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UPSC CDS 1 2022 GK Paper

Question : 20 of 120

Marks: +1, -0

Which one of the following statements is true with regard to a greenhouse?

The shorter wavelength infrared radiations from the Sun can enter into the greenhouse while longer wavelength infrared radiations from the ground and the plants inside the greenhouse cannot pass back out through the glass.
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.
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.
No infrared radiation can pass through the glass of the greenhouse.

Solution:

Concept:

The greenhouse effect relies on glass being transparent to incoming shortwave solar radiation but opaque to outgoing longwave thermal radiation.

Explanation:

Sunlight contains shorter wavelength infrared radiation that easily passes through glass into the greenhouse.

Inside, the ground and plants absorb this energy and re-emit it as longer wavelength infrared radiation (heat).

Glass is opaque to these longer wavelengths, so the heat cannot escape and remains trapped, warming the interior.

This is similar to Earth’s atmosphere where greenhouse gases like

\mathrm{CO}_2

\mathrm{CH}_4

, and

\mathrm{N}_2\mathrm{O}

allow sunlight in but block outgoing infrared radiation.

Therefore, the true statement is that shortwave infrared from the Sun enters, while longwave infrared from inside cannot pass back out through the glass.

Answer:

Option A: "The shorter wavelength infrared radiations from the Sun can enter into the greenhouse while longer wavelength infrared radiations from the ground and the plants inside the greenhouse cannot pass back out through the glass."

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