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    Human Eye and Colourful World
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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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    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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    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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    A beam of white light falling on a glass prism gets split up into seven colours marked 1 to 7 as shown in the diagram.
    A student makes the following statements about the spectrum observed on the screen.
    (a)  The colours at positions marked 3 and 5 are similar to the colour of the sky and the core of a hard boiled egg respectively.
    Is the above statement made by the student correct or incorrect? Justify.
    (b) Which two positions correspond closely to the colour of (i) a solution of potassium permanganate?
    (ii) ‘danger’ or stop signal lights?   

    The colours of the spectrum observed on the screen are in the order:

    V

    I

    B

    G

    Y

    0

    R

    7

    6

    5

    4

    3

    2

    1
     

    (a) The statement made by the student is incorrect. He is stating the colours in the reverse order. Color at position 3 is yellow which is the color of a boiled egg and color at position 5 is blue, which is the color of the sky. 

    (b) (i) A solution of potassium permanganate is violet. So position 7, corresponds to the color of  potassium permanganate solution.

         (ii) Danger or stop signal lights are red in colour. So position 1 corresponds to this colour.

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