Glass has a special property of allowing infra-red radiation of shorter wavelengths to pass through it. But the glass does not allow the infra-red radiation of longer wavelengths to pass through it. A very hot body like the sun emits infra-red radiation of shorter wavelength or large energy. While a body at low temperature emits infra-red radiation of longer wavelengths or small energy. This property of glass has been used by man in constructing glasshouses to protect the plants from the severe cold. 

What is the Green House Effect?

Infra-red radiation emitted by the sun has short wavelengths and large energy. This heat radiation passes easily through the window panes and roofs of the glasshouse. The objects like plants inside the house absorb this heat radiation. Thus, the temperature of the air inside the glasshouse increases by conduction and convection. 

The warm objects like plants inside the glasshouse also radiate energy in the form of infra-red radiation of longer wavelengths and small energy. This infra-red radiation of longer wavelengths inside the house is not allowed by the glass to escape. Hence, the interior of the glasshouse and the plants placed inside it remain warm. This is known as the Green House effect. 

Thus, the greenhouse effect is simply defined as, the warming of Earth’s atmosphere due to the trapping of heat radiation (infra-red radiation).

The greenhouse effect is the consequence of the variation of λ_max with temperature (i.e. Wien’s law). 

In a greenhouse, the incident radiation from the sun is most intense at m = 4.83 × 10 m and passes through glass more easily. This radiation has absorbed the objects inside the greenhouse which in turn re-emit the radiations Generally, the temperature in the greenhouse is about 300 K, therefore the wavelength (λ_max) corresponding to the maximum intensity of radiation is given by 

                                  λ_max  = b / T= 2.898 x 10^-3 mK T /300K = 96.6 x 10^-7 m. 

This wavelength lies in the infra-red part of the radiation spectrum and does not pass through the glass easily. The greenhouse warms up due to the entrapping of infra-red radiations.

Green House Gases 

The earth is a huge greenhouse where the glass in the artificial greenhouse is replaced by the atmosphere. The water vapor and greenhouse gases like carbon dioxide (CO₂), nitrous oxide (N₂O), methane (CH₄), chlorofluorocarbons (CFC), and Ozone (O₃) in the troposphere are good absorbers of infra-red radiation. The sunlight passes more easily through the atmosphere as compared to the infra-red radiations. Due to this fact, the temperature of the earth’s surface increases.

Consequences of Greenhouse Effect

• Temperature Rise: The greenhouse effect heats up the earth’s surface and atmosphere. Human activity (like transport, industries, etc.) has increased the greenhouse gases, resulting in the increase in the average temperature of the earth by 0.3 to 0.6°C in this century. It has been estimated that if such activities continue, then the temperature will increase by 1°C to 3°C after 50 to 60 years. 
• Global Warming: Global warming due to the greenhouse effect leads to major climate changes, which are dangerous to the survival of human and animal life. Coastal areas will be submerged and desert areas will expand. Therefore, efforts must be made to decrease global warming for the survival of human life.
• Ozone Layer Depletion: The ozone layer is found in the upper regions of the stratosphere it protects us from harmful ultraviolet rays from the sun. With the depletion of the ozone layer entry of the harmful UV rays takes place to the earth’s surface which leads to skin cancer and can also change the climate.

Examples of Green House effect 

     1. The interior of a car parked in the sunshine with its windows closed becomes very hot due to the greenhouse effect. 

     2. Rooms of a house in cold countries whose windows are closed and sunlight falls on them become warm due to the greenhouse effect.

     3. Solar cookers and solar furnaces are heated due to the greenhouse effect. 

How the atmosphere of the Earth is a huge greenhouse?

The atmosphere of the earth acts as a greenhouse. The water vapors and carbon dioxide in the atmosphere are good absorbers of infra-red radiation of longer wavelengths than that of the infra-red radiation of shorter wavelengths. The solar radiation (visible rays, infra-red radiation of shorter wavelength, and ultraviolet radiation) pass easily through the atmosphere. The heat radiation passing through the atmosphere is absorbed by the earth’s surface and various objects like plants, water, and rocks. 

Thus, the temperature of air and the earth’s surface increase. The earth’s surface and various objects on it emit infra-red radiation (i.e., heat radiation) of longer wavelengths. These infra-red radiation of longer wavelengths are absorbed by the water vapors and carbon dioxide in the atmosphere. Due to these absorbed heat radiation, the warming of the earth’s atmosphere takes place (Figure 1).

Figure 1: The warming of the earth’s atmosphere

Prevention of greenhouse effect

     1. Avoid Use of Plastic: One should minimize the use of polythene and plastics for packaging, so recycle and reuse household waste.

     2. Plant Tree: One should plant and trees to reduce the carbon dioxide gas in the atmosphere.

     3. Avoid AC: One should avoid AC as much as possible.

     4. Use of Bicycle: One should use bicycles for short distances, the use of fewer cars and bikes will reduce the burning of fuel which is one of the major cause of global warming.

Sample Problems

Problem 1: Write the name of five greenhouse gases.

Solution:

Carbon dioxide (CO₂), nitrous oxide (N₂O), methane (CH₄), chlorofluorocarbon (CFC) and Ozone (O₃).

Problem 2: Mention the two consequences of the greenhouse effect.

Solution:

    1. Global Warming: The global warming due to green house effect leads to major climate changes, which are dangerous to the survival human and animal life. Coastal areas will be submerged and desert areas will expand. Therefore, efforts must be made decrease the global warming for the survival of human life.

    2. Ozone Layer Depletion: Ozone Layer is found in the upper regions of the stratosphere it protects us from harmful ultraviolet rays from the sun. With the depletion of the ozone layer entry of the harmful UV rays takes place to the earth’s surface which leads to skin cancer and can also change the climate.

Problem 3: What is the role of the greenhouse effect in climatic change?

Solution:

Green house effect increasing the temperature of the earth surface which is reducing the percentage of the rain.

Problem 4: Write two examples of the greenhouse effect.

Solution:

1. The interior of a car parked in sunshine with its windows closed becomes very hot due to green house effect.

2. Rooms of a house in cold countries whose windows are closed and sunlight falls on them become warm due to green house effect.

Problem 7: Which infra-red radiation absorbed by the earth, longer or shorter?

Solution:

 Earth is good absorbers of infra-red radiation of longer wavelengths than that of the infra-red radiation of shorter wavelengths.