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Blue eyes
Blue eyes are common in infants in whom dark pigments such as melanin have not been formed, and only blue is seen against the dark interior of the eye.
We even see the same scattering phenomenon in the blue color of eyes, particularly in the intense blue of most infants. Other eye colors are formed by melanin, for example, combined with a little yellow melanin, a green color of iris results; if melanin dominates, brown and black colors appear. In the absence of melanin in albinos, the blue scattering adds to the red of the underlying blood vessels to produce pink; the pink skin of light-skinned individuals is similarly produced.
Other kinds of scattering
Careful observation of the sky reveals that the color of the sky varies in its intensity depending on the angle of observation. The blue color gets lighter close to the horizon (picture p.14). This cannot be explained in the framework of the Rayleigh theory which describes scattering on particles much smaller than the wavelength of the incident light. The usual molecules with dimensions of approximately 1 nm (1 nanometer equals one billionth of a meter) and the visible light of wavelengths between 400 to 750 nm fulfill this condition nicely. However, the molecules of gases are not the only particles contained in the atmosphere. The aerosols, e.g. particles of dust, smoke, droplets of liquids, minuscule crystals or bacteria and pollens float in the lower atmosphere. The typical dimensions of such particles can lie anywhere between several hundreds and more than ten thousand nm. This is comparable to or even bigger than the wavelength of the incident light. This kind of scattering was investigated by the German physicist Gustaf Adolf Mie (1868-1957) who established that the intensity of the scattered light (Mie-scattering) is practically independent of the its wavelength, e.g. white light remains white after having been scattered on bigger particles. The concentration of the aerosols is highest close to the ground and thus the lightening of the blue color of the sky is most pronounced close to the horizon. The aerosols are washed out by rain causing a deepening of the blue color of the sky. The blue of the sky appears deeper in the mountains due to lower amount of aerosols in higher altitudes.
If the size of the scattering particles approaches the wavelength of light or exceeds it, then the complex Mie scattering theory applies and permits colors other than blue; white is scattered at the largest sizes, as in fog and clouds. This theory also applies to scattering particles that are electrically conducting, as previously mentioned. Rayleigh and Mie scattering are called elastic scattering, with no change of the wavelength. Forms of inelastic scattering, in which there is a shift in the wavelength, include Raman scattering and Brillouin scattering; both of these can be used for laser operation.
