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    Name two daily life phenomena based on scattering of light by atmospheric particles.

    The interaction of sunlight with atmospheric particles gives rise to the following phenomena:

    1. Blue colour of the sky.
    2. Redishness of the sun at sunrise and sunset.

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    How does the colour of the scattered light depend on the size of the colloidal particles?
    The size of the scattering particles do affect the color of the scattered particles. Very fine particles having sizes, less than the wavelength of visible light mainly scatter blue light of shorter wavelength of the visible spectrum. Colors with longer wavelength i.e., red is scattered be particles of relatively larger size. Particle which are very large like raindrops, dust and ice particles scatter white light. 
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    Why do we observe the apparent random wavering or flickering of objects when seen through a turbulent stream of hot air rising above fire, a stove or radiator?
    Random wavering or flickering of objects when seen through a turbulent stream of hot air rising above the fire is due to atmospheric refraction i.e., refraction of light by the earth’s atmosphere. The air just above the fire becomes hotter than air further up. Hotter air is less denser than the cooler air (denser) above it. Therefore, due to decreasing density or increasing temperature, there is decrease in refractive index. Since the physical conditions of the refracting medium (air) are not stationary, the apparent positions of the objects, as seen through the hot air, fluctuate. Consequently, the objects seen through such air show a wavering effect. 
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    The sun is visible to us about two minutes before actual sunrise and about two minutes after the actual sunset. Give reason.
    Advance sunrise and delayed sunset. Apparent shift in the position of sun at sunrise and sunset. The sun is visible before actual sunrise and after actual sunset, because of atmospheric refraction. With altitude, the density and hence refractive index of air-layer decreases. As shown in Fig., the light rays starting from the sun travel from rarer to denser layers. They bend more and more towards the normal. To an observer on the earth, light rays appears to come from position S’.The sun which is actually in position S below the horizon, appears in position S' above the horizon. Thus the sun appears to rise early by about two minutes and for the same reason, it appears to set late by about two minutes. This increases the length of the day by about four minutes.

    Advance sunrise and delayed sunset. Apparent shift in the position of
    Fig. Atmospheric refraction effect at sunrise
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    The sun near the horizon appears flattened at sunset and sunrise. Explain why?
    Apparent flattening of the sun’s disc at sunrise and sunset is due to atmospheric refraction. The density and the refractive index of the atmosphere decreases with altitude, so the rays above and below the sun on the horizon are refracted by different degrees. This causes the apparent flattening of the sun. But the rays from the sides of the sun on a horizontal plane are generally refracted by the same amount, hence the sun still appears circular along its sides.
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