When an object is heated up, it transfers heat energy to the surroundings of that object. There are three ways to transfer heat from one place to another conduction, convection, and thermal radiation. Conduction occurs by physical contact while convection occurs due to differences in the density of molecules.
Thermal radiation is electromagnetic radiation that travels through electromagnetic waves. These radiations are generated by the motion of electrons and particles in a heated material. This results in charge acceleration and dipole oscillation which generates electromagnetic radiation.
Materials can emit thermal radiation that has a greater temperature than absolute zero. Emitted thermal radiations do not require any medium to transfer from the source to the receiver. Thermal radiation is a volumetric phenomenon for gases and semi-transparent materials. For example, this phenomenon can be used to study the behavior of lenses in optical devices.
In the electromagnetic spectrum, thermal radiations have a wavelength range from 0.1-100 μm. This includes a small region of ultraviolet rays (UV) and a complete region of visible (VI) and infrared rays (IR).
characteristics of thermal radiation
- Thermal radiation is the transmission of thermal energy without any medium.
- A body that emits thermal radiation has a wide range of frequencies.
- Materials that have nonzero temperatures can emit thermal radiation.
- Thermal radiation energy shifted to a higher level at a higher temperature.
- The color range of radiation varies with temperature. Higher temperature results in red, green, and blue color flame.
- The wavelength of thermal radiation ranges from 0.1 micrometers to 100 micrometers.
- Unlike conduction and convection, thermal radiation does not transfer by contact.
Black body radiation
A black body is an ideal body that allows all the incident radiation to pass through it. There is no reflected energy and all the energy absorbs internally. That’s why a black body is a perfect absorber for radiation.
When thermal radiation has the same characteristics as a black body, they are called black body radiation. These are three laws that explain black body radiation.
- Planck’s law
This law explains the spectrum of black body radiation which depends on the temperature of the object.
- Wien’s displacement law
Wien’s displacement law tells us about the frequencies of the emitted radiation from any object.
- Stefan Boltzmann law
It explains how much power is radiated from a black body in terms of temperature.
The concept of black body radiation is used to study radiative energy transfer. It can serve as a standard absorber with which real absorbers can be compared. It also emits the maximum radiant energy so, this can be compared with emissions from the real bodies.
These are some examples of materials that act as a black body:
- Black carbon
- Black platinum
- Black gold
- Black paints
Interaction of thermal radiation with materials
The interaction of radiation at the surface of the material does not depend on the characteristic of the surface but also depends on the properties of the material beneath the surface. For example, when radiation stikes on the surface of a homologous object, some of them are reflected back and some are penetrates through the surface and enter into the body of the object.
When the thickness of the material is larger than the thickness of the dimension of that body, this results in the transmission of absorbed radiation through the body. However, that material that is strong internal absorbers converts radiation into internal energy.
Materials can be differentiated on the basis of the ability to let radiation pass through the material surface and totally absorbed the radiation after passing into the surface of the material. For example, polished metals can reflect a small portion of radiation but the remaining radiