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Definition of black body radiation, Absorption of radiation by a black body, Characteristics of black body radiation, Laws of black body radiation, Stefan- Boltzman law, Stefan-Boltzmann formula

To study the qualitative ideas of black body radiation, you must know that the ideal black body notion is of chief importance in the theory of thermal radiation and electromagnetic radiation, where energy is transferred together with wavelength bands.

## Definition of a Black Body

A black body can be defined as a body that allows the incident radiation to pass through itself without any energy reflection. It steadily absorbs radiation inside itself without passing into any other. This property has validity in every type of radiation of wavelengths and angles of incidence. Thus, the black body acts as an ideal absorbent of incident radiation.

## Characteristics of black body radiation:

Black bodies are ideal radiation absorbents and also have many other properties that hold an important place in the study of physics.

Some important characteristics of these black body radiation are as follows:

1. A black body emits radiation of all wavelengths in the ultraviolet, visible and infrared radiation.
2. With the increase in temperature of the black body, the overall amount of radiation emitted also increases.
3. The energy distribution isn’t uniform. At a wavelength of λm, the energy emitted is maximum.
4. The wavelength λm decreases when the energy emitted is maximum, and the temperature is higher.

## Absorption Of Radiation by Black Body

The radiation absorbed from a heated body depends on the body itself. To visualise this, let us consider how different materials can absorb radiation. You can observe that radiation absorption doesn’t happen. This situation arises in the case of ordinary glass, as no frequencies correspond to light.

## Laws of Black Body Radiation:

Considering the general law of thermodynamics, many scientists proposed different laws on black body radiations. The laws of black body radiation are as follows-

1. The Planck law
2. The Wien displacement law
3. The Kirchhoff law
4. The Stefan Boltzmann law

## Stefan-Boltzmann Law

According to this law, the emission of radiant heat energy is directly proportional to the fourth power of its ideal temperature. It was formulated in 1879 by an Austrian physicist named Josef Stefan after his experiment. An equivalent law was also derived in 1884 by a physicist named Boltzmann from his thermodynamic considerations. According to him, if E is the radiant heat emitted from a unit area in one second and T be the temperature (in kelvins),

then, E = σT4

The Greek letter sigma (σ) stands for the proportionality constant, also called the Stefan-Boltzmann constant. The value of Stefan Boltzmann constant is 5.670374419 × 10-8 watt per metre2 per K4. This law applies only to black bodies that absorb all incident heat radiation.

## Stefan- Boltzmann law formula:

Stefan Boltzmann law deals with the temperature of the black body to the quantity of power it emits in per unit area. One can obtain the overall power that gets radiated on the wavelength of a black body by integrating Plank’s radiation formula.

The light beams which go inside the insulated cavity lose their intensity after corresponding reflections get captivated there. It happens as the heated walls of the cavity are sources of thermal radiation, and only an insignificant part of it leaves it. There exists an ideal equilibrium density of radiation within the cavity. Standing waves are produced within the cavity. As for standing waves during a string, the utmost wavelength of undulation is decided by the dimensions of the cavity. Also, the minimum wavelength is set by the discrete character of the substance from which the walls are made. Thus, it can be said that the density of standing waves is finite in the insulated cavity. After detailed consideration, one can obtain ω2/4π2c2.