MHT-CET : Physics Entrance Exam

### MHT - CET : Physics - Radiation Glossary

 ·         Electromagnetic Waves: Waves with electric and magnetic fields at right angles to the medium of propagation of wave and also mutually perpendicular are called electromagnetic waves. ·         Diathermanous Surface: A surface which transmits most of the radiations incident on it. For a diathermanous surface, t = 1 ·         Athermanous or Opaque Surface: A surface which does not transmit radiations at all. For an opaque surface t = 0 ·         Perfectly Black Body: A surface (or body) which absorbs all the radiations of all wavelengths and at all temperatures is called a perfectly black body. For a perfectly black body, a = 1, r = 0 and t = 0. ·         Emissive Power (E): It is the amount of radiations emitted per unit area per second at a given temperature. ·         Emissivity (e): It is the ratio of emissive power of a surface to the emissive power of a perfectly black body at the same temperature. e = (E/Eb) ·         Prevost's Theory of Heat Exchanges: Every material body, at any temperature above absolute zero, radiates heat to the surroundings and at the same time absorbs heat from the surroundings. ·         Kirchhoff's Law: At any given temperature, the emissivity of a body is equal to its coefficient of absorption. ·         Stefan's Law: The rate of emission of heat from unit area of a perfectly black body is directly proportional to the fourth power of its absolute temperature. ·         Newton's Law of Cooling: The rate of loss of heat from a body is directly proportional to the excess temperature of the body over the surroundings, provided the excess is small. ·         Radiation Correction: The correction to be applied to the observed temperature of a body in order to compensate for the change in its temperature due to loss or gain of heat by radiation.

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