# Refractive Index

## Introduction

The index of refraction or the refractive index of an optical medium is determined as the number that does not have any dimension. This dimensionless number gives an indication of the ability of light bending of that particular medium (optical medium). The phenomena of the refractive index determine how much the light path has bent or got refracted while entering into the optical medium (Luk’yanchuk et al. 2017).

The Refractive Index is mainly considered to be the ratio between the light speed in an empty place or vacuum and the speed of light in a specific space. The refractive index in a vacuum is denoted as the number 1. The refractive index varies on the basis of the wavelength of the lights. It can be stated that in the case of white light, the light gets refracted to various constituent colours. This process is known as dispersion and can be seen in rainbows and prism.

## What is Refractive Index?

The index of refraction is also determined as the refractive index. This refractive index is generally used in order to measure the passing of light rays from one medium to another medium. Refractive index can be calculated according to the phenomena where the ratio between the light speed in the specific medium and the light speed in an empty space, or vacuum. In a vacuum, we can find the refractive index as 1 (Xu et al. 2019).

The formula of the refractive index can be stated as η = c/v. Here in this formula, η is denoted as the refractive index, c as light velocity in the vacuum that is (3 × 108 m/s), and v is considered to be the light velocity in a particular medium.

Figure1: Refractive Index

The refractive index of a substance gradually increases with a greater extent to which a beam of light gets refracted or reflected upon leaving the substance. This refractive index of a particular substance also depends to a certain extent upon the light frequency that passes by the medium has the highest frequencies with the highest value of n.

## SI unit of Refractive Index

In the case of the refractive index there is no SI unit. This is because the ratio of the velocity of light in an object and the velocity of light in an empty space or vacuum has the refractive index of 1 (Liberal et al. 2017).

Here, all units of light velocity are cancelled and the thing that is left here is the number that is showing the ratio of the refractive index.

Figure 2: Refractive Index in Vacuum and Medium

This index of refraction is also determined as equal to the light velocity, c has a certain wavelength in a vacuum or in an empty space that is divided by the velocity, v in a medium. This can be stated as a form like n= c/v. A typical index of refraction for the light that shows yellow colour has a wavelength which is equal to 589 nanometres [10−9 metre]. The wavelength of yellow light in the air is about 1.0003, in the crown glass is nearly 1.517, in water is around 1.333, in diamond is about 2.417, and in the flint glass that is dense is about 1.517.

## Snell’s Law

The refractive index varies on the basis of the wavelength of the lights. It can be stated that in the case of white light, the light gets refracted to various constituent colours. This index of refraction denotes how much the night path will be bent or get refracted while entering a medium that can be the optical medium (microscopyu, 2021).

This can be explained by Snell’s law of refraction. The amount of light that will get reflected from the medium is also determined by this refractive index by a formula η = c/v. The concept of refractive index according to Snell’s law applies across the full spectrum of the electromagnet that is from X-rays to the radioactive waves.

## Refractive index: significance

In order to mention the significance of the refractive index, it can be stated that if the index of refraction gets high then the speed of light is found as increasing correspondingly with the change in direction of light travels through the medium (Liu et al. 2021). The thinner lens has a higher refractive index. These lenses are often denoted as the polycarbonate lenses that have an index of about 1.59. The refractive index of concave lens material is high. Here a parallel light beam is an incident on this concave lens and has the emergent path as per the laws of optics.

## Conclusion

As per laws of optics, it can be stated that the concave lens material reserves its behaviour when it is placed in a higher refractive index medium. Thus, the concave lens generally shows the diversion of light but when it is placed in the medium of a higher index of refraction it gradually gets converged and forms parallel light rays to a particular focal point. For example, it can be stated that in the case of ordinary glass, the refractive index for the light having a violet colour has a percentage that is 1% more than that of the red light in a glass medium.

## FAQs

Q1. What do you mean by index of refraction?

The calculated ratio of the speed of the light travelling in an empty space or in a vacuum is divided by the speed of light travelling through a medium having greater density is denoted as the index of refraction. The formula of the refractive index can be stated as η = c/v.

Q2: In which medium-light travels the fastest?

Light travels rapidly in a vacuum or in an empty space. The speed of light in a vacuum has been reported to be 3×108 m s–1.

Q3.What is the Refractive index of the lens?

When the ratio of light speed in a vacuum or in an empty space is divided by the light speed in the material of the lens, then this phenomenon is determined as the refractive index of the lens. In the case of the CR-39 plastic lens, the refractive index is 1.49.

Q4. Why the refractive index is important?

In order to measure how the light rays are propagating through a medium or optical medium, this refractive index is used. If the refractive index of a medium is high then the speed of light travelling through it will be low.

## References

### Journals

Liberal, I., & Engheta, N. (2017). Near-zero refractive index photonics. Nature Photonics, 11(3), 149-158. Retrieved from: https://robobees.seas.harvard.edu

Liu, M., Plum, E., Li, H., Li, S., Xu, Q., Zhang, X., ... & Zhang, W. (2021). Temperature‐controlled optical activity and negative refractive index. Advanced Functional Materials, 31(14), 2010249. Retrieved from: https://onlinelibrary.wiley.com

Luk’yanchuk, B. S., Paniagua-Domínguez, R., Minin, I., Minin, O., & Wang, Z. (2017). Refractive index less than two: photonic nanojets yesterday, today and tomorrow. Optical Materials Express, 7(6), 1820-1847. Retrieved from: https://www.osapublishing.org

Xu, Y., Bai, P., Zhou, X., Akimov, Y., Png, C. E., Ang, L. K., ... & Wu, L. (2019). Optical refractive index sensors with plasmonic and photonic structures: promising and inconvenient truth. Advanced Optical Materials, 7(9), 1801433. Retrieved from: https://www.researchgate.net

### Websites

microscopyu, (2021). About: Refractive Index or Index of Refraction. Retrieved from: https://www.microscopyu.com[Retrieved on June 11, 2022]

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Updated on: 21-Aug-2023

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