Blackbody radiations
                     Blackbody radiations
By,
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 
favourable conditions. But a black body absorbs all the electromagnetic radiation 
falling on it without limit from all directions. Consequently, it is black.
Jayam chemistry adda
 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. 
 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.
Jayam chemistry adda
 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. 
 Besides, the hotter bodies emitting electromagnetic radiation under thermal 
equilibrium conditions are also considered partial black bodies. 
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 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 state 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.
Jayam chemistry adda
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.
Examples of black bodies:
 Sun light
 The filament of light bulb.
 The heating element of a toaster.
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.
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 Blackbody radiation comes out from the cavity of the blackbody enclosure. Hence 
they are called cavity radiations.
 When heated, the charged particles of the hypothetical body receive energy. It 
enhances their kinetic energy, in other terms, their oscillations in the closed 
enclosure. 
 Then, the energetic particles radiate thermal energies out as blackbody radiations. 
By measuring the temperature of the emitted thermal radiations by a pyrometer, 
we can calculate the total amount of energy for all wavelengths radiated per unit 
time per unit area of the blackbody. 
Jayam chemistry adda
Properties of black body radiations:
 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.
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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. 
Jayam chemistry adda
 Experimental observations confirm that the 
blackbody spectrum is a continuous emission 
spectrum of emitted electromagnetic radiation 
wavelength with the temperature. 
 A graph between emissive power and radiation 
wavelength at a fixed temperature T is known as a 
blackbody curve. 
 The wavelength range lies in between (λ-½) and 
(λ+½). At different source temperatures, we 
observe different black body curves.
Jayam chemistry adda
Here are the observations for the above experiment:
(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.
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. 
(i) The emissivity of the blackbody is the highest of all other earthly objects. Hence 
only a blackbody can emit all absorbed radiations without reflection and transmission. 
Jayam chemistry adda
Applications of black body radiations:
1. Used for lighting and heating purposes.
2. Used in thermal imaging.
3. Used for calibrating and testing the radiation thermometers.
4. Used in optical sensors.
5. Used to provide security, such as burglar alarms.
Conclusion:
Apart from all these, the major drawback of the blackbody hypothesis is that it is unreal. It has no 
physical existence. Hence, no chemical structures and properties. Therefore the blackbody 
emissions tendency depends wholly on temperature.
Jayam chemistry adda
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Jayam chemistry adda