In the experiment transport of heat we saw that the transfer of energy in the air via normal heat conductivity is not as easy as e.g. in water. The energy of light is transported in the air as electromagnetical radiation. Only a small fraction of this energy spectrum (visible light) can be experienced by our eyes. Other parts are invisible and damage e.g. our skin (as ultraviolet light) or we can feel it as warm radiation (infrared radiation). The temperature in the atmosphere is considerably governed by the capability of the air molecules to absorb this radiation.
An important fraction of the sunlight reaching the earth’s surface is absorbed, transformed into thermal energy and emitted again as infrared radiation. Greenhouse gases in the atmosphere are these gases who efficiently absorb this infrared radiation emitted by the earth in order to keep the heat in the atmosphere like in a greenhouse.
In order to investigate this phenomenon we carry out the following experiment:
Glass vessels containing air (right) and CO2 (left)
two lights of identical output
two pans with water
Temperature feeler (data registration by a computer)
A The lights in the experiments represent the sun, which emits light towards the earth.
B A pan filled 1cm with water is needed in order to absorb the thermal energy emitted by the lights, which would disturb the experiment. Normal bulbs emit a major fraction of their energy as heat but only the radiation should pass and is required for the processes below.
C1 The left hand side glass vessel is filled with pure carbon dioxide.
C2 The right hand side vessel is filled with normal air containing only about 0,037% of carbon dioxide. Therefore the absorbing effect of CO2 is by far stronger in the left vessel and the capability to act as greenhouse gas can be estimated through this experimental setup.
D Below the glass vessels (covered by the sign) is a layer of black cardboard, representing the dark surface of the earth. After switiching on the light, the temperature is measured in both bessels and plotted on the computer screen as a function of the time.
Sketch of the experimental setup
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|left: carbon dioxide|
the infrared radiation emitted by the cardboard, is absorbed by the CO2 and causes an increase of heat in the vessel.
the infrared radiation emitted by the cardboard leaves the vessel without any significant absorption. The increase in tempearture is less.
1) The lights are switched on
2) Infrared light (heat) is absorbed by the water in the pan above the vessel and does not reach the air or the CO2 below.
3) The visible light emitted by the bulbs passes the water and reaches the black cardboard.
4) The cardboard becomes warmer and emits infrared radiation.
5) The infrared light is absorbed by the pure carbon dioxide (left) much stronger than by the air (right).
6) In the vessel with carbon dioxide the gas is heated due the absorption.
Temperature as a function of time:
|Time||T (air)||T (CO2)|
The blue series in front shows the course of temperatures in the right vessel.
The violet series behind shows the temperature course in the left vessel containing carbon dioxide.
Due to the infrared radiation absorbed by the pure carbon dioxide this gas is heated by far stronger than the air consisting mainly from nitrogen and oxygen. The fraction of carbon dioxide in the air is acting as an greenhouse gas.