Physics · Thermal Properties Of Matter · NEET
The Sun is very hot (about 5800 K), so by Wien's law its radiation peaks at short wavelength (visible light). The atmosphere is fairly transparent to visible light, so it reaches the ground. The Earth is much cooler (about 288 K), so it re-radiates at long wavelength (infrared, around 10 micrometre). Greenhouse gases like CO2 and water vapour strongly absorb these longer infrared wavelengths. So light comes in easily but the outgoing infrared is absorbed and partly sent back, keeping the surface warm.
No. The greenhouse effect is a natural and necessary process — without it Earth's average temperature would be about -18 degrees C instead of about +15 degrees C, and life would not exist. Global warming is the extra, unwanted rise in temperature caused when human activity adds more greenhouse gases (mainly CO2 from burning fuel). So greenhouse effect = the mechanism; global warming = too much of it.
The main ones are carbon dioxide (CO2), water vapour (H2O), methane (CH4), nitrous oxide (N2O), and ozone. They all absorb infrared radiation. Nitrogen (N2) and oxygen (O2), which make up most of the air, are NOT greenhouse gases because their molecules do not absorb infrared strongly. Water vapour actually contributes the most, but CO2 is the gas humans keep increasing.
The whole effect is a Wien's displacement law story. Wien's law says peak wavelength is inversely proportional to temperature (lambda_m T = constant = 2.9 x 10^-3 m K). Hot Sun gives short-wavelength (visible) radiation; cool Earth gives long-wavelength (infrared) radiation. The atmosphere behaves like a filter that passes one band and absorbs the other. That is why NEET puts this topic right after thermal radiation, blackbody, Stefan-Boltzmann and Wien's law.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is the trapping of heat near Earth's surface when greenhouse gases absorb the infrared radiation emitted by the warm Earth and re-radiate part of it back down.
A glass greenhouse lets sunlight in to warm the plants but the glass blocks the warm air and infrared from leaving. The atmosphere does the same job for the whole planet, so the process shares the name.
The atmosphere is transparent to short-wavelength visible light from the hot Sun, but greenhouse gases like CO2 and water vapour strongly absorb the long-wavelength infrared radiation emitted by the cooler Earth.
Carbon dioxide (CO2) from burning coal, oil and gas is the gas humans increase the most. Water vapour absorbs the most infrared naturally, but CO2 is the driver of the recent rise in temperature.
Yes. It appears in Thermal Properties of Matter under heat transfer by radiation, linked with blackbody radiation, Stefan-Boltzmann law and Wien's displacement law.
