Shu Vidyut Sachit Teukay Che

Author(s): Mahendramuni
Publisher: Anekant Bharati Prakashan

Page 146

They do not heat the intervening medium through which they pass. They obey inverse square law i.e. their intensity varies inversely as the square of the distance from the source. They can be reflected and refracted according to the laws of
reflection and refraction of light. 6.: Thermal radiations also exhibit the phenomena of interference,
diffraction and polarisation as do the light radiations. The only major difference between the thermal radiations and light radiations is in their wavelength. Whereas the wavelength of thermal radiations lies in the range 8 x 10 metre to 4 x 104 metre, wavelength of visible light lies in the range 4 x 10-7 m to 8 x 10 m. Thus thermal radiations are of longer wavelength or smaller frequency and
hence smaller energy as compared to the visibile light." 103. Text-book of Physics (XII Std.) (Part I), pp. 161, 162— "(2) Convection of Heat : In fluids, the heat-transfer takes place
mainly by the process of heat convection. When a fluid is heated from below, it expands and hence its density decreases. So the hotter fluid comes above under the effect of buoyancy and the heavier, cooler fluid sinks under the force of gravity. This way, the entire mass of fluid gets heated. Hot and cold air currents in the atmosphere are due to non-uniform heating of the atmosphere and the associated effects of heat convection. Thermal Radiation : Any substance radiates electromagnetic radiations of differnet frequencies to an extent that depends on its temperature. This radiation is called the thermal radiation. Energy associated with the electromagnetic radiation of the
thermal radiation is called the radiation energy. In a given thermal radiation how the radiated energy is distributed at different frequencies depends upon the temperature and the nature of the radiating body. For example, piece of iron when heated, first appears dark reddish, then as it gets more hot it shines yellowish red, and at still higher temperature, whitish. When we warm ourselves near a coal stove, we absorb energy contained in different frequencies of electro magnetic radiations, being by hot charcoals. Here the heat energy propagates through radiation.
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