Blackbody radiation

A blackbody is a rigid, closed unreal body that can absorb all electromagnetic radiation that falls on it regardless of its frequencies, but it reflects none. Hence, it is a perfect emitter and absorber of thermal radiations following Kirchhoff's law.
An ideal black body is an imaginary perception of Gustav Kirchhoff that played a significant role in the development of quantum mechanics. Every object in the universe absorbs and emits electromagnetic energy to a certain extent under favorable conditions. But a black body absorbs all the electromagnetic radiation falling on it without limit from all directions. Consequently, it is black.

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1. Blackbody radiations JAYAM CHEMISTRY ADDA
2. 01-JAYAM CHEMISTRY ADDA What is a black body? A blackbody is a rigid, closed unreal body that can absorb all electromagnetic radiation that falls on it regardless of its frequencies, but it reflects none. Hence, it is a perfect emitter and absorber of thermal radiations following Kirchhoff's law. An ideal black body is an imaginary perception of Gustav Kirchhoff that played a significant role in the development of quantum mechanics. Every object in the universe absorbs and emits electromagnetic energy to a certain extent under favorable conditions. But a black body absorbs all the electromagnetic radiation falling on it without limit from all directions. Consequently, it is black. The surface of a blackbody is opaque. It neither transmits light nor reflects it. So the reflective and transmitting powers of an ideal blackbody are zero. The following relationship helps calculate the absorbing power of a perfect blackbody. 1. 2. 3.
3. Formula to measure blackbody absorbing power: 02-JAYAM CHEMISTRY ADDA (Absorbing power + transmitting power + reflecting power) =1 (Absorbing power+0+0)=1 Absorbing power=1 So, the absorbing power of a blackbody is one. It implies that the amount of light absorbed is the same as the total amount of light that falls on it. Hence, it became an absolute absorber of heat and light. Ordinary objects lose a portion of absorbed light in reflection, transmission, or other means. It shows a difference between the absorbed light with the incident light amount. Hence absorptive power of ordinary objects is always less than the ideal blackbody.
4. 03-JAYAM CHEMISTRY ADDA On heating, the blackbody releases all thermal electromagnetic radiation that it absorbed previously. These are blackbody radiations. A blackbody radiates heat energies without affecting the intervening medium. It has the highest emissive power of all other bodies at that particular temperature and wavelength conditions. Consequently, the emissivity of a perfect blackbody becomes one. The blackbody is a hollow enclosure with a pinhole to emit its radiations. The secure covering of the blackbody prevents the absorbed light from escaping. A blackbody is a solid closed unreal body that is inexistent. But lamp black, platinum black, and graphite-coated surfaces are non-ideal black bodies for laboratory purposes. An object with above 0.95 emissivities is an approximate blackbody.
5. Why is the blackbody emit radiant energies only on heating? Blackbody withheld a portion of incident light while absorbing and radiating energies in thermal equilibrium conditions. It cannot emit detained radiant energies by disturbing its thermal equilibrium conditions under ordinary conditions. Heating disturbs its thermal equilibrium status, and the blackbody is at a higher energy state than the surrounding. Consequently, it releases heat assimilated light energies. The thermal electromagnetic energies transfer to the cold bodies in this situation. In this way, it attains its thermal equilibrium state again. For example- When a polished shiny metal surface with black spots on its surface is heated to a high temperature and cooled immediately emits all absorbed thermal radiations. Under those circumstances, only the black specks act as blackbodies. 04-JAYAM CHEMISTRY ADDA
6. Example of blackbodies Characteristics of black bodies It is an ideal emitter. It emits more radiant energy than any other object at the same temperature. It is a diffuse emitter. It radiates energy isotropically independent of direction. Sun light The filament of light bulb. The heating element of a toaster. Warm blooded animals Electric heaters 1. 2. 3. 4. 5. 05-JAYAM CHEMISTRY ADDA
7. 06-JAYAM CHEMISTRY ADDA What is black body radiation? These are heat-combined electromagnetic radiations that emerge at constant temperature conditions. They comprise discrete photons whose energy depends on the frequency of emitted heat radiation following Planck law. Blackbody releases all absorbed light intermittently on heating, including all wavelengths of the entire electromagnetic spectrum. Additionally, blackbody ejection is a spontaneous process that varies with the temperature only. These are known as temperature radiations since they are the strongest at a particular temperature. Blackbody radiation comes out from the cavity of the blackbody enclosure. Hence they are called cavity radiations. 1. 2. 3. 4. 5.
