Atmospheric Refraction.

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Atmospheric Refraction

Twinkling of Stars

  • The twinkling of stars is also due to atmospheric refraction.
  • The air in the earth's atmosphere is such that it has increasing refractive index
  • When starlight enters earth's atmosphere it undergoes multiple refraction continuously before it reaches the earth.
  • Because of the increasing refractive index, the starlight bends downwards (ie towards the normal) and thereby the apparent position of the star is slightly higher than its actual position.
  • Also, this apparent position of the star is not fixed. It keeps on changing slightly since the atmospheric conditions of the earth keep varying
  • But however the stars are so distant that they appear fixed to the naked eye

    • Early sunset and Delayed sunrise

    • Atmospheric Refraction is the same reason that the sun is seen about 2 minutes before actual sunrise and is seen for about 2 minutes after sunset.
    • The actual position of the sun is slightly shifted from the actual position of the sun.

  • Tyndall Effect

    • The earth's atmosphere is a mixture of many minute particles such as smoke, water droplets, dust etc. When a beam of light strikes these fine particles, the path taken by that beam becomes visible. Light gets reflected continuously by these particles and then reaches us.
    • This phenomenon of scattering of light by particles is the Tyndall effect.
    • Tyndall effects explain why we see the sky as blue, forests as green and so on.


  • Why  does the Sky appear clear blue?

    • The air molecules and other fine particles have sizes smaller than the wavelength of visible light.
    • Such particles scatter light of shorter wavelengths ie blue more effectively than longer wavelengths( red)
    • When sunlight passes through the atmosphere, the blue end of the spectrum is scattered more.
    • This enters our eyes and hence we see the sky as blue

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