With reference to the Earth's atmosphere, which one of the following statements is correct?

(a) The total amount of insolation received at the equator is roughly about 10 times of that received at the poles.
(b) Infrared rays constitute roughly two-thirds of insolation.
(c) Infrared waves are largely absorbed by water vapour that is concentrated in the lower atmosphere. ✓ UPSC's answer
(d) Infrared waves are a part of visible spectrum of electromagnetic waves of solar radiation.

Why the answer is (c)

• Option (c) is correct: the infrared part of solar radiation is largely absorbed by water vapour, which is concentrated in the lower troposphere, along with carbon dioxide; this is a key part of the greenhouse effect.

• Option (a) is wrong: the equator receives roughly two to two-and-a-half times the insolation of the poles, not ten times, because the Sun's rays are more direct and pass through less atmosphere.

• Option (b) is wrong: infrared makes up a little under half of insolation (about 45–49%), with visible light about 42–46% and ultraviolet under 10%; it is not two-thirds.

• Option (d) is wrong: infrared lies beyond the red end of the visible spectrum and is invisible.

• Hence option (c).

Why the other options are wrong

(a) The total amount of insolation received at the equator is roughly about 10 times of that…: The equator receives about twice, not ten times, the polar insolation.
(b) Infrared rays constitute roughly two-thirds of insolation.: Infrared is a little under half of insolation, not two-thirds.
(d) Infrared waves are a part of visible spectrum of electromagnetic waves of solar radiation.: Infrared is outside the visible spectrum.

Asked in the GS Paper I of the UPSC Civil Services Preliminary Examination 2023, held on 28 May 2023. Question and answer key: Union Public Service Commission. Explanation: UPSC Answer Check.