What is refractive index?

Refractive index, n, is the ratio of the speed of light in vacuum to its speed in a material. A value of 1.5 means light travels 1.5 times slower in that medium than in vacuum. Higher n bends light more strongly at a surface, which is why diamond (n = 2.42) sparkles so much.

Why is the wavelength 589 nm specified?

Refractive index depends on wavelength, an effect called dispersion. The values here are quoted at 589 nm, the bright yellow sodium D line, which is the conventional reference for optical glass. At blue wavelengths n is slightly higher, and at red slightly lower.

How is the critical angle calculated?

The critical angle is θc = arcsin(n₂/n₁), valid only when light travels from a denser medium (n₁) into a rarer one (n₂), so n₁ is greater than n₂. Beyond this angle of incidence all the light reflects internally, the principle behind optical fibres and prisms.

How does refractive index relate to Snell's law?

Snell's law, n₁·sin θ₁ = n₂·sin θ₂, governs how light bends at an interface. The larger the index difference, the more the ray bends. Setting θ₂ to 90 degrees gives the critical angle, the steepest incidence that can still escape into the second medium.

Does temperature change the refractive index?

Yes, modestly. As a material warms it usually becomes slightly less dense and its index falls a little; liquids change more than solids. The values here are near 25°C, suitable for general work, but precision optics account for both temperature and wavelength.

Refractive Index Reference

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Refractive index measures how much a material slows and bends light. This reference lists indices at the sodium D line (589 nm) for more than thirty-five glasses, crystals, liquids, and plastics, and includes a critical-angle calculator for total internal reflection.

How it works

The refractive index n of a medium is n = c / v, the speed of light in vacuum divided by its speed in the medium. Because v is always less than c, n is at least 1. When light crosses a boundary it bends according to Snell’s law, n₁·sin θ₁ = n₂·sin θ₂. The bigger the index difference, the sharper the bend. When light travels from a denser medium into a rarer one and the angle of incidence is large enough, it cannot escape and reflects entirely — total internal reflection. The threshold is the critical angle:

θc = arcsin(n₂ / n₁)   (requires n₁ > n₂)

Reading the table

Gases sit just above 1, water is 1.33, and ordinary window glass is around 1.52. High-index crystals like sapphire, cubic zirconia, and diamond climb past 2, while semiconductors such as silicon exceed 3 in the visible-to-infrared range. A higher index bends light more and, in gemstones, increases the brilliance and fire that make diamond so distinctive.

Example and notes

For light going from diamond (n₁ = 2.42) into air (n₂ = 1.00), the critical angle is arcsin(1.00 / 2.42) ≈ 24.4° — unusually small, so most light striking a facet from inside is trapped and reflected, giving diamond its sparkle. All values here are at 589 nm and about 25°C; index varies with wavelength (dispersion) and temperature, so use wavelength-specific data for precise lens and fibre design.