BLACK BODY AND BLACK BODY RADIATION
Hello Everyone, Am Manjeet Singh and Today we will Discuss about Black Body And Black Body Radiation... So Let's Start...
To understand the concept of black body we consider
following two situation.
(1) Absorption of radiation by a body.
(2) Emission of radiation by a body.
When a radiation falls on the surface of a body, a part of it is reflected, some port of it is absorbed and the rest is transmitted. Whole of radiant energy is not absorbed because generally the surface of the objects are not perfect Absorbers of radiation. However if the radiation falls on a blackened metallic surface, it is almost completely absorbed. So, a body which completely absorbs all the radiations falling on it is called a black body.
Now we take the case of emission of radiation by the object. Radiation is emitted by any solid body at any temperature as a result of vibrations of its particles. At low temperature the emitted radiations are of low frequency. As the temperature is raised the emitted radiation goes from a lower frequency to the higher frequency. Different solids emit radiation at different rates at same temperature, the emission rate is maximum when the solid is perfect black. In other words, when a body is heated, it radiates maximum amount of radiant energy. So a black body is a perfect emitter of radiant energy. At a given temperature, a black body radiates maximum amount of radiant energy. Thus a black body is defined as an object that completely absorbs all the radiations falling on it, it is also a perfect emitter of radiation.
The radiation emerging from a black body is called " black body radiation".
Experimentally, a nearest approach to black body is a hollow sphere blackened from inside and having a small hole for the entry of radiations. The inner surface of the sphere is blackened with lamp black. This blackened surface helps in complete absorption of radiations falling on it. The black body designed by Fery is shown in figure. It consists of a hollow double walled metallic sphere having a hole. The inner surface has a projection to prevent the direct reflection of the radiations entering from the hole. The inside surface is blackened with lamp black. Its exterior is completely insulated from surroundings so as to avoid the loss of radiation in the form of heat. Any radiation that enters black body through a small hole is reflected repeatedly from the walls and reabsorbed inside the sphere untill all of the energy is absorbed.
When a black body is heated, it glows from inside and cannot lose any energy to the outside world. It soon attains a state of thermal equilibrium and no external heat energy is required to maintain a constant temperature. But the inner walls are radiating all types of wavelength. The radiation emitted by a black body at a particular temperature are called Black Body Radiation. The origin of these black body radiations lies in the vibrations of particles of the body ( the oscillators) that emit the radiation energy in the form of electromagnetic waves.
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Created By Manjeet Singh...
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