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1.3 Thermal radiation
Radiation is a heat transferred by the wave energy. These waves are called

Electromagnetic Waves, because the energy travels in a combination of electric and magnetic

waves. The radiation energy transfer is through energy-carrying electromagnetic waves

that are emitted by atoms and molecules due to change in their energy content. It means:

does not depend on an intermediate material. Radiative heat transfer does not require a

medium to pass through; thus, it is the only form of heat transfer present in vacuum. It

uses electromagnetic radiation (photons), which travels at the speed of light and is

emitted by any matter with temperature above 0 degrees Kelvin (-273 °C). This energy is

released when these waves are absorbed by an object. Radiative heat transfer occurs when

the emitted radiation strikes another body and is absorbed. We all experience radiative

heat transfer everyday; solar radiation, absorbed by our skin, is why we feel warmer in

the sun than in the shade.

The electromagnetic spectrum classifies radiation according to wave lengths of the

radiation. Main types of radiation are (from short to long wavelengths): gamma rays, x-

rays, ultraviolet (UV), visible light, infrared (IR), microwaves, and radio waves. The

energy a wave carries is related to its wavelength (measured from crest to crest). Shorter

wavelengths carry more energy than longer wavelengths. Wavelengths are measured in

terms of meters:

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 1 (millimeter) mm = .001 m = 10 m
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 1 (micrometer) μm = .000001 m = 10 m
 1 μm is 1 millionth of a meter. One-hundredth the diameter of a human hair.

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 1 (nanometer) nm = .000000001 m = 10 m
When talking about electromagnetic waves it is sometimes easier to give them

characteristics of particles, we call these particles photons. A photon of x-ray radiation
carries more energy than a photon of visible light.

Radiation with shorter wavelengths are more energetic and contains more heat. X-
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rays, having wavelengths ~10 m, are very energetic and can be harmful to humans,
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while visible light with wavelengths ~10 m contain less energy and therefore have
little effect on life. A second characteristic which will become important later is that

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