Blackbody-a brief synopsis-Jayam chemistry learners

By V. Sai Kalyani

 Blackbody-Blackbody radiation, definition & overview

A blackbody is a solid closed unreal body capable of reciprocating the whole electromagnetic spectrum at a uniform temperature. It is an impracticable assumption of Gustav Kirchhoff, who studied the electromagnetic emissions of physical objects at a particular temperature.

It depicts the blackbody and its emissions.
Blackbody explanation

It radiates thermal electromagnetic radiations called blackbody radiations. How does the blackbody release thermal electromagnetic radiation? You will get the answer in a few moments. This blog article discusses the blackbody and its emissions at different temperatures.

Table of contents:

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.

(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.

The image shows the difference between the black body and the gray body.

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. 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.

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.

Doubt-1: 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.

Doubt-2: Who introduced the term blackbody?

Gustav Kirchhoff introduced the term black body in 1860. And he studied about thermal energies of substances and envisioned an ideal physical body as a perfect emitter and absorber of light.

What is a 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.

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. 

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.

It shows the color variation of blackbody emissions.

(For a high resolution image of color of blackbody radiation, click on the highlighted link.)

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. 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.

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. Moreover, the blackbody emissions have peak intensity at a definite wavelength for every temperature, after which temperature rise decreases thermal emissions.

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.

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.

It shows experimental setup to measure the temperature of blackbody radiations

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.

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.

The image shows a pictorial representation of a blackbody curve.

(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.

(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.

(f) The peak wavelength of the black body curve becomes shorter (moves toward the blue end of the visible light) at higher temperatures.

 (g) Area under each black body curve gives the radiance emittance of the body at that temperature. And they vary directly.

Fatafat check list of blackbodies:

A blackbody is an imaginary perception of the German physicist Gustav Kirchhoff. An ideal blackbody absorbs all light radiations that fall on its surface without any reflection. Hence, the absorbing power of a perfect blackbody is always equal to one. And all other objects possess less absorbing power than one due to the loss of radiant energy by transmission and reflection. Heating causes the blackbody to emit all absorbed radiation without holding anything. Radiant energy intake and emission at regular periodic intervals maintain thermal equilibrium in the rigid enclosure. 

Besides, the emitted thermal electromagnetic radiation is called blackbody radiation. Blackbody radiation is also named temperature radiation due to the thermal radiant energy of blackbody emissions peak at particular temperature conditions. Wien displacement law discusses in detail the temperature influence on wavelengths of emitted blackbody radiations.

We designed a fatafat list of all simple questions and answers of blackbody and its radiations for better understanding.

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Factors affecting blackbody radiation emissions:

The factors affecting blackbody emissions are temperature, emissivity, and surface area.

Temperature:

An object emits blackbody radiation only on heating. At lower temperatures, the blackbody emissions are in the infrared region of the electromagnetic spectrum. Further rise of temperature moves the radiation frequency to visible and then to the ultraviolet zones of the electromagnetic spectrum. It shows that the radiation frequency is directly proportional to the temperature. Consequently, brighter objects possess high energies than dull ones. As a result, white stars have the highest temperature than the blue stars in the universe.

The inherent cause is temperature enhances the kinetic energies of the charged particles of a blackbody. It escalates collisions between the oscillatory particulate matters of blackbodies that lead to blackbody emissions.

It shows the factors affecting blackbody emissions.

Emissivity:

It means the amount of energy radiated per unit time per unit area of the blackbody. Blackbody emissions escalate with emissivity till λmax, a wavelength position for peak emissivity. Next, emissivity changes confirm only a limited number of photons possess shorter wavelengths. Yes, only a few light chunks carry infinite radiant energies that pass towards zero emissivity.

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.

Surface area:

The surface area of an object shows a directly proportional relationship with its radiation emissions. The larger the surface, the greater the rate of blackbody emissions. Smaller bodies have little space for radiation loss, which impacts their emission rate. It even applies to the absorbing powers of bodies.

For example- Polished metal surfaces are poor absorbers and emitters of thermal energies than coarse ones that are identical in every physical aspect under the same temperature and wavelength conditions.

Straight to the point, polished metal surfaces have less surface area than rough surfaces. As a result, a smoothly bright metal surface cannot be a superior radiator for thermal light to the irregular one in thermal equilibrium conditions.

Difference between the blackbody and gray body:

Generally, all earthly objects are grey bodies with an emissivity of less than one. A blackbody is an ideal imagination of physicist Gustav Kirchhoff that has no physical existence. 

Kirchhoff's imaginary body absorbs all wavelength incident radiation without reflection and transmission. But for existent objects surface absorbs a part of incident radiation with reflection and transmission abilities. Hence, the absorbing power of a perfect blackbody is always one. On the other side, the earthly bodies absorbing capacity is less than one.

Further, the blackbody emits all absorbed light without any loss on heating. However, the actual substances emit a portion of absorbed radiation depending upon several factors, such as the nature of the material, its size, and composition. Not necessary to say the emissivity of the perfect imaginary object is one. But the terrestrial bodies lag in their emissivity when compared to it.

The energy density of blackbody emission depends only on enclosure temperature. Yet the earthly object's energy density depends on the material physical aspects, such as size, shape, density, and composition.

Here is an infographic discussing the difference between a hypothetical blackbody and a real earthly object.

Laws governing blackbody radiations:

Planck quantum law:

Planck law calculates the spectral energy density of blackbody radiation at a particular frequency and temperature conditions. Besides, it considered that each oscillation mode of charged bodies has a specific number of energy particles. Quantum energy repeats after regular intervals, and it is an integral multiple of its frequency. In this way, it quantized the energies of blackbody radiations Hence, this theory was widely known as Planck's quantum theory.

E=nhν

The average energy of each mode by Planck distribution law is

It shows a formula for the mode's average energy in a blackbody.
Average energy of each oscillation mode of blackbody

Where,

h is Planck’s constant. And it has a value of 6.626 X 10-34 J s.

Wien displacement law:

Wien displacement law explained the correlation between the maximum intensity blackbody radiation wavelength and the absolute temperature. It gave accurate results for shorter wavelength blackbody emissions.

It states that the wavelength of emitted black body radiation with maximum intensity varies inversely to the body's absolute temperature.

λm x T = b

Where,

λm = wavelength of radiation emitted with maximum intensity

T= absolute temperature of the body on the Kelvin scale

b= Wien constant. And it has a value of 2.89 X 10-3 mK.

Stefan-Boltzmann law:

Stefan-Boltzmann law shows the relationship between the radiant emittance of a blackbody with its temperature. Moreover, it elucidated that the energy radiated by the blackbody per unit time per unit area varies directly as the fourth power of the absolute temperature of the blackbody.

E = σT4

Where,

E= radiant emittance of a black body

σ= Stefan’s constant. It has a value 5.7 X 10-8 Wm-2K-4

T= absolute temperature on Kelvin scale

Mind map of blackbody radiation:

The mind map of blackbody radiation discusses its definition, applications, and laws governing blackbody emissions. Even though a blackbody is an ideal object, it is helpful in thermal imaging and optical sensors. 

It is mind map of blackbody and its radiation

(For a high-resolution picture, click the link to the mind map here.)

Besides, crucial quantum mechanics laws such as Planck's quantum theory were invented by studying energy curves of blackbody emissions. And they laid the foundation to relate the interaction of matter with energy. 

Frequently asked questions and answers on blackbody topic:

1.Why is it called blackbody radiation?

A blackbody is an idealized physical body that absorbs all colored radiations incident on it regardless of their frequencies and angle of incidence. And on heating, it emits all absorbed radiations without any reflection. 

Hence, the radiations emitted by a blackbody are called blackbody radiations. And it includes radiations of all wavelengths ranging from zero to infinity.

At thermal equilibrium conditions, an ideal blackbody is a good absorber and emitter of electromagnetic radiations that follows Planck’s law.

2.Why are blackbodies hotter?

Blackbody, an imaginary perception of Gustav Kirchhoff, is a perfect absorber and emitter of thermal electromagnetic radiations in thermal equilibrium conditions.

The blackbody constantly intakes and releases radiant energies even at constant temperature conditions. And the rate of blackbody emissions varies directly with its temperature only. And it is independent of the nature of the material, size and shape of the enclosure, and other physical aspects around there.

On heating, the blackbodies emit hot light radiations to attain their equilibrium state again. All these facts hint at the correlation between the blackbody and its temperature conditions. Moreover, the blackbody cannot stick to room temperature conditions due to the dynamic nature of thermal equilibrium. Indeed, the black bodies are hotter when emitting radiation.

3.Why does the blackbody radiate even at thermal equilibrium?

Two bodies are said to be in thermal equilibrium when there is no net flow of thermal energy between them. 

By Prevost’s theory, thermal equilibrium is a dynamic equilibrium state but not static equilibrium. So, every object in thermal equilibrium consistently interchanges heat energies.

The same thing applies to blackbody even. So, the blackbody at constant temperature exchanges radiant energies without disturbing its thermal equilibrium. It is possible only when the amount of energy absorbed and released is equal in magnitude.

4.What are the characteristics of blackbody radiations?

A blackbody is an ideal emitter that emits more thermal energy than any other body at the same temperature.

The distribution of blackbody radiation is uniform in all directions obeying Lambert’s law.

The blackbody radiation spectrum depends only on the temperature of the body. 

5. Will an object absorb all wavelength light radiations if it can emit blackbody radiations?

A blackbody is an unreal object that can absorb radiations of all wavelengths that fall on its surface. Its opaque surface did not allow any of the incident radiation to reflect or transmit from it. Hence it is a perfect absorber and emitter of radiations so far.

But blackbody radiation is nothing but simple heat-assimilated electromagnetic radiation of definite wavelength. Not necessary to mention a blackbody can emit radiant energy only on heating. So, the emitted light radiations pair up with the heat. Moreover, the hotter bodies emitting blackbody radiations under thermal equilibrium conditions are called partial blackbodies. Their absorbing capacities lie below the absorbing power of a blackbody.

Hence, an object that emits thermal electromagnetic radiation of a particular wavelength cannot accept light radiations of all wavelengths since being not an ideal absorber like the blackbody.

Conclusion:

Provided that Kirchhoff's blackbody concept served as a reference to measure the absorbing and emissive powers of ordinary objects. Scientists like Wein and Stefan explained the effect of temperature on blackbody radiation's intensity and radiant emittance.

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.