Which one of the following statements with regard to greenhouse effect is correct?

Correct — C, the version in which short wavelength infrared radiation can pass through into the chamber and long wavelength infrared radiation cannot pass back out. Sunlight arrives as short-wavelength radiation because the Sun's surface is near 5,800 kelvin, and ordinary silicate glass is transparent to it. That radiation is absorbed by the soil and plants inside, which are near 300 kelvin, so they re-radiate at much longer infra-red wavelengths — around 10 micrometres. Glass is opaque at those wavelengths, so the outgoing radiation is trapped and the interior warms. The one-way behaviour of the glass is the whole mechanism, and only this option states it in the right direction.

• (a)It can take place inside a glass chamber where no radiation can pass into it or pass back through it — A chamber that lets no radiation in at all would receive no solar energy and could not warm above its surroundings. This describes an opaque box, not a greenhouse.
• (b)It can take place inside a glass chamber where long wavelength infrared radiation can pass through into it and short wavelength infrared radiation cannot pass back through it — This reverses the two wavelengths. Long-wavelength infra-red is exactly what glass blocks; if it entered freely and short-wavelength radiation were trapped, the described chamber would not behave like a greenhouse.
• (d)It can take place inside a glass chamber where all infrared radiation can pass into it or pass back through it — If all infra-red passed both ways the glass would be doing nothing, and there would be no net trapping of energy. The interior would simply track the outside temperature.

Wien's law fixes where a body radiates. A hot source such as the Sun peaks in the visible and near infra-red; a warm surface such as soil or a leaf at about 300 kelvin peaks near 10 micrometres. Silicate glass and the atmosphere's greenhouse gases share a useful property — both are largely transparent at the short incoming wavelengths and largely opaque at the long outgoing ones. That asymmetry is what raises the surface temperature.

The four options differ only in which wavelength goes which way, so the item is really asking whether you know the direction of the asymmetry. Anchor it on physical intuition instead of memory: a car left in the sun gets hot inside even though its windows are clear, which can only happen if energy enters more easily than it leaves. One footnote worth carrying: in a real garden greenhouse a large part of the warming also comes from the glass stopping convection, that is from trapping warm air rather than radiation. The atmospheric greenhouse effect has no such lid, and works by radiation alone.

• The Sun radiates mostly at short wavelengths; the Earth's surface, at about 300 kelvin, radiates in the long infra-red near 10 micrometres.
• Ordinary silicate glass transmits visible and near infra-red radiation but is opaque to long-wave infra-red.
• Carbon dioxide, methane, water vapour and nitrous oxide play the same role in the atmosphere that glass plays in a greenhouse.
• Without any greenhouse effect the Earth's mean surface temperature would be about 33 degrees Celsius lower.
• Wien's law: the wavelength of peak emission is inversely proportional to absolute temperature.

• Reversing the two wavelengths, which is what three of the four options are built to catch.
• Assuming glass blocks all infra-red, including the incoming near infra-red.
• Treating the atmospheric greenhouse effect as identical in mechanism to a garden greenhouse.

A four-way statement item where the options are near-identical sentences with two terms swapped. The examiner is testing the direction of the asymmetry, nothing else.

• practice — not a real PYQ

  The Earth's surface radiates energy chiefly in which part of the spectrum?

  • (a)Ultra-violet
  • (b)Visible light
  • (c)Long-wave infra-red
  • (d)Microwave

  Answer(c) Long-wave infra-red — a surface near 300 kelvin peaks close to 10 micrometres.

• practice — not a real PYQ

  Which one of the following is not a greenhouse gas?

  • (a)Methane
  • (b)Nitrogen
  • (c)Nitrous oxide
  • (d)Water vapour

  Answer(b) Nitrogen — a symmetric diatomic molecule with no infra-red active vibration.