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
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.