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Infrared radiation

All bodies at all temperatures emit radiation, the intensity and wavelength distribution depending on the nature of the body itself and its temperature. At temperatures below about 500 oC (773 K) the radiation emitted is in the Infrared region of the electromagnetic spectrum. This radiation is invisible to the human eye but its detection is much used in Earth resource satellites, in remote control devices for televisions and stereos, by the military in night glasses, in heat seeking missiles, by the rescue services in locating buried people in collapsed buildings, by the fire service for "seeing" in smoke filed rooms and for detecting heat loses from buildings and power cables.

Snakes have infra red detectors below their eyes so that they can detect the heat emitted by their potential prey! Reflecting capes and the silvering on a vacuum flask exploit the low emissive properties of shiny surfaces. Infrared radiation was first detected by Herschel in 1820, when he showed that there was radiation beyond the red end of the visible spectrum.

The nature of infrared radiation

Infrared radiation can be shown to be electromagnetic in nature and to have a wavelength rather longer than that of visible light. In fact the infrared region of the spectrum extends from about 750 nm to some 400 000 nm (400 mm, 0.4 mm).

We can demonstrate the reflection of infrared radiation quite easily because the surfaces do not need to be very flat to give good reflection. Refraction is more difficult to show because many materials are opaque to infrared. Glass is one of these, only short wave radiation passing through. Glass will actually transmit up to 3000 nm, fluorite up to 9000 nm and rock salt up to 15 000 nm so clearly lenses and prism designed to refract infrared should be made of rock salt.
An infrared filter that will transmit infrared but be completely opaque to visible light can be made from a solution of iodine in carbon disulphide. The velocity of infrared can be inferred from a solar eclipse; infrared radiation and light are cut off at the same instant. This simple observation suggests that in free space the velocity of infrared radiation is the same as that of light.
Infrared radiation

All bodies emit radiation, the intensity and wavelength distribution depending on the nature of the body itself and its temperature. Infrared radiation is invisible to the human eye. The detection of infrared radiation is much used, however, in Earth resource satellites, by the military in night glasses, for spotting areas of high heat loss from buildings and by the electricity boards in detecting hot spots in power cables.

Prévost's theory of exchanges

It had been thought that only 'hot' bodies emitted radiation, but of course what may be 'hot' when compared with one set of surroundings may be 'cold' when compared with another. For example, a candle flame (700 oC) seems hot compared with your hand (37 oC) but cold when compared with the surface of the Sun (6000 oC).

In 1792. Prévost suggested that all bodies radiate energy, but that those with a higher temperature radiate more energy than those at lower temperatures.

If we consider two isolated bodies A and B initially at different temperatures (T1 and T2) with A being hotter than B (Figure 1), then each body will radiate heat to the other. The result will be equal temperatures (T), a kind of 'smearing out' of heat energy over the whole system.
Notice that it is finally a case of dynamic equilibrium, both bodies radiating an equal amount of energy to each other.

Kirchhoff's law

When radiation falls on a surface three things may happen to it:(a) a certain amount R will be reflected,
(b)a certain amount A will be absorbed, and
a certain amount Twill be transmitted.

The total amount of incident energy (I) is divided between these three possibilities. We can therefore write:

For a shiny surface such as silver R will be large and both A and T small.
For a black surface such as coal R will be small, A large and T small.

For glass R will be average, A small and T large. The energy absorbed by a body can be emitted later, and clearly if a surface cannot absorb radiation strongly it will be unable to emit strongly. Kirchhoff summarised this by saying:

The black body

The amount of infrared radiation emitted by a body depends on three things:
(a) the surface area of the body
(b) the type of surface
(c) the temperature of the body

Consider first the type of surface. Simple experiments like those with Leslie's cube show that rough black surfaces makes the best emitters and absorbers of radiation at a given temperature.
An ideal absorber would be one that absorbed all the radiation that fell on it and also emitted the maximum amount of radiation possible for that area at that temperature. Such a body is known as a black body and the radiation emitted by it as black body radiation.
It is important to realise the difference between this equation and Newton's law of cooling. Stefan's law applies to the loss of energy by radiation while Newton's law applies to loss of energy by convection. Both laws are found to hold for temperature differences of hundreds of degrees.

Some further effects of heat radiation

Satellites – passing from bright sunlight into the earth's shadow. The change in the incident radiation would cause expansion and contraction of the satellite and so it must be reflective coating to prevent excessive heating/cooling .
Bedouin wear black clothes - absorb heat and then give good thermal convection currents between layers of clothing.
Reflective capes are given to the runners at the end of a marathon to prevent/reduce heat loss from the body by radiation
© Keith Gibbs 2007