Emission and Absorption of Radiation

Emission of Radiation:

The rate of emission of radiation from a unit surface area of a body depends only on two properties of the body:

i. Nature of the surface: The rate of emission depends on the color of the surface, whether the surface is rough or shiny, and whether is it smooth or porous, etc. This dependence is elaborated by Kirchhoff’s law.

ii. Temperature of the body: Higher the temperature of a body, more is the rate of emission. The rate falls to zero only when a body is at the temperature of absolute zero, at T = 0. This dependence is described by Stefan’s law.

iii. The material and the internal structure of a body: As long as the nature of the surface and the temperatures of two or more bodies are the same, the rate of emission from a unit surface area remains identical. The internal properties of the bodies and their materials have no effect on this rate.

iv. Presence of other bodies in the surroundings: The objects in these surroundings, whatever the temperatures they are at have no effect on the rate of emission. However, the change in the temperature of a body, of course, depends on its surroundings. A body emits radiation and simultaneously absorbs radiation from its surrounding objects.

Its temperature would naturally increase, decrease or remain the same, if the rate of emission is less than, greater than, or equal to the rate of absorption, respectively. It is described by Prevost’s theory of heat exchanges.

Absorption of Radiation:

When radiation is incident on a surface, it is, in general, divided into three parts:

i. Reflection from the surface: A part of the incident radiation is reflected. The reflective power or reflectance (r) is defined as

r = amount of reflected radiation/amount of incident radiation

ii. Transmission across the surface: A part of the incident radiation is transmitted across the surface. The transmitting power or transmittance (t) is defined as,

t = amount of transmitted radiation/amount of incident radiation

iii. Absorption by the surface: A part of the incident radiation is absorbed by the surface. They are absorbed by the surface. The absorptive power (a) of a surface is defined as,

a = amount of radiation absorbed/amount of incident radiation