Index of Refraction Calculator

Created by Bogna Szyk and Steven Wooding
Reviewed by Dominik Czernia, PhD candidate
Last updated: Nov 03, 2021

The index of refraction calculator will allow you to calculate the refractive index of any medium. It is also a valuable tool for determining the speed of light in any translucent material. Continue reading to discover the index of refraction formula and learn how to find the index of refraction.

Refractive index definition

The index of refraction, also called the refractive index, describes how light propagates through a medium. It is a dimensionless quantity, and it determines how much light is bent(refracted) when entering a different medium. In essence, refraction means a change in the speed and wavelength of the wave.

Index of refraction formula

Index of refraction of any medium is defined as the proportion between the speed of light in vacuum and in the investigated medium. The refractive index equation is:

  • n = c / v


  • c — Speed of light in vacuum – 299,792.46 km/s;
  • v — Speed of light in the medium; and
  • n — Refractive index.

The typical values for the refractive index are between 1 and 2, but there are some higher values. As nothing can travel faster than with the speed of light, there are no materials with the index of refraction lower than 1.

How to find the index of refraction

  1. Determine the speed of light in the analyzed medium. For example, let's assume 228,850 km/s.
  2. Divide the speed of light by this value. 299792.46 / 228850 = 1.31.
  3. The obtained value is the refractive index of the medium.
  4. You can use this value to calculate the angle of refraction, using our Snell's law calculator.

You can also take the value of the refractive index directly from the list below.

  • Vacuum: 1 (by definition)
  • Air: 1.000293
  • Water at 20 °C: 1.333
  • Ethanol: 1.36
  • Ice: 1.31
  • Diamond: 2.419


What does the index of refraction directly measure?

The index of refraction is a measure of how fast light travels through a material compared to light traveling in a vacuum. For example, a refractive index of 2 means that light travels at half the speed it does in free space.

What is the refractive index of water?

The refractive index of water is 1.333 if the temperature of the water is 20 °C. As you increase the temperature, the water becomes less dense, light travels faster, and the refractive index decreases.

What is the refractive index of glass?

The glass used for windows has a refractive index of 1.52. The refractive index for other types of glass are:

  • Fused silica — 1.458
  • Pyrex — 1.470
  • Crown glass — 1.50-1.54
  • Flint glass (pure) — 1.60-1.62
  • Flint glass (impure) — 1.523-1.925

What is the SI unit of refractive index?

Refractive index doesn't have any SI units, as it is a unitless, dimensionless quantity. This result is because you calculate it by dividing the speed of light in a vacuum by the speed of light in the material.

How to calculate speed of light using refractive index?

You can calculate the speed of light by multiplying the refractive index of a material by the speed at which light travels through the same material. That means by carefully choosing the right material, measuring the speed of light is easier.

How does sugar concentration affect the refractive index of water?

The addition of sugar to water will make it denser, slowing light down and so increasing the refractive index of the solution.

Bogna Szyk and Steven Wooding
Index of refraction illustration with a light beam traveling from a vacuum into a medium material.
Absolute index of refraction
Speed in medium (v)
Index of refraction
To calculate the angle of refraction, use our Snell's law calculator 🐌
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