Page 1 - Lesson note atmospheric refraction
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SAI INTERNATIONAL SCHOOL
                                                      PHYSICS
                                                      CLASS X
                          Chapter: LIGHT(Human eye and the colourful world)
                                                    MODULE-39


               LESSON NOTES:    Atmospheric refraction
               Atmospheric refraction
               Definition: The refraction of light caused by the Earth’s atmosphere is called
               atmospheric refraction.

               Cause:- All air in the atmosphere is not at the same temperature .The cooler air
               layer of the atmosphere behave as optically denser medium and the warmer air layer
               of the atmosphere behave as optically rarer medium for the ray of light. So in the
               same atmosphere we have air layer of different optical densities.

               The air in the upper atmosphere is optically rarer and as we come down the air in the
               lower atmosphere is gradually denser. So when light rays pass through the
               atmosphere the refraction of light takes place .


               The following are the examples of atmospheric refraction of light.

               Twinkling of stars:

               The twinkling of stars is due to the atmospheric refraction of stars light. When the
               light coming from star enters the Earth’s atmosphere it undergoes refraction due to
               the varying optical densities of air at various altitudes. The continuously changing
               atmosphere refract the light from the stars by different amounts from one moment to
               the next. When the atmosphere refracts more star -light toward us, the star appears
               to be bright and when the atmosphere refracts less star- light, then the star appears
               to be dim. In this way the star-light reaching our eyes increases and decreases
               continuously due to atmospheric refraction. And the stars appear to twinkle at night.


               Planets don’t twinkle:

               Planets considered to be a collection of a very large number of point sources of light.

               The dimming effect produced by some of the point sources in one part of the planet
               is nullified by the brighter effect produced by the point sources in its other part. Thus
               on the whole the brightness of a planet always remains same hence it does not
               twinkle.The continuously changing atmosphere is not able to cause variation in the
               light coming from a big sized planet due to refraction, because of which the  planets
               do not twinkle.(see diagram from book)

               2) Stars appear higher than they are





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