By Zencalcy Editorial TeamLast reviewed About us
The refractive index tells you how much slower light travels in a material than in vacuum. Water is about 1.33 and ordinary glass about 1.5, which is why a straw looks bent at the waterline and why lenses can focus light.
Refractive index from the speed of light in a medium.
n = c / v
c is the speed of light in vacuum, 299,792,458 m/s, and v is the phase speed in the medium. The index is dimensionless and depends on wavelength.
| Symbol | Meaning |
|---|---|
| n | Refractive index, dimensionless and at least 1. |
| c | Speed of light in vacuum, 299,792,458 m/s. |
| v | Speed of light in the medium, in m/s. |
n = 299,792,458 / 200,000,000 = 1.499. That is close to ordinary crown glass, whose index is about 1.52 at visible wavelengths.
Continue with the Single Slit Diffraction Calculator, Snell's Law Calculator, or the Telescope Magnification Calculator — or browse more optics and light tools and all physics calculators.
Not for a bulk passive material. Phase speed can exceed c in special structured media, but the signal speed still obeys relativity.
Results are computed from the standard equations of classical mechanics. Unit conversions use the SI definitions published by NIST. Air resistance is ignored unless the calculator names a drag model.
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Last reviewed: September 2026. See how we build and check every tool on the about page.
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Content updated: September 2026