BlogNCERTBlack Body Radiation

Black Body Radiation

The black body is an idealized depiction of a body that absorbs all electromagnetic radiation that strikes it regardless of frequency or angle.

How does Black Body Radiation work?

Ideally, a black body must emit as much radiation as it absorbs to remain thermally balanced. Therefore, it should also be a sound emitter of radiation, emitting electromagnetic waves at as many frequencies as it can absorb.

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    Radiation characteristics of blackbody radiation:

    Using the following laws, we can explain the characteristics of blackbody radiation:

    1. Wien’s Law on Displacement
    2. Planck’s law
    3. Stephan-Boltzmann law

    Wien’s Displacement Law:

    Vienna’s displacement law states that “Temperature-dependent blackbody radiation curves have peaks at wavelengths that are inversely proportional to the temperature”.

    Stefan-Boltzmann Law Formula:

    E ∝ T 4

    1. The energy emitted is referred to as E
    2. The absolute temperature is T

    Wien’s Displacement Law Example:

    1. Easily, we can calculate that a wood fire, which is approximately 1500K hot, emits a peak emission at 2000 nm. Therefore, the wood fire radiates most of its radiation out of sight of the human eye. A campfire is excellent for keeping warm, but it is not suitable for providing light.
    2. The surface of the sun has a temperature of 5700 °C. The peak radiation output at a wavelength of 500 nm can be calculated using the Wien displacement law. The wavelength falls within the green region of the visible light spectrum. This particular wavelength of visible light is highly sensitive to the eyes. The sun’s radiation falls in a relatively small portion of the visible spectrum, which is something we should be grateful for.
    3. When a piece of metal is heated, it first becomes ‘red hot.’ It has the longest wavelength visible to the human eye. It changes colour from red to orange to yellow upon further heating.
    4. A white glow is visible when the metal reaches its hottest point. Shorter wavelengths dominate the radiation.

    Radiation Definition:

    Radiating heat without heating the intervening medium is defined as the transfer of heat from one place to another. Releasing heat without heating the intervening medium is known as radiation. A wave is caused by the superposition of electric and magnetic fields perpendicular to each other and carries energy.

    Radiation has, on the whole, been adopted after the phenomenon of waves radiating from a source.

    Important Types Of Radiation:

    Radiation is the study of different types of energy. Some of the common ones are;

    1. The electromagnetic spectrum includes various types of radiation, including microwaves, infrared, ultraviolet, radio waves, x-rays, gamma rays (?), and visible light.
    2. The particle radiation category comprises alpha radiation (α), beta radiation (β), and neutron radiation.
    3. The most common types of acoustic radiation are sound, ultrasound, and seismic waves.
    4. There is a type of radiation called gravitational radiation, which often manifests in gravitational waves, ripples in space-time, etc.

    Radiation is described chiefly or classified as either ionizing radiation or non-ionizing radiation. Energy levels of radiated particles determine whether or not the radiation is ionizing.

    Properties of Radiation:

    (a) All objects emit radiation simply because their temperature is above absolute zero, and all things absorb some of the radiation that falls on them from other objects.

    (b) Maxwell, based on his electromagnetic theory, proved that all radiations are electromagnetic waves and their sources are vibrations of charged particles in atoms and molecules.

    (c) More radiation is emitted at a higher temperature of a body and less at a lower temperature.

    (d) The wavelength corresponding to the maximum emission of radiation shifts from a longer wavelength to a shorter wavelength as the temperature increases. Due to this, the colour of a body appears to be changing. Radiations from a body at NTP have predominantly infrared waves.

    (e) Thermal radiations travel with the speed of light and in a straight line.

    (f) Radiations are electromagnetic waves and can also travel through a vacuum.

    (g) Like light, thermal radiation can be reflected, refracted, diffracted, and polarized.

    (h) Radiation from a point source obeys an inverse square law (intensity a ).

    Perfectly black body:

    Almost all wavelengths of heat radiation on a perfect black body are absorbed. It does not reflect or transmit any of the incident radiation, so it appears black irrespective of the colour of the incident radiation.

    Any natural object does not strictly possess the properties of a pure black body. A good approximation of the black body is the lamp-black and platinum black. These materials absorb about 99 percent of the incident radiation. Ferry’s black body is one of the simplest and most commonly used. Black paint is used to paint the inside of the enclosure, which has a small opening. Painting the opening black creates an illusion of an enclosed body.

    Radiation that falls on the opening inside the enclosure will get absorbed by multiple reflections before escaping. Radiation cannot be reflected directly by the cone opposite the entrance.

    Applications of Radiation:

    1. Diagnostic, therapeutic, and research purposes often require radiation and radioactive substances. Examples include X-rays. Cancer treatments also often involve radiation.
    2. In almost all modern communication systems, electromagnetic radiation is used.
    3. Carbon dating, for example, involves the use of radioactive atoms to determine the age of materials.
    4. Radiation is also used to determine the composition of materials. Such an analysis is called neutron activation analysis.

    Also read: Thermal Expansion

    Frequently Asked Questions (FAQs):

    Q. Wien’s Law tells us about a Blackbody; what does it mean?

    Ans: In principle, the wavelength of the black body will peak inversely proportional to the temperature since the law of Wien’s displacement states that for temperatures that differ, the radiation curve will peak at a different wavelength.

    Q. Are black bodies capable of emitting radiation?

    Ans: Any object that emits electromagnetic radiation based on its temperature. Black bodies are idealized objects that absorb all electromagnetic radiation in their vicinity.

    Q.Who came up with the expression radiation from the Blackbody?

    Ans: The radiation of Planck’s law represents a mathematical relationship formulated in 1900 by a German physicist named Max Planck to explain the energy of spectral distribution of radiation emitted by a blackbody. This hypothetical body absorbs all radiant energy falling upon it and thus reaches the same equilibrium temperature.

    Q: What is the Radiation Formula?

    Ans: Radiation is the electromagnetic waves transfer energy directly due to a difference in temperatures. Sir Stefan-Boltzmann explained the law of radiation: This was expressed as follows:

    Qt = σe A T 4,

    where σ symbolizes Stefan-Boltzmann constant,

    capital A represents object surface area,

    capital T represents absolute temperature, and

    capital e represents emissivity.


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