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A student claims that because of atmospheric refraction, the sun can be seen after it has set, and the day is, therefore, longer than if the earth had no atmosphere.(a) What does the student mean by saying that the sun can be seen after it has set?(b) Do you think that the students conclusion is correct?
(a) By saying that the sun can be seen after it has set, the student means that due to atmospheric refraction we are able to see the sun about two minutes even after it has set below the horizon.
(b) Yes, the student's conclusion about the longer duration of the day due to atmospheric refraction is correct. The atmospheric refraction causes the sun to appear two minutes before the actual sunrise and two minutes after sunset. Hence, the time from sunrise to sunset is increased by about four minutes. Thus, the day gets longer by about four minutes.
If the earth had no atmosphere, then there will be no refraction, eventually, the day wouldn't be longer.
Explanation
Due to the refraction of sunlight caused by the atmosphere, we can see the sun 2 minutes before it's actually above the horizon (advanced sunrise), and 2 minutes even after the actual sunset (delayed sunset).
This happens because when the sun is slightly below the horizon, then the sun's light coming from the less dense air to more dense air is refracted downwards as it passes through the atmosphere. Thus, due to this atmospheric refraction, the sun appears to be raised above the horizon when actually it is slightly below the horizon.
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