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Radiation

Radiation is 'transfer of heat energy at a distance' by electromagnetic waves. When someone feels the heat of a fire, they are usually experiencing the heat of the radiation – people notice the heat most readily from radiation in the infra red region of the spectrum. This is the region in which lies most radiation from objects at those temperatures experienced on Earth. Every body above the absolute zero of temperature radiates electromagnetic waves.

Materials do not need to be in contact for radiation to be able to transfer heat energy between them. They may be billions of light years apart in fact: the only way heat energy can travel across the vastness of space is by electromagnetic radiation.

The following points should be noted:

  • All objects at any temperature above absolute zero both absorbs and radiates energy. If the body is at a constant temperature then the rates are the same.

  • A good emitter of radiation is also a good absorber.

  • Light shiny surfaces are bad absorbers and bad emitters of radiation.

  • Dark dull surfaces are good absorbers and good emitters of radiation.

  • If the temperature increases then the typical frequency of radiation increases. The totoal rate at which radiation is absorbed and emitted will also increase.

  • Radiation is the only means of heat transmission in a vacuum.

Our bodies show up especially well as sources of infra red radiation. This is used in thermal imaging cameras. When the human body is photographed in the infra red, it will show up clearly against a cold background. Even if the background is at body temperature, the human body will be clearly recognisable when it starts to move.

Infra red, or night vision cameras have many law enforcement and military applications, and in disaster zones to detect life under collapsed buildings.