# Index of Refraction Calculator

URL: https://speedycalculator.com/calculators/physics/index-of-refraction/
Last updated: 2026-10-30

> The index of refraction calculator finds a medium's refractive index from the speed of light in it, or the speed of light from the index, with presets for common materials.

## Inputs

- Mode
- Medium
- Speed in medium v (km/s, m/s, km/h, mph)

## Results

- Index of refraction n
- Speed of light in the medium
- Critical angle to air

## Formula

```
n = c / v
```

## Explanation

Light slows down in transparent materials. The ratio of its speed in vacuum to its speed in the material is the refractive index: 1.33 for water, about 1.5 for glass and 2.42 for diamond – which is why diamonds sparkle so much.

### How to use the Index of Refraction Calculator

- **Mode**
- **Medium**
- **Speed in medium v**

Results update as you type, every field has its own unit menu, and everything is calculated in your browser.

### How it works

Choose a medium or enter the speed of light in it to find n.

### Inputs explained

FieldWhat to enterModeChoose one: Index from a medium preset, Index from the speed in the mediumMediumChoose one: Vacuum, Air, Water at 20 °C, Ethanol …Speed in medium vUnits: km/s, m/s, km/h, mph

### Example values

How the main result changes when **Speed in medium v** changes and everything else stays at the example values above:

Speed in medium v (km/s)Index of refraction n112,5001.333168,7501.333225,0001.333281,2501.333337,5001.333450,0001.333

## Worked example

Inputs — Mode: Index from a medium preset; Medium: Water at 20 °C.

Results — **Index of refraction n = 1.333**; Speed of light in the medium = 224,900.5686 km/s; Critical angle to air = 48.6257 °.

## Key terms

- **Refractive index**: Ratio of the speed of light in vacuum to that in the medium.

## FAQ

**Can the refractive index be less than 1?**
For visible light in ordinary materials, no.

**Does n depend on colour?**
Yes – dispersion makes n slightly higher for blue light.

**What is the index of air?**
About 1.000293.

**What is the critical angle?**
The incidence angle above which total internal reflection occurs.


## Sources

- [OpenStax – University Physics Volume 3](https://openstax.org/details/books/university-physics-volume-3)
- [HyperPhysics (Georgia State University)](http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html)