Question 30
How does refraction takes place in the atmosphere? Why do stars twinkle but not the planets?
Question 30
How does refraction takes place in the atmosphere? Why do stars twinkle but not the planets?
When sunlight enters the earth's atmosphere, it continuously goes from rarer to the denser medium and hence refraction of light takes place. The refraction of light taking place in the atmosphere is known as atmospheric refraction.
Stars twinkle on a clear night : The twinkling of a star is due to the atmospheric refraction of star's light. When the light coming from a star enters the earth's atmosphere, it undergoes refraction due to the varying optical densities of air at various altitudes. The atmosphere is continuously changing (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. As the amount of refraction caused by atmosphere changes, the star-light reaching our eyes increases and decreases continuously. And the star appears to twinkle at night.
Planets do not twinkle at night: The atmospheric refraction is unable to cause variations in the intensity of light coming from much nearer, big looking planets due to which the planets do not twinkle at night.
The planets appear to be quite big to us (because they are much nearer to the earth). So, a planet can be 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 of light in one part of the planet is nullified by the brighter effect produced by the point sources of light in its other part. Thus, on the whole, the brightness of a planet always remains the same and hence it does not appear to twinkle.
When sunlight enters the earth's atmosphere, it continuously goes from rarer to the denser medium and hence refraction of light takes place. The refraction of light taking place in the atmosphere is known as atmospheric refraction.
Stars twinkle on a clear night : The twinkling of a star is due to the atmospheric refraction of star's light. When the light coming from a star enters the earth's atmosphere, it undergoes refraction due to the varying optical densities of air at various altitudes. The atmosphere is continuously changing (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. As the amount of refraction caused by atmosphere changes, the star-light reaching our eyes increases and decreases continuously. And the star appears to twinkle at night.
Planets do not twinkle at night: The atmospheric refraction is unable to cause variations in the intensity of light coming from much nearer, big looking planets due to which the planets do not twinkle at night.
The planets appear to be quite big to us (because they are much nearer to the earth). So, a planet can be 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 of light in one part of the planet is nullified by the brighter effect produced by the point sources of light in its other part. Thus, on the whole, the brightness of a planet always remains the same and hence it does not appear to twinkle.
