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Scattering of Light

When light passes through a substance or gas, a part of it is absorbed and the rest scattered away. The basic process in scattering is absorption of light by the molecules followed by re-radiation in different directions. The strength of scattering can be measured by the loss of energy in the light beam as it passes through the medium. In absorption the light energy is converted into the internal energy of the medium and in scattering the light energy is radiated in other directions. The strength of scattering depends on the size of the particle causing the scattering and the wavelength of light. The scattering is proportional to 1/h4. This is known as Raleigh's law of scattering. So the red light is scattered the least and the violet is scattered the most. This explains why red signals are used to indicate danger.

Among the shorted wavelengths the color blue is present in the larger proportion in sunlight. This explains why the sky appears blue. When we look at the sky we see it blue because, blue is scattered the most. Another natural phenomenon related to the scattering of light is the red appearance of the sun at the sunset and sunrise. At these times the sunlight has to travel a large distance through the atmosphere. The blue and the neighboring colors are scattered away and the red light reaches our eye. All these scattering is done by the atmospheric particles. Hence if the earth had no atmosphere the sky would appear black. Not only the air molecules, the water particles and dust particles also scatter the sunlight. The change in the quality of color of sky is due to the various sizes of the scattering medium namely the water or the dust particles.

Scattering is a general physical process whereby some forms of radiation, such as light or moving particles, for example, are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which it passes.

scattering of light from sun

A large number of molecules are present in the earth’s atmosphere. These molecules scatter light in various directions. The air is composed of many tiny particles including dust and water vapour. As the sunlight passes through the air, the shorter blue light waves are reflected and refracted by the particles while the other coloured light waves being longer are unaffected and are not reflected by the water vapour or dust in the air. Blue, therefore, is scattered the most and this explains the bluish colour of the sky. At sunset or sunrise, the sunrays have to cover large atmospheric distances to reach us and most of the blue light gets scattered and doesn’t reach us. The sky as well as the sun, at sunrise and sunset, therefore looks reddish.

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Tyndall Effect

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The earth's atmosphere is a heterogeneous mixture of minute particles. These particles include smoke, tiny water droplets, suspended particles of dust and molecules of air. When beam of light strikes such air particles, the path of the beam becomes visible. Similarly the path of a beam of light passing through a true solution is not visible. However, its path becomes visible through a colloidal solution where the size of the particles is relatively larger. The phenomenon of scattering of light by the colloidal particles gives rise to Tyndall effect.

 

tyndall effect scattering of light phenomenon

 

Tyndall effect is the visible scattering effect of light on particles along the path of a beam of light passing through a colloid system.


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