The blue color of the sky is caused by scattering of sunlight. Colors of more energetic shorter wavelengths (blue, violet) scatter a lot more than less energetic longer wavelengths. Also scattered light disperses equally in all directions. Hence blue light which is at the short wavelength end of the visible spectrum will be scattered much more strongly than red light which is at the long wavelength end.
The concept of scattering of light or any other electromagnetic radiation by particles much smaller than the wavelength of the light is called Rayleigh scattering.
Particles such as dust and smoke are significantly smaller than 0.4 μm (the wavelength of the blue/violet or lower limit of the visible spectrum). These particles can also scatter visible radiation. The intensity of scattering is inversely proportional to the fourth power of wavelength. Reddish colors at sunset and sunrise result from Rayleigh scattering. The longer wavelengths pass directly through the atmosphere to the observer, while particles in the air scatter out radiation of shorter wavelengths.
The scattering of light that happens when the light passes through colloids is called ‘Tyndall effect’. A colloid is a type of homogeneous mixture in which the dispersed particles do not settle out. For example milk, paint, smoke, fog and ink are colloids.
The iris of the human eye does not contain any blue pigment. The darkness of brown eyes is determined by the amount of a substance called colloidal melanin. Melanin meaning ‘dark black’ is a group of natural pigments found in many organisms. In case, if iris contains little melanin, the front layer appears blue due to preferred scattering of light with short wavelengths.
Blue eyes are example of Tyndall scattering. The visible beam of headlights in fog is also caused by the Tyndall effect.
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