Sun emits light of different wavelengths consisting of ultra-violet, visible and infrared radiations. Out of these, the harmful ultra-violet (UV0 radiations are largely absorbed by ozone layer. The visible and infrared radiations freely pass through the atmosphere and reach the surface of the earth.
The infrared rays, which come from the sun, are of short wavelength and pass through the atmosphere easily. The sunlight heats up the surface of the earth. The heated surface of the earth emits (or reflects) infrared radiations of longer wavelength. These infrared radiations (heat waves) are trapped by carbon dioxide, water vapours and ozone in the atmosphere.
Although these constituents are less than 1% of the earth’s atmosphere, they absorb about 90% of the heat radiated from the earth’s surface, causing the atmosphere to heat up. The heated atmosphere in turn warms the earth’s surface to a higher temperature than it would have in the absence of an atmosphere. The heating of earth due to trapped radiation is called greenhouse effect.
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The name ‘greenhouse’ is derived from the glass house in which the green plants are kept. The glass house (known as greenhouse) maintains a high temperature inside. The glass allows visible light from the sun to pass through it but traps the infrared radiations reflected by the inner surface of the glass house, much like the carbon dioxide in the atmosphere. The trapped infra-red rays keep the greenhouse warm.
Green house effect in an automobile
The heating due to greenhouse effect can also be observed in a car (or any other automobile) parked in the sun with all its windows closed. The glass windows of the car allow the sun’s rays to enter the car. Some of these rays are reflected in the form of infra-rd rays from the metal and the upholstery inside the car. These rays are trapped by glass window, just like CO2, and cause the interior of the car to heat up. Hence, the inside of the car becomes considerably hot.
Greenhouse Gases
The gases such as carbon dioxide (CO2), water vapours (H2O) and ozone (O3) which are responsible for the greenhouse effect are called greenhouse gases.
Out of these greenhouse gases, water vapours and ozone do not contribute much to the greenhouse effect of the earth’s atmosphere because water vapour is found only near the surface of the earth whereas ozone is present only in the upper atmosphere. On the other hand, carbon dioxide is much more uniformly distributed in the atmosphere and hence makes more contribution towards the greenhouse effect. Thus, carbon dioxide is the most important greenhouse gas.