Find a material’s refractive index. This free calculator uses Snell’s law (from the angles of incidence and refraction) or the speed of light (n = c/v), and shows the critical angle too.
Method
°
°
Refractive index of the second medium from Snell’s law: n₂ = n₁ · sin θ₁ ÷ sin θ₂. Angles are measured from the normal.
m/s
Refractive index from the speed of light: n = c ÷ v, where c = 299,792,458 m/s (speed of light in a vacuum).
Refractive index n₂
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Enter your values
For reference: vacuum 1.0000, air ≈ 1.0003, water ≈ 1.333, typical glass ≈ 1.5, diamond ≈ 2.42. The refractive index has no units.
Use this free refractive index calculator to find the refractive index of a material. Use Snell’s law from the angles of incidence and refraction, or calculate it from the speed of light in the medium.
What this refractive index calculator does
A refractive index calculator finds n, the number describing how much light bends and slows in a material. You can calculate it two ways: from the angles of incidence and refraction (Snell’s law) or from the speed of light in the medium (n = c/v).
How refractive index is calculated
Snell's law: n₂ = n₁ · sin θ₁ ÷ sin θ₂
Speed: n = c ÷ v (c = 299,792,458 m/s)
Light entering glass at 45° and refracting to 28° gives a refractive index of about 1.51. A medium where light travels at 2.25×10⁸ m/s has an index of about 1.33 (water).
Refractive index of common materials
Material
Refractive index (n)
Vacuum
1.0000
Air
1.0003
Water
1.333
Glass (typical)
1.5
Diamond
2.42
What is the critical angle?
When light travels from a denser to a less dense medium, beyond a certain critical angle it reflects entirely (total internal reflection). It equals arcsin(n₂/n₁) where n₁ > n₂ — for glass to air, about 41.8°. The calculator shows it in Snell’s-law mode.
How to use the refractive index calculator
Choose a method — Snell’s law or speed of light.
For Snell’s law, enter n₁ (blank = air), and both angles.
For the speed method, enter the speed of light in the medium.
Read the refractive index and related values.
Refractive Index FAQ
How do you calculate refractive index?
Two common ways: with Snell's law, n₂ = n₁ · sin θ₁ ÷ sin θ₂ from the angles of incidence and refraction; or from speed, n = c ÷ v, the speed of light in vacuum divided by its speed in the material.
What is refractive index?
It's a number describing how much light slows down and bends when entering a material. Higher values mean light slows more and bends more — water is about 1.33, glass about 1.5.
What is Snell's law?
Snell's law relates the angles and refractive indices at a boundary: n₁ sin θ₁ = n₂ sin θ₂. Rearranging it lets you find an unknown refractive index from the angles.
What is the critical angle?
The angle of incidence (going from a denser to a less dense medium) beyond which light is totally internally reflected. It equals arcsin(n₂/n₁); for glass to air it's about 41.8°.
Does refractive index have units?
No. Because it's a ratio (of speeds or of sines), the refractive index is a pure number with no units.
Is the refractive index calculator free?
Yes, this refractive index calculator is completely free, needs no sign-up, and gives instant results in your browser.
