Thermal Radiation
When the body is heated, it emits radiation called thermal radiation.
The nature of radiation depends upon the temperature. All the bodies not only emit radiations but also absorb radiations from their surroundings. At low temperature, a body emits radiation which are of longer wavelength in invisible infrared region. At high temperature, the proportion of shorter wavelength radiation increases. If a body is hotter than its surroundings it emits more radiation than it radiation from their surroundings.
Black Body and Black Body Radiation
Ideal Black Body
A body which absorbs all the radiations incident upon it, is called a perfect black body.
How to get a black body?
Consider a non reflecting object such as a solid having a hollow cavity in it. It has a small hole and the radiation can enter or escape only through this hole. It is blackened with lamp black inside it to make it a good absorber. The radiation that enters, is reflected from inside wall many times and partly absorbed at each reflection until none remain. Such a body is called black body. It is an ideal absorber as well as ideal radiator.
Black Body Radiation
The radiation emitted from a black body, are called cavity radiations, temperature radiations or black body radiations. The wavelength of emitted radiations decreases with the increase in temperature.
For example
When pplatinum wire is heated, it appears;
- Dull red at 500″C
- Cherry red at 900″C
- Orange red at 1100″C
- Yellow at 1300″C
- White at 1600″C
This shows that as the temperature raises, the radiation becomes richer in shorter wavelength.
Intensity Distribution Diagram
Lummer and Pringsheim measured the intensity of emitted energy with wavelength radiated from black body at different temperatures by the apparatus. The amount of radiation emitted with different wavelengths is shown by energy distribution curves for each temperature.
Results of energy distribution curves
The curve tell the following interesting facts;
1. Non uniform distribution of energy
The energy is not uniformly distributed in the radiation spectrum of the body at a given temperature.
2. Wien’s displacement law
At aa given temperature T, the emitted energy has maximum value for a certain wavelength and product remains constant. This is called Wien’s displacement law. The value of this constant is 2.9×10^-3 mK. So this equation shows that as temperature increases shift to shorter wavelength.
3. Radiation intensity varies with wavelength
For all wavelength, an increase in temperature causes an increase in energy emission. The radiation intensity increases with the increase in wavelength and at a particular wavelength, it has maximum value. With further increase in wavelength, the intensity decreases.
4. Area under energy discrimination curve
The area under each curve represents the total energy E radiated per second square meter over all wavelengths at a particular temperature.
Stephan-Boltzman Law
The area under the curve represents the intensity of the radiations. Boltzman’s can be stated as the total energy ‘E’ radiated per second per unit area is directly proportional to fourth power of absolute temperature.
