The blackbody radiation experiment led to the discovery of a new field of science. It is a theoretical concept in quantum mechanics. Ideally, no material can be a black body completely, but some have come close, such as carbon. But what is blackbody radiation? Learn about the concept in detail, its examples, curves, and more.

A theoretical concept refers to the behaviour of a system or substance that completely absorbs all radiations incident upon it. As per the laws of thermodynamics, it must re-radiate the same amount of light as it had absorbed. However, re-radiated energy is a system’s characteristic and doesn’t depend on the energy that strikes the system. Rather, the emitted energy depends on the system’s temperature.

A perfect black body absorbs all the heat radiation incident on it irrespective of its wavelength without transmitting any portion of the incident radiation. Therefore, it appears black.

The substance/ system/ object on which the radiation strikes is called a black body and the radiation that it emits is called blackbody radiation. It is an ideal radiator and absorber that absorbs all radiation incident upon it is also called cavity radiation. Although there isn’t a material that can truly satisfy the characteristics of a black body, materials like graphite are up to 96% efficient in light absorption.

Other examples of a blackbody include the sun, which gives off enormous amounts of sunlight, but it is only 70% of a perfect black body.

Understanding a BlackBody

It’s hard to imagine a body that absorbs and emits all radiation, ensuring equal probability but not magnitude. This condition implies that the object is capable of giving off any light wavelength but would not because different wavelengths possess different energies.

So, physicists consider a black body to be a hollow metal box with a small hole. The box is assumed to be in thermal equilibrium. Once it absorbs rays, they move around in the box and get emitted by the metallic walls. The small hole in the box wall is an escape point for some of the radiation.

Blackbody radiation spectrum refers to light spectra emitted by any heated object. Common examples you can study are a toaster’s heating element and the light bulb’s filament.

A common question is whether blackbody radiation is a continuous spectrum.

Yes. According to the theoretical concept, a black body is an object absorbing all electromagnetic radiation that it encounters. It then gives off thermal radiation in a continuous spectrum depending on its temperature. Blackbody emission corresponds to an electron jumping from a higher to a lower energy state. Since the electron in an excited state has several energy levels below, it can jump into any and give a continuous spectrum.

Studying the blackbody radiation spectrum gives insights into the emissions and helps decide the use of blackbodies such as for heating, lighting, thermal imaging, and so on.

The characteristics can be explained using the following laws:

Wien’s displacement law

According to the law, the radiation curve of blackbody radiation for different temperatures rises to a maximum at a wavelength inversely proportional to the temperature. The Wien’s law formula is as follows:

λmax = b/T

Where λmax is the maximum wavelength, b is Wein’s displacement constant (Wien’s displacement constant = 2.8977*103 m.K), and T is the temperature in Kelvin.

Planck’s Law

Planck’s law is used to determine the spectral energy density of the black body emission for each wavelength at a particular temperature. The Planck’s law formula is as follows:

Where Eλ is the wavelength, h is Planck’s constant, c is the speed of light, and T is the absolute temperature.

The Stefan-Boltzmann Law

It provides a relationship between total emitted energy and the absolute temperature with the following formula:

E ∝ T4

Where E is the total emitted energy and T is the absolute temperature.

A black body is like a real-world object in thermodynamic equilibrium. It has no net energy flow as the inflow and outflow rate is the same. So, when the temperature of the black body changes, the blackbody radiation curve shifts to create a balance between the emitted curve and the absorbed energy.

These curves allow you to explain what light wavelength is being radiated by the black body by checking out the peak wavelength via Wien’s displacement law.

The black body curves are shown in the figure convey that hotter blackbodies emit their peak energies at shorter wavelengths. So, the wavelength at which the body strongly radiates and the total radiation amount of a black body depends on its temperature.

While the total energy emitted by a black body can be determined from the area of the curve or using the Stefan-Boltzmann law. With a rise in temperature, the total radiation by the black body increases, but the wavelength at which the radiation peaks gets smaller.

Blackbody radiation is a commonly occurring phenomenon that you can observe when the temperature of a body rises. Electromagnetic radiation covers a wide spectrum, including the visible and invisible regions as per the temperature of the object and radiation amount.

For instance, a filament bulb converts electrical energy into light energy when switched on. It goes from red to yellow to almost white light when completely energised. However, when it is not completely heated, or it is cooling down, it gives off energy in the infrared regions.

Similarly, suppose you observe a metal piece while welding. You will note that as the welding temperature of the metal piece increases, it glows red. In that case, orange and the colour continuously change until it becomes bluish. The glow at very high temperatures gets intense, so welders put on dark goggles to prevent eye damage.

Since a very hot object emits radiation in the infrared region, the night vision equipment uses this concept to detect infrared radiation while converting it into a visible image. It allows them to detect warm-blooded animals and people in the dark. Blackbody radiation from animals falls mostly in the infrared radiation category that cannot be seen with the naked eye.

The blackbodies suit security, testing, lighting, heating, thermal imaging, and measurement applications. Blackbody radiation examples that emit visible light or their radiation are used for various processes as follows:

• Electric heaters
• Stoves
• Incandescent light bulbs
• Sun
• Stars
• Burglar alarms
• Warm-blooded animals
• Night vision equipment

Using Planck’s law of radiation, one can quickly determine the intensity of the energy at any wavelength and temperature. So, a blackbody radiation source with a known temperature can be used for various purposes, such as testing and calibrating the radiation thermometers.

Selecting a blackbody radiation source depends on major factors, including

• Emissivity
• Temperature
• Size of Emissive area
• Cooling time
• Warm-up time
• Regulation stability

The Ultraviolet Catastrophe

The predictions in Physics suggested that an ideal black body would emit radiation with infinite power at thermal equilibrium. This theory came forward in the light of Rayleigh-Jeans Law, which states energy is proportional to λ−4. The law predicted that the sum of all the energies at all wavelengths gives the system an infinite amount of energy. This paradox is referred to as the ultraviolet catastrophe.

Max Planck addressed this paradox and predicted that energy comes in discrete amounts. He called these discrete amounts quanta. His prediction forms the basis of quantum mechanics. His equation was not proportional to λ−4 but took the form given below:

In the above equation, ν is the energy frequency, h is Planck’s constant, kB is Boltzmann’s constant, and T is the temperature in Kelvin.

1. Which objects resemble a blackbody closely?

The lamp-black and platinum black closely resemble the characteristics of a black body as they absorb about 99% of the radiation incident on them. Fery designed the most simple and commonly used black body that consisted of an enclosure with a small opening. He painted it black from the inside and acted like a black body.

2. What is gray body radiation?

A gray body is an imperfect black body that partly absorbs the incident radiation. It emits radiant energy. A gray body has the same relative spectral energy distribution at a given temperature as a black body but in a smaller amount.

3. What is white body radiation?

In Physics, a white body is a hypothetical system/substance with a surface that absorbs no electromagnetic radiation of any wavelength. It exhibits zero absorptivity irrespective of the wavelengths of rays incident on it.

Conclusion

Blackbody radiation is an important theoretical concept that can be applied to various systems to study their emission and absorption rates. A perfect blackbody absorbs all the incident radiation irrespective of its wavelength. It neither transmits nor reflects any portion of the incident radiation and thus appears black. However, in real life, no object is strictly a perfect black body.