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Infrared radiation, often abbreviated as IR, is the part of the electromagnetic spectrum which has a longer wave-length than visible light ranging from the red-edge of the visible light at 700 nanometers (frequency 430 THz) to 1000000 nm (300 GHz). Additionally to molecular motion  this part of the electromagnetic spectrum is the main reason for heat transfer between objects as stones, liquids and gases. One example is a fire which heats the skins of the living beings around it. Another example is how the heat transfers from the Sun to the Earth is to a large extent provided due to the IR from the Sun .
IR and global warming
Knowledge of the the dependence of Earth's climate on IR is also very important to understand global warming due to greenhouse gas emissions. At any time there is a balance of incoming radiation from the Sun and outgoing radiation from the Earth into space. The equilibrium temperature on Earth depends on both but the latter is altered by the concentration of greenhouse gases in the atmosphere. As a consequence, an increasing concentration of greenhouse gases shifts the equilibrium to higher average temperatures.