What is the refractive index?

The refractive index (n) of a medium is the ratio of the speed of light in vacuum (c ≈ 2.998 × 10⁸ m/s) to the phase velocity of light in that medium (v). It is always ≥ 1 for normal materials.

What is the refractive index of common materials?

Vacuum: 1.000, Air: ~1.0003, Water: ~1.333, Glass (crown): ~1.52, Diamond: ~2.417. Higher values mean light slows down more in that medium.

What is the critical angle?

The critical angle is the minimum angle of incidence (measured from the normal) at which total internal reflection occurs when light travels from the medium into vacuum or air. It equals arcsin(1/n).

Can the refractive index be less than 1?

For most natural materials n ≥ 1. Values slightly below 1 can occur near absorption resonances or in engineered metamaterials, but the phase velocity cannot exceed c for energy transport.

What does the wavelength factor mean?

Inside a medium the wavelength of light shrinks by a factor of 1/n. For example, in glass with n = 1.5 the wavelength is 2/3 of its vacuum value.

Refractive Index Calculator

Name: Refractive Index Calculator
Availability: InStock
Author: Nham Vu

Compute the refractive index n = c / v, or find the speed of light in a medium from its refractive index.

Refractive Index Calculator

Calculation Mode

Speed of Light in Medium (v)

c in vacuum = 299,792,458 m/s

Common Materials

Results

Enter a value and click Calculate

Summary

Compute the refractive index n = c / v, or find the speed of light in a medium from its refractive index.

How it works

Choose a calculation mode: find the refractive index n, or find the speed of light v.
Enter the known value (speed of light in the medium, or refractive index).
Click Calculate to see the result instantly.
The tool applies the formula n = c / v using c = 299,792,458 m/s.
Results include the refractive index, phase velocity, wavelength factor, and critical angle.

Use cases

Physics and optics homework involving Snell's law.
Verifying refractive indices for glass, water, diamond, or other materials.
Computing the phase velocity of light in fiber optic cables.
Finding the critical angle for total internal reflection.
Teaching or learning the relationship between speed of light and optical density.
Engineering calculations for lens and prism design.

Frequently Asked Questions

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Last updated: 2026-05-23 · Reviewed by Nham Vu