The way light bounces off tiny particles or structures in a material, sometimes producing a visible colour without any pigment being present.
Shorter (blue) wavelengths scatter more than longer (red) wavelengths when light passes through a medium containing particles smaller than or comparable to the light's wavelength — a Tyndall- or Rayleigh-type effect. In a low-melanin iris stroma, this scattering effect is what produces the visible blue or grey appearance, not a blue pigment.
Light scattering is a physical/optical effect, not a chemical pigment — it's the same broad family of phenomenon behind why the sky looks blue, applied at the scale of the iris stroma.
Blue and grey eyes get their colour mainly from light scattering in a low-melanin iris, not from a blue or grey pigment.
Explains why 'blue eyes' and 'blue paint' work completely differently, and why blue/grey eye colour is unusually sensitive to lighting and photography compared with brown.
Scattering is described by how its strength varies with wavelength and with direction, and both depend on the size of the scatterer. Particles much smaller than the wavelength, such as air molecules, scatter in proportion to the inverse fourth power of wavelength, which strongly favours blue. Particles of about the wavelength's size, such as cloud droplets, scatter all visible wavelengths about equally and look white. In instruments, scattering shows up as haze in a transmission measurement and as the diffuse part of a reflection.
| Wavelength | Scattered, relative to 700 nm |
|---|---|
| 400 nm | 9.4× |
| 450 nm | 5.9× |
| 550 nm | 2.6× |
| 650 nm | 1.3× |
| 700 nm | 1.0× |
Light at 450 nm is scattered 5.9 times as strongly as light at 700 nm, and at 400 nm the factor is 9.4. That steep bias towards short wavelengths is enough to colour a material that contains no blue pigment at all, provided there is something dark behind it to absorb the light that is not scattered back.
How this was calculated: Relative strength is (700 ÷ wavelength)⁴, the inverse-fourth-power rule for particles much smaller than the wavelength. Larger particles scatter all wavelengths more evenly and look white.
A white paint or plastic is formulated with a cheaper filler in place of some titanium dioxide.
Scattering depends on the gap in refractive index between particle and binder. Fillers close to the binder's index scatter little, so the product turns translucent and greyer, and needs more coats to hide.
Distant hills are painted or rendered in the same colours as the foreground.
The air between scatters blue light into the line of sight, so real distance looks paler and bluer. A landscape without that shift reads as flat.