Case 1
- Speed of light in the medium
- 225000000m/s
1.332411
Open with these valuesRefractive Index Calculator
1.332411
Result: 1.332411The refractive index is the speed of light in vacuum divided by its speed in the medium. It is a plain number with no unit and never below 1, because nothing outruns light in vacuum. Water sits near 1.33, ordinary glass near 1.50 and diamond at 2.42.
Worked examples
Case 2
- Speed of light in the medium
- 200000000m/s
1.498962
Open with these valuesCase 3
- Speed of light in the medium
- 124000000m/s
2.417681
Open with these valuesHow it's calculated
n = c ÷ v
- StepEnter how fast light travels inside the material, in metres per second.
- StepThe value must stay at or below 299792458 m/s, the speed in vacuum.
- ResultRead the index; 1.33 is water, 1.50 window glass, 2.42 diamond.
Reference table
| Speed in the medium in m/s | Medium | Refractive index |
|---|---|---|
| 299792458 | Vacuum, the defining value | 1 |
| 299700000 | Air at sea level | 1.000309 |
| 225000000 | Water | 1.332411 |
| 200000000 | Window glass | 1.498962 |
| 124000000 | Diamond | 2.417681 |
Questions
How do I calculate the refractive index?
Divide the speed of light in vacuum by the speed in the medium. With c = 299792458 m/s, a material in which light moves at 225000000 m/s has an index of 1.332411, which is close to water.
What does the refractive index tell me?
It says how many times slower light travels in the material than in empty space. An index of 1.5 for glass means light moves one and a half times slower there, and that slowdown is what makes the glass bend light.
Why is the index never below 1?
Nothing carries a signal faster than light in vacuum, so the speed in any medium is at most c and the ratio c ÷ v is at least 1. Vacuum itself is exactly 1, and a speed above c is rejected as physically impossible.
What unit does the index have?
None at all — it is a ratio of two speeds, so the units cancel. Just enter the speed in metres per second, so it matches the vacuum value used in the formula.
Is the speed of light really exact?
Yes. Since the 2019 revision of the SI, c is a defined constant of exactly 299792458 metres per second, and the metre is derived from it. There is no measurement uncertainty left to carry into this calculation.
Sources and last check
Information, not professional advice.