Question

Assertion : The Sun is visible to us about 2 minutes before the actual sunrise and about 2 minutes after the actual sunset. Reason : The time difference between actual sunrise and apparent sunrise is about 2 minutes.
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Answer 1

Answer:

Both Assertion and Reason are True. The Reason is the correct explanation of Assertion.

Explanation:

We are able to see the sunrise 2 min before the actual sunrise and the sunset 2 minutes after the actual sunset due to the phenomenon called atmospheric refraction.

What is Atmospheric Refraction?

Ans) The Refraction of light by the Earth's atmosphere is known as atmospheric refraction.

Advance sunrise:

• The advance sunrise is due to atmospheric refraction.
• The figure shows the actual position of the sun at the time of sunrise, just below the horizon and its apparent position just above the horizon appears to us.
• When the sun is slightly below the horizon, the light rays move through the varying refractive indices of different layers of the air, get bent towards the normal, and appear to come from the apparent position of the sun. ( It bends towards the normal as light rays travel from rarer medium to denser medium)
• That’s why the sun is visible to us about 2 minutes before the actual sunrise.

Delayed Sunset:

• The delayed sunset is due to atmospheric refraction.
• The figure shows the actual position of the sun at the time of sunset, just below the horizon, and its apparent position just above the horizon appears to us.
• After sunset when the sun is slightly below the horizon, the light rays move through the varying refractive indices of different layers of the air, get bent towards the normal. and appear to come from the apparent position of the sun.
• That’s why the sun is visible to us about 2 minutes after the actual sunset.

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Answer 2

Answer:

(a) Both the Assertion and Reason are True and the Reason is the correct explanation of assertion.

Explanation:

• The Sun is visible to us about 2 minutes before the actual sunrise and about 2 minutes after the actual sunset.
• This is the result of atmospheric refraction done by the Earth's Atmosphere.
• When light moves from a rarer medium to a denser medium it slows down and hence bends towards the normal.
• When light moves from a denser medium to a rarer medium it speeds up and bends away from the normal.
• As vacuum has a refractive index slightly lesser than that of our atmosphere it is an optically rarer medium in comparison to our atmosphere.
• Hence, when the Sun is slightly below the horizon, as the light from the sun moves through vacuum and enters the atmosphere, it bends towards the normal as it is entering a denser medium from a rarer medium, and the Sun appears to be above the horizon when in actual reality it is slightly below the horizon.
• The same phenomenon is responsible for a 'delayed sunset'.