Scattering Of Light
Most of the beautiful phenomena like ‘Blue color of sky’, ‘white color of clouds’, ‘red hues of sunrise and sunset’, have been explained in terms of scattering of light.
As sunlight travels through the earth’s atmosphere, it gets scattered by the large number of molecules present. Scattering represents basically change in direction of light.
Lord Rayleigh was the first to deal with scattering of light from air molecules. If
A = amplitude of incident light,
λ = wavelength of incident light,
V = volume of the scattering particle,
a = amplitude of the scattered light at a distance r from the scattering particle,
then, according to Rayleigh, a = AV/rλ2
as intensity of scattered light (Is) varies directly as square of amplitude of scattered light (a2), therefore,
Is ∝ 1/ λ 4
i.e. intensity of scattered light varies inversely as the fourth power of the wavelength of incident light.
These conclusions were verified experimentally by Tyndall.
Rayleigh established further that the rays do not undergo any change in wavelength on scattering.
However, in case of scattering of light by transparent media, the scattered light might contain some frequencies or wavelengths, which might not be present in the incident radiations.
The scattered light has a longer wavelength, when some energy from incident light is imparted to the scattering molecule. On the contrary, the scattered light has a smaller wavelength, when the scattering molecule is in a higher energy state before the collision than after the collision. Therefore, some energy is received from the scattering molecule.
However, when there is no exchange of energy between the incident light and the scattering molecule, there is no change in wavelength of frequency on scattering, as postulated by Rayleigh.
Another important factor in scattering is relative size of the wavelength of light ( λ ) and the scatter (of typical size, say, a). for a < < λ , Rayleigh scattering is valid and for a>> λ , Rayleigh scattering is not valid. In that event, all wavelengths are scattered nearly equally.
(a) Blue color of sky: is due to scattering of sunlight. Light from the sun, while travelling through earth’s atmosphere, gets scattered by large number of molecules in earth’s atmosphere. As a << λ , Rayleigh scattering is valid. The intensity power of wavelength of light. As blue color has a shorter wavelength than red, therefore, blue colour is scattered much more strongly. Hence the sky looks blue.
(b) White colour of clouds: the clouds are at much lower height. They are seen due to scattering of light from lower parts of the atmosphere, which contains large dust particles, water droplets, ice particles etc. In this case, size of scatterer a >> λ , Therefore, all wavelengths are scattered nearly equally. All colors scattered equally merge to give us the sensation of white. Hence clouds generally appear white.
(c) The sun looks reddish at the time of sunrise and sunset: at the time of sunrise and sunset, the sun is near the horizon. The rays from the sun have to travel a larger part of the atmosphere, and intensity of scattered light ∝ 1/ λ4 , therefore, most of the blue light is scattered away. Only red colour, which is least scattered enters our eyes and appears to come from the sun. Hence the sun looks red both at the time of sun rise and sun set.
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