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
| Field | What to enter |
|---|---|
| Mode | Choose one: Index from a medium preset, Index from the speed in the medium |
| Medium | Choose one: Vacuum, Air, Water at 20 °C, Ethanol … |
| Speed in medium v | Units: 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 n |
|---|---|
| 112,500 | 1.333 |
| 168,750 | 1.333 |
| 225,000 | 1.333 |
| 281,250 | 1.333 |
| 337,500 | 1.333 |
| 450,000 | 1.333 |
Formula
n = c / v
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.
Frequently asked questions
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.