How Greenhouse effect works...

A brief description of Wien's displacement law:
The wavelength of radiation emitted by any blackbody is directly proportional to the temperature of the blackbody.
(Black body is a body that absorbs all radiation that falls on it and emits it back - No object in the world is an ideal blackbody but for all practical purposes, all objects are considered as black bodies as they emit radiation when their temperature is increased.)
The constant of proportionality is called Wien's displacement constant.
Now the Sun is a blackbody.. So is the earth.
The temperature of the sun's surface is approximately 5800 K. Thus, according to Wien's displacement law, the wavelength of radiation emitted by this will be less - i.e more of visible light and some infrared radiation. The atmosphere which acts as a transparent cover, allows radiation of this wavelength to pass through. When this falls on the surface of the earth, the earth gets heated due to the infrared radiation. The temperature of the earth's surface is around 286 K. According to Wien's law, the wavelength of radiation emitted is very high. Hence the intensity of infrared radiation emitted is high. This heats the atmosphere. While on its way back, the atmosphere does not allow radiation of this wavelength to escape. Most of it is reflected back to the earth, the reflected light having lower energy. The gases in the atmosphere (CO2, CH4 etc.) absorb the infrared radiation and emit heat thus heating the earth and its atmosphere and making life possible. (This is analogous to the glass house where glass allows radiation of lower wavelength from sun to get inside but does not allow the radiation of higher wavelength, emitted by the surface inside the glass, to get out.)

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