Atmospheric Refraction ,Advance sunrise and Delayed Sunset class 10 -cbse24

 The refraction of light caused by the earth's atmosphere(having air layers of varying optical densities is called atmospheric refraction)

[1] Twinkling of Stars:(due to Atmospheric Refraction)

The twinkling of a star is due to the atmospheric refraction of starlight. The scientific name of the twinkling of stars is “stellar scintillation”. Stars twinkle when we see them through the thick layer of turbulent(moving) air in the earth's atmosphere.

The twinkling of stars is due to the atmospheric refraction of their light

• Also, planets do not usually twinkle, because they are so close to us, they appear big enough that the twinkling is not noticeable(except when the air is extremely turbulent)
• Star closer to the horizon appear to twinkle more than stars that are overhead, this is because the light of stars near the horizon has to travel through more air than the light of stars near the horizon has to travel through more air than the light of stars overhead.
• Star would not appear to twinkle if we viewed them from outer space(or from a planet/moon that did have an atmosphere)

[2] Apparent Position of Sun (Advance Sunrise and Delayed Sunset)

The apparent position of the sun is because of atmospheric refraction, we do not see the sun (or the stars) in its true position when it is directly overhead.

Effect of atmospheric reflection at sunrise

As a ray of light from the sun enters the earth's atmosphere at B. It continuously bends towards the radius of the earth. The ray of light will read the observer at O as if it had come in the direction AO instead of on its true direction BO.

• It is due to atmospheric refraction that the sun is visible before the actual sunrise and after the actual sunset.
• The sun appears to rise 2 minutes before the actual rise, and it continues to be seen 2 minutes after it has actually set therefore, the day becomes longer by 4 minutes due to atmospheric refraction.
• 2 minutes(after rise)+2 minutes (before sunset)= 4 minutes(Day is longer than night due to atmospheric refraction)

[3] The stars seem higher than they actually are

Due to atmospheric refraction, the stars seem to be higher in the sky than they actually are.

Star seems to be higher in the sky due to atmospheric refraction

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 star appears to be at a higher position.