Kirchhoff's Identity Explained
Kirchhoff's Law of Thermal Radiation establishes a fundamental connection between how a body emits and absorbs thermal radiation.
The law specifically applies when a body is in thermal equilibrium with its surroundings. In this state, the condition of the body is stable, and there is no net flow of heat.
Applying Kirchhoff's Law
When a body is placed inside a perfectly black enclosure and reaches thermal equilibrium, its properties simplify:
- Emissivity ($\epsilon$): This measures how effectively a surface emits thermal radiation compared to a perfect blackbody at the same temperature and wavelength.
- Absorptivity ($\alpha$): This represents the fraction of incoming radiation at a specific wavelength that the body absorbs.
Under the conditions described (thermal equilibrium within a black enclosure), Kirchhoff's Law states that the emissivity of the body at any given wavelength is exactly equal to its absorptivity at that same wavelength.
Mathematically, this is expressed as:
$ \epsilon(\lambda, T) = \alpha(\lambda, T) $
Therefore, according to Kirchhoff's identity, the absorptivity of the body at a given wavelength equals its emissivity.
Important Questions from Radiation
Stefan Boltzmann's constant is expressed in the unit-
A body whose absorptivity does not vary with temperature and wavelength of the incident ray is known as
The process of heat transfer from a hot body to a cold body in a straight line, without affecting the intervening medium, is known as ______.
Heat is transferred from an electric bulb by ______.
- Radiosity is defined as _______.