8. Color of blackbody radiations: Blackbody emits most portion radiations in the infrared region at room temperature. Therefore, the human eye is not able to see them. But, we can feel the sensation of heat. Hence, it is also known as heat radiation. Blackbody radiations are colored only when their wavelengths lie in the visible region when ranging from 400 to 750 nm approximately. But we cannot detect color due to color mixture affecting eyes differently. But, the cooled object emits red color predominantly, rather than the hotter ones are bluish. While heating an object to a temperature of 1500 K gives pale red radiation in the visible region. Sun surface has above 5000 K temperature that emits a good proportion of visible light and appears white. 07-JAYAM CHEMISTRY ADDA
9. 08-JAYAM CHEMISTRY ADDA Most sunlight reaching the earth's surface is in the infrared region. And sunlight seems yellow-green as the sun's emission spectrum has intensified spectral lines at that wavelengths. Finally, blackbody radiation is temperature dependent. With temperature, the radiation frequency increases, and the radiation color changes as per the visible spectrum. Sun's outer atmosphere
10. Blackbody radiations and temperature influence The blackbody, under normal conditions, absorbs and releases radiations to maintain thermodynamic equilibrium conditions in the enclosure. At constant temperature, the magnitude of heat energy is equal in both the absorption and emission processes of the blackbody. While heating, the blackbody is out of thermal equilibrium state. Hence, it releases thermal electromagnetic radiation of all wavelengths at a particular temperature. All these suggest blackbody radiation outflow is wholly dependent on the temperature conditions of the enclosure. But remaining factors such nature of the material, its size and shape, or other physical dimensions cannot influence blackbody emissions as it is unreal and inexistent. 09-JAYAM CHEMISTRY ADDA
11. The black body radiations are homogeneous. It is a steady state equilibrium radiation in the cavity of a rigid body at a uniform temperature. These are isotropic in nature. The black body spectrum depends only on the temperature. And it is independent of the shape, structure, and composition of blackbody. 1. 2. 3. 4. Properties of black body radiations: 10-JAYAM CHEMISTRY ADDA
12. Experimental study of blackbody radiations: B is a heated blackbody at constant temperature T in thermodynamic equilibrium conditions. The hot electromagnetic emissions of body B pass through the slit and then fall on the prism. Thermopile placed there converts the thermal energy of hot radiations into voltage. A galvanometer connected with it measures the electric current. 1. 2. 3. 4. 11-JAYAM CHEMISTRY ADDA Hot blackbody Slit Prism Thermopile Galvanometer Blackbody radiation
13. 12-JAYAM CHEMISTRY ADDA Observations for the above experiment A graph between emissive power and radiation wavelength at a fixed temperature T is known as a blackbody curve. At different source temperatures, we observe different black body curves. (a) The energy distribution in the black body spectrum is non-uniform over a wide range of wavelengths. (b) At constant temperature, the emissive power of the black body initially increases with an increase in wavelength. (c) At a particular wavelength, the emissive power shows the peak value. And the intensity of black body radiation is maximum at this wavelength. (d) Beyond the peak wavelength, the emissive power of the body drops slowly with a further rise in wavelength. It gives a hill-shaped curve for the black bodies.
14. 13-JAYAM CHEMISTRY ADDA (e) At higher wavelengths, the intensity of blackbody radiation is high due to heavy photon releases. (f) Light intensity approaches zero at shorter wavelengths, suggesting only a few photons can possess infinite energies. It contradicts the Rayleigh- Jeans law. (g) The peak wavelength of the black body curve becomes shorter (moves toward the blue end of the visible light) at higher temperatures. (h) Area under each black body curve gives the radiance emittance of the body at that temperature. And they vary directly.
15. Applications of black body radiations: Used for lighting and heating purposes. Used in thermal imaging. Used for calibrating and testing the radiation thermometers. Used in optical sensors. Used to provide security, such as burglar alarms. 1. 2. 3. 4. 5. 14-JAYAM CHEMISTRY ADDA
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