Refractive Index Calculator
Enter the speed of light inside a material to get its refractive index (n = c / v) — and see how it compares to air, water, glass, and diamond.
One speed, one index
Enter the speed of light in the medium and the calculator divides the speed of light in a vacuum by it to return the dimensionless refractive index.
Use metres per second
The speed in the medium must be in m/s and never above 299,792,458 m/s — no material lets light travel faster than it does in a vacuum.
What is the refractive index?
How much a medium slows light
The refractive index calculator turns one measurement — the speed of light inside a material — into the refractive index n, the number that says how much slower light travels there than in empty space. Light moves fastest in a vacuum, at exactly 299,792,458 m/s, and slows down in any material it passes through. The refractive index is simply the ratio of those two speeds, so it is a pure number with no units and is always at least 1. It is the quantity behind why a straw looks bent in a glass of water, why lenses focus light, and why a diamond sparkles.
Enter the speed of light in a medium in metres per second to get its refractive index instantly.
The refractive index is the speed of light in a vacuum divided by the speed of light in the medium.
n = c / vHere c is the speed of light in a vacuum (299,792,458 m/s, fixed exactly by the SI definition) and v is the speed of light in the medium. Suppose light travels at 225,000,000 m/s inside a sample, roughly the speed it has in water. Dividing 299,792,458 by 225,000,000 gives a refractive index of about 1.332411. Because v can never exceed c, the index can never drop below 1: a vacuum has n = 1.0, while denser, slower media push it higher.
The refractive index places your material on a simple scale. Air is barely above a vacuum at about 1.0003, water sits at 1.33, common glass at 1.5, and diamond at a high 2.42. The larger the number, the more light slows down and the more sharply it bends when it enters or leaves the material — which is exactly what lenses, prisms, and fibre-optic cables exploit. A value near 1 means light passes through almost unchanged; a value above 2 means the medium bends light dramatically, trapping much of it inside by total internal reflection. That trapping is why a cut diamond throws back so much sparkle, and why a stick poking into a pond appears to snap at the surface. If your result comes out below 1, the speed you entered was faster than light in a vacuum, which is physically impossible — the calculator returns no value in that case.
The formula is exact, but a couple of practical points are worth keeping in mind.
One wavelength, ordinary materials
The refractive index depends slightly on the colour (wavelength) of the light — this dispersion is what splits white light into a rainbow through a prism — so a single value describes one wavelength, usually yellow sodium light. The simple n = c / v relation also assumes an ordinary transparent material; exotic engineered metamaterials can behave differently. Keep the speed in metres per second and at or below 299,792,458 m/s, or the result will be meaningless.