Atmospheric Refraction.

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