CDS (I) 2022 General Knowledge · Question 20 of 120

Correct — A, the shorter wavelength radiation from the Sun can enter while the longer wavelength radiation from the ground and the plants inside cannot pass back out through the glass. Ordinary window glass treats the two kinds of radiation quite differently: it is practically transparent to the short-wave radiation arriving from the Sun and almost opaque to the long-wave infrared given off by objects at ordinary temperatures. So sunlight passes in, is absorbed by the soil, the benches and the plants, and warms them. Those warmed objects then radiate too, but because they are hundreds of degrees cooler than the Sun their radiation comes out at a far longer wavelength — and at that wavelength the glass is a barrier. Energy has an easy way in and a poor way out, and the interior warms until the losses balance the gains.

• (b)The shorter wavelength infrared radiations from the Sun can enter into the greenhouse and the longer wavelength infrared radiations from the ground and the plants inside the greenhouse can also pass back out through the glass. — Lets the energy leave as easily as it arrived. If the glass were equally transparent both ways nothing would accumulate, and the greenhouse would end up no warmer than the air outside — which is not what a greenhouse does.
• (c)The shorter wavelength infrared radiations from the Sun cannot enter into the greenhouse while longer wavelength infrared radiations from the ground and the plants inside the greenhouse can pass back out through the glass. — The situation exactly reversed. It blocks the incoming energy and releases the outgoing energy, which would make the inside colder than the outside rather than warmer.
• (d)No infrared radiation can pass through the glass of the greenhouse. — Would leave the interior with no source of energy at all. Sunlight has to be able to get in for anything to be warmed; a structure opaque in both directions is not a greenhouse but a cupboard.

Every body radiates, and the hotter it is the shorter the wavelength at which most of its radiation comes out. The Sun's surface, at about 5800 K, radiates mainly as visible light with substantial infrared alongside; soil and leaves at ordinary temperatures radiate in the far infrared. A material can be transparent at one of those wavelengths and opaque at the other, and glass is. The same asymmetry, achieved by gases rather than glass, is what warms the planet: the atmosphere absorbs only about 23 per cent of the incoming short-wave solar radiation but about 90 per cent of the long-wave infrared the surface sends back up.

The whole item turns on noticing that the two arrows are not symmetric, and only one option is asymmetric in the right direction. Two honest qualifications belong on this answer. The paper describes the incoming radiation as 'shorter wavelength infrared radiations from the Sun', which is loose — most of what arrives from the Sun is visible light, with infrared a large minority — and what actually matters is the contrast in wavelength rather than the label attached to it. The second qualification is bigger. In a real horticultural greenhouse the larger share of the warming comes from the roof and walls simply preventing the warmed air from escaping by convection, not from the radiative trapping this option describes. The radiative account is the correct one for the ATMOSPHERIC greenhouse effect, where there is no roof to stop convection — and it is the account CDS is testing here.

• Window glass is practically transparent to the short-wave radiation arriving from the Sun and almost opaque to the long-wave infrared radiated by objects inside.
• The hotter a body is, the shorter the wavelength at which most of its radiation is emitted — which is why solar and terrestrial radiation occupy different bands.
• Sunlight is absorbed by the soil and plants inside, and it is the warmed objects, not the glass, that emit the long-wave radiation.
• In the atmosphere the same asymmetry is produced by gases: about 23 per cent of incoming short-wave radiation is absorbed against about 90 per cent of outgoing long-wave radiation.
• In a real greenhouse a large part of the warming also comes from the roof and walls stopping the warmed air from escaping by convection.

• Assuming a greenhouse works by keeping cold air out rather than by trapping energy.
• Reading the two radiation arrows as symmetric, so that whatever enters can also leave.
• Treating the glass itself as the thing that is heated; it is the soil and plants inside that absorb and then re-radiate.

As a mechanism statement to be judged, as here, or through the atmospheric analogy — which gases trap outgoing radiation, and which band of radiation they act on.

• practice — not a real PYQ

  The Earth's atmosphere is heated mainly by

  • (a)direct absorption of incoming solar radiation
  • (b)long-wave radiation emitted by the Earth's surface
  • (c)solar radiation reflected from clouds
  • (d)heat conducted upward from the Earth's interior

  Answer(b) long-wave radiation emitted by the Earth's surface — sunlight passes largely through the air and warms the ground, and greenhouse gases then absorb the infrared the ground sends back up.

• practice — not a real PYQ

  Which one of the following best explains why the temperature in the troposphere falls with increasing height?

  • (a)The Sun is further away at greater heights
  • (b)The atmosphere is heated from below by the Earth's surface
  • (c)Air pressure increases with height
  • (d)Ozone at higher levels absorbs infrared radiation

  Answer(b) The atmosphere is heated from below by the Earth's surface — the source of the heat is the ground, so air further from it is cooler.