Absorption, reflection & transmission of heat radiation

Heat can be transferred by conduction, convection and radiation. The first two modes of heat transfer require a material medium for transmission of heat but radiation does not. As the term ‘radiation’ refers to one mode of transfer of heat, the term ‘radiation’ also refers to continuous emission of energy from the surface of any body because of its thermal energy. This emitted energy is termed as radiant energy. Radiation is therefore the fastest mode of transfer of heat. 

Interaction of Thermal Radiation and Matter:

Whenever thermal radiation falls on the surface of an object, some part of heat energy is reflected, some part is absorbed and the remaining part is transmitted. Let Q be the total amount of thermal energy incident on the surface of an object and Qa , Qr and Qt be the respective amounts of heat absorbed, reflected and transmitted by the object: 

Coefficient of absorption or absorptive power or absorptivity (a): The ratio of amount of heat absorbed to total quantity of heat incident is called the coefficient of absorption.

Coefficient of reflection or reflectance (r): The ratio of amount of radiant energy reflected to the total energy incident is called the coefficient of reflection.

Coefficient of transmission or transmittance (tr): The ratio of amount of radiant energy transmitted to total energy incident is called the coefficient of transmission.

Since all the three quantities a, r and tr are ratios of thermal energies, they are dimensionless quantities. If r = 0 and a = 0, then t = 1, all the incident energy is transmitted through the object i.e., it is a perfect transmitter. A substance through which heat radiations can pass is known as a diathermanous substance. For a diathermanous body, t is not equal to 0. A diathermanous body is neither a good absorber nor a good reflector. On the other hand, if t = 0 and a + r = 1, i.e., the object does not transmit any radiation, it is said to be opaque to the radiation. Substances which are largely opaque to thermal radiations i.e., do not transmit heat radiations incident on them, are known as athermanous substances. If r = 0 and t = 0 then a = 1, all the incident energy is absorbed by the object. Such an object is called a perfect blackbody.