of 4 nm small gold nanoparticles that generate the cranberry glass colour. This work led directly to Zsigmondy's Nobel Prize for chemistry. Comparison with Rayleigh scattering Rayleigh scattering is defined by a mathematical formula that requires the light-scattering particles to be far smaller than the wavelength of the light. For a dispersion of particles to qualify for the Rayleigh formula, the particle sizes need to be below roughly 40 nanometres (for visible light), and the particles may be individual molecules. Colloidal particles are bigger and are in the rough vicinity of the size of a wavelength of light. Tyndall scattering, i.e. colloidal particle scattering, is much more intense than Rayleigh scattering due to the bigger particle sizes involved. The importance of the particle size factor for intensity can be seen in the large exponent it has in the mathematical statement of the intensity of Rayleigh scattering. If the colloid particles are spheroid, Tyndall scattering can be mathematically analyzed in terms of Mie theory, which admits particle sizes in the rough vicinity of the wavelength of light. Light scattering by particles of complex shape are described by the T-matrix method. Blue irises A blue iris in an eye is blue due to Tyndall scattering by a translucent layer of turbid media in the iris that contains numerous small particles, which are about 0.6 micrometers in diameter, that are finely suspended within the fibrovascular structure of the stroma or front layer of the iris. Some brown irises have the same layer, except with more amount of melanin in it. Moderate amounts of melanin make hazel, dark blue & green eyes. However, most brown & black eyes are only colored by melanin because it is the only functionally important eye color, which therefore lacks this turbid media due to its unnecessary function in protecting the eye from harmful UV rays. In eyes that contain both particles and melanin, melanin absorbs light. In the absence of melanin, the layer is translucent (i.e. the light passing through is randomly and diffusely scattered by the particles) and a noticeable portion of the light that enters this translucent layer re-emerges via a radial scattered path. That is, there is backscatter, the redirection of the light waves back out to the open air. Scattering takes place to a greater extent at shorter wavelengths. The longer wavelengths tend to pass straight through the translucent layer with unaltered paths of