which the velocity in a medium is decreased in relation to the velocity of light in a vacuum. 

 Vm = C/n

Thus, refractive index (n)= C/Vm

Where Vm= Velocity of light in a medium m

C= Velocity of light in vacuum

The assumption here to simplify calculations is that linear propagation of light waves takes place. Thus, the refractive index is the ratio of the speed of light in a vacuum divided by the speed of light in a medium. 

A Refractometer is used to calculate this in everyday life and complex applications. A Refractometer works on the principle of bending light in different mediums.

Refractive index using Snell’s law.

Snell’s law states that the ratio of the sine of the angle of the incident ray with the normal to the sine of the angle of the reflected ray with the normal is constant. This constant is the refractive index.

n1/n2 = sin α2/sin α1

Here n1 = Incident index 

n2 = Refractive index

α2 = Refracted angle

α1 = Incident angle

This ratio is constant and called the refractive index. 

Factors affecting refractive index-

The Refractive index is dependent on 

1. Nature of the medium
2. Wavelength colour

Wavelength is another factor that dictates the refractive index. It has been seen that a linear relationship exists between different wavelengths of light. Thus, a monochromatic light wave must be used in a Refractometer to prevent any kind of dispersion. 

3.   Temperature

The refractive index of a medium is calculated at normal room temperature. When the temperature is high, there is a change in the density of the particular medium. This means higher temperature causes a faster travelling of light. When the temperature is lower, there is a slowing down of the velocity of light because of increased density and velocity. 

Refractive index of commonly used medium

Glass

The refractive index of glass differs based on its thickness, composition, and wavelength. Commonly used crown glass has a refractive index of 1.52, and flint glass has a refractive index of 1.63.

Water

The refractive index of water is 1.333. 

Importance of refractive index

1. The Refractive index helps understand the behaviour of light in different mediums. It shows how the medium can change the velocity and direction of the propagation of light waves. 
2. The Refractive index is also important to determine the focusing power of optical materials such as lenses.
3. It is essential for designing accurate and precise optical instruments such as glasses, microscopes, telescopes, and magnifying glasses. 
4. It is also used to determine the concentration of alcohol in solutions like alcoholic beverages. 
5. The Refractive index is an important test for the food safety industry, and it helps in testing adulteration and purity in food. 
6. It is used to differentiate between liquid samples. It can help set a standard for a particular sample, and other samples can be measured in relation to that specific sample. 
7. The Refractive index influences the amount of light scattering in fillers and pigments. 
8. It is also used for the estimation of several thermophysical properties in petroleum. 
9. The focusing power of several lenses is calculated and checked using a refractive index.
10. Different Rock concentrations and bioactive components in supplement and pharmaceutical industries are also calculated using the refractive index formula.

Types of Refractive index

Index of refraction can be classified into two types-

Absolute refractive index 

It can be understood as the ratio of the velocity of light in a vacuum compared to the ratio of light in a given medium. The absolute refractive index of any medium can never be less than 1. 

Relative refractive index

Is the refractive index of one medium when compared to another medium. It is thus the ratio of the velocity of light in different mediums.

It is denoted by n21= v1/v2

This shows the refractive index of the velocity of light in the medium compared to the velocity of light in medium 1. 

Conclusion

Thus, the Refractive index is a dimensionless number that affects the phenomenon of refraction. Some mediums have a higher refractive index, while others have a smaller 

refractive index. This RI is responsible for the extent of bending of light in different mediums and is widely used. RI finds various industrial applications and can be seen in everyday life examples.