Calculates a material’s refractive index from how much light slows down while traveling through it compared to a vacuum.
How it works
The exact speed of light in a vacuum is divided by the measured speed of light within the material.
What this does not include
This does not include using a known refractive index to find a refraction angle — for that, use this site’s Snell’s law calculator instead.
How to use this calculator
- Enter the speed of light as measured within the material.
A worked example
Light traveling at 200,000,000 m/s through a medium: index of refraction = c/v = (299,792,458 ÷ 200,000,000) ≈ 1.499 — close to typical glass.
Light at 150,000,000 m/s: index ≈ 1.9986 — a denser medium, close to some types of diamond-adjacent materials.
What the variables mean
| Variable | Meaning |
|---|---|
| Speed | Speed of light within the medium, in m/s |
| c | Speed of light in a vacuum, a fixed constant (299,792,458 m/s) |
Edge cases worth knowing
A higher index of refraction means light slows down more inside the material — the second example’s slower speed corresponds to a denser optical medium than the first.
A speed of zero makes the index infinite, which has no physical meaning for real light propagation, so the calculator declines to show a result.
Frequently asked questions
Why is a material’s refractive index always greater than 1?
Because light always travels slower in a material than in a vacuum, so dividing the (faster) vacuum speed by the (slower) material speed always gives a number above 1.
What has a higher refractive index, air or glass?
Glass — its refractive index (around 1.5) is noticeably higher than air’s (very close to 1), meaning light slows down significantly more inside glass.
Does refractive index depend on the color of light?
Yes, slightly — this effect (dispersion) is why a prism splits white light into a rainbow, though this calculator gives a single average value.
