Atmospheric refraction

• Atmospheric Refraction: In the same atmosphere we have air layers having different optical densities. The refraction of light caused by the earth’s atmosphere (having air layers of varying optical densities) is called atmospheric refraction.
• Examples of atmospheric refraction:
  • Twinkling of stars
  • The stars seem higher than they are actually are
  • Advanced sunrise and sunset

TWINKLING OF STARS

• The apparent position of a star is slightly different from the actual position due to refraction of starlight by the atmosphere.
• When the light coming from a star enters the earth’s atmosphere, it undergoes refraction or bending due to the varying optical densities of air at various altitudes which results in the change of the position.
• The atmosphere is continuously changing as it is affected by turbulent winds and varying temperatures due to which the optical densities of air at different levels in the atmosphere keep on changing.
• The continuously changing atmosphere refracts the light from the stars by different amounts from one moment to the next.
• The speed of star-light reaching our eyes increases and decreases continuously as it travels from one medium to another while passing through the atmosphere. Due to the atmospheric refractions, the star appears to twinkle at night.

THE RAINBOW

• It is a beautiful spectrum seen in the sky when the sun shines on raindrops during or after a rain shower.
• A rainbow is produced by the dispersion of white sunlight by raindrops (or water drops) in the atmosphere.
• The raindrops in the atmosphere act like many small prisms splitting into a spectrum.

The stars that are overhead twinkle more than those seen on the horizon from a large empty field

This is because the stars overhead are nearer than those seen on the horizon and hence there are more layers of atmosphere in the horizontal direction, resulting in more series of refractions.

The planets do not show twinkling effect

As the planets are much closer to the earth, the amount of light received from them is much greater and the fluctuations caused in the amount of light due to atmospheric refraction are negligible as compared to the amount of light received from them.

THE STARS SEEM HIGHER THAN THEY ACTUALLY ARE

• Light from a star is refracted as it leaves space (a vacuum) and enters the earth’s atmosphere.
• Air higher up in the sky is rarer but that nearer the earth’s surface is denser. So, as the light from a star comes down, the dense air bends the light more.
• Due to this atmospheric refraction of star’s light, the star appears to be at a higher position than they actually are.

ADVANCE SUNRISE AND DELAYED SUNSET

• The earth’s atmosphere is associated with temperature changes, winds and gases making it more dense than vacuum..
• When the sun is slightly below the horizon, then the sun’s light coming from less dense air to more dense air is refracted downwards as it passes through the atmosphere. Because of this atmospheric refraction, the sun appears to be raised above the horizon when actually it is slightly below 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.