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HEAT TRANSMISSION BY RADIATION |
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Radiation is a form of energy of whose existence (as far as a small fraction of it is concerned, namely the light and the heat) man has been aware due to his senses.
At present, it is known that radiant energy covers a very wide range, and its effects in the sensitive world differ according to the class it belongs to. The energy propagates through space without a material support in the ordinary sense of the word, and this is most obvious in a vacuum space, where it travels at a rate of 300,000 km/s, regardless of the emission range (thermal, light, cosmic radiation), without being altered qualitatively or quantitatively by the temperature of the space it goes through.
Thermal radiation can be transmitted at any distance without any material support. It depends but on the temperature of the radiating body, not on the temperature of the medium it goes through.
While in the case of heat transmission by conduction and convection, the heat maintains its feature of gradual molecular movement (at speeds much inferior to those of radiation passing through vacuum), thermal radiation keeps this feature only at the moment of emission and reception, the rest of the way having an electromagnetic nature.
To explain the way in which thermal energy turns into radiant electromagnetic energy at the emitter and then how electromagnetic energy (the infrared range of the spectrum) turns into thermal energy (molecular movement energy) at the receiver, Max Planck introduced the term energy quantum, which is a intermittent, pulsatory emission of energy grafted onto a material corpuscle which L. de Broglie credited with vibratory properties. Thus the energy quantum with its two features, corpuscular and energetic, represents two different images of the same physical reality (the material corpuscle is the elementary unit of matter, the same for all bodies and substances).
In this respect, the radiation is released from the structure of the atom, grafted onto an elementary unit of matter. This is why irradiation implies loss of matter. Thus the sun loses 4,000,000 tons of its mass (corresponding to the energy it dissipates in space) every second.
According to the length of the electromagnetic waves emitted, radiant energy can be classed into the following categories::
under 0.05 nm - cosmic radiations;
between 0.5 nm and 10 nm - gamma rays;
between 10 nm and 20 nm - Roentgen rays;
between 20 nm and 0.38 µm - ultraviolet rays;
between 0.38 µm and 0.72 µm - light radiation;
between 0.72 µm and 0.8 mm - thermal or ultrared radiation (short wavelength radiations within this spectrum are called infrared rays);
between 0.2 mm and X km - electric waves.
When a radiation is emitted on a single wavelength, that radiation is called monochromatic.
The wavelength varies as it passes from one medium to another, while the frequency of the radiation remains unaltered, therefore, the speed at which the radiation propagates depends on the material environment.
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