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)Reference values across material families
| Material | n at 589 nm | Notes |
|---|---|---|
| Air (dry, STP) | 1.0003 | Effectively 1.00 for most calculations |
| Water (liquid) | 1.333 | Drops slightly with temperature |
| Ice | 1.309 | Lower than liquid water |
| Fused silica (quartz) | 1.458 | Standard optical fibre material |
| Crown glass (BK7) | 1.517 | Common camera and microscope glass |
| Flint glass | 1.62–1.70 | Higher dispersion than crown glass |
| Sapphire (Al₂O₃) | 1.762 | Hard crystal, used in watch faces |
| Cubic zirconia | 2.15 | Gemstone diamond simulant |
| Diamond | 2.417 | Critical angle ≈ 24.4° in air |
| Silicon (near IR) | ~3.5 | Much higher in infrared than visible |
| Common plastics (PMMA) | ~1.49 | Acrylic / Perspex |
| Polycarbonate | ~1.586 | Impact-resistant optical plastic |
Gases sit just above 1, liquids between 1.3 and 1.5 for most common solvents, and crystals reach well above 2.
The critical angle in practice
For light going from diamond (n₁ = 2.417) into air (n₂ = 1.00):
θc = arcsin(1.00 / 2.417) ≈ 24.4°This unusually small critical angle means most light inside a diamond facet reflects internally when it strikes at any angle steeper than 24.4° from the surface. Gemstone cutters exploit this by designing facet angles that ensure most light bounces internally multiple times before exiting the crown facing upward — creating the brilliance and fire that distinguish well-cut diamonds.
For an optical fibre made of fused silica (n = 1.458) surrounded by a silica cladding of slightly lower index (for example, 1.444 for a doped outer layer):
θc = arcsin(1.444 / 1.458) ≈ 82°The very high critical angle means light entering within a narrow acceptance cone stays trapped through total internal reflection, enabling long-distance signal transmission.
Dispersion: why the index depends on wavelength
All values here are quoted at 589 nm (the sodium D line) because refractive index is not a single number — it is a function of wavelength. Blue light (around 450 nm) experiences a higher refractive index than red light (around 700 nm) in most glass. This dispersion is what prisms and raindrops use to separate white light into a spectrum. Precision lens design requires index data at multiple wavelengths, not just at 589 nm.
Temperature also shifts the index slightly. Liquids change more than solids; water’s index drops about 0.0001 per degree Celsius near room temperature, which matters for precision refractometry but is negligible for most optical calculations.