JEE Main & Advanced Physics Transmission of Heat Radiation

(1) The process of the transfer of heat from one place to another place without heating the intervening medium is called radiation.

(2) Precisely it is electromagnetic energy transfer in the form of electromagnetic wave through any medium. It is possible even in vacuum e.g. the heat from the sun reaches the earth through radiation.

(3) The wavelength of thermal radiations ranges from \[7.8\times {{10}^{-7}}\,m\] to \[4\times {{10}^{-4}}\,m\]. They belong to infra-red region of the electromagnetic spectrum. That is why thermal radiations are also called infra-red radiations.

(4) Medium is not required for the propagation of these radiations.

(5) They produce sensation of warmth in us but we can't see them.

(6) Every body whose temperature is above zero Kelvin emits thermal radiation.

(7) Their speed is equal to that of light i.e. \[(=3\times {{10}^{8}}\,m/s)\].

(8) Their intensity is inversely proportional to the square of distance of point of observation from the source (i.e. \[I\propto 1/{{d}^{2}}\]).

(9) Just as light waves, they follow laws of reflection, refraction, interference, diffraction and polarisation.

(10) When these radiations fall on a surface then exert pressure on that surface which is known as radiation pressure.

(11) While travelling these radiations travel just like photons of other electromagnetic waves. They manifest themselves as heat only when they are absorbed by a substance.

(12) Spectrum of these radiations can not be obtained with the help of glass prism because it absorbs heat radiations. It is obtained by quartz or rock salt prism because these materials do not have free electrons and interatomic vibrational frequency is greater than the radiation frequency, hence they do not absorb heat radiations.

(13) Diathermanous Medium : A medium which allows heat radiations to pass through it without absorbing them is called diathermanous medium. Thus the temperature of a diathermanous medium does not increase irrespective of the amount of the thermal radiations passing through it e.g., dry air, \[S{{O}_{2}}\], rock salt \[(NaCl)\].

(i) Dry air does not get heated in summers by absorbing heat radiations from sun. It gets heated through convection by receiving heat from the surface of earth.

(ii) In winters heat from sun is directly absorbed by human flesh while the surrounding air being diathermanous is still cool. This is the reason that sun's warmth in winter season appears very satisfying to us.

(14) Athermanous medium : A medium which partly absorbs heat rays is called a thermous medium  As a result temperature of an athermanous medium increases when heat radiations pass through it e.g., wood, metal, moist air, simple glass, human flesh etc.