12 Radiation

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L.D.D Advanced Building Physics - Exchange of energy by Radiation THERMAL RADIATION It is called thermal radiation the radiation emitted or absorbed by the body when it is only due to interaction with macroscopic hidden coordinates (due to molecular thermal agitation) and not due to the interaction with macroscopic electrical modes. In essence thermal radiation is the radiation that is produced or produces thermal agitation of the molecules of a body; it is therefore also connected to the temperature of the body. Every body emits radiation in greater or lesser extent depending on its thermal agitation and independently from the presence of other objects in the universe. It is important to distinguish the volumetric emission, characteristic of gases and vapours, and surface emission, characteristic of most solids and liquids. In this second case the radiation which is emitted by the internal molecules is strongly absorbed by the adjacent ones, which implies that radiation emitted externally by solid or liquid is only the one emitted by the molecules that are whithin the distance of 1 micrometer from the surface itself . Here we will treat only the phenomenon of surface emission (solids / liquids). Two different theories have been proposed to explain the radiation phenomena: 1 - Wave theory Interprets the radiation as an electromagnetic wave, governed by Maxwell's equations. 2 - Corpuscular theory Assumes that the radiant energy in exchanged from the bodies in 'packets' of discrete quantities, wich have corpuscolar characteristics; this theory explains the phenoenon of emission of electrons from an area hit by radiation. L.D.D Figure: Momochrometic emisseve power Ebλ as a function of wave length and the surface temperature of a black body. (experimental values) Planck…