yellow light, and then encounter the next layer further back in the iris, which is a light absorber called the epithelium or uvea that is colored brownish-black. The brightness or intensity of scattered blue light that is scattered by the particles is due to this layer along with the turbid medium of particles within the stroma. Thus, the longer wavelengths are not reflected (by scattering) back to the open air as much as the shorter wavelengths. Because the shorter wavelengths are the blue wavelengths, this gives rise to a blue hue in the light that comes out of the eye. The blue iris is an example of a structural color because it relies only on the interference of light through the turbid medium to generate the color. Blue eyes & brown eyes, therefore, both seem to be anatomically different from each other in a genetically non-variable way because of the difference between turbid media and melanin also both kinds of eye color can remain functionally separate despite being "mixed" together. Similar phenomena different from Tyndall scattering When the day's sky is overcast, sunlight passes through the turbid layer of the clouds, resulting in scattered, diffuse light on the ground (sunbeam). This exhibits Mie scattering instead of Tyndall scattering because the cloud droplets are larger than the wavelength of the light and scatters all colors approximately equally. When the daytime sky is cloudless, the sky's color is blue due to Rayleigh scattering instead of Tyndall scattering because the scattering particles are the air molecules, which are much smaller than the wavelengths of visible light. Similarly, the term Tyndall effect is incorrectly applied to light scattering by large, macroscopic dust particles in the air; however, due to their large size, they do not exhibit Tyndall scattering. Gallery See also Light scattering Transparency and translucency Ultramicroscope References Scattering Scattering, absorption and radiative transfer (optics) Smoke Optical phenomena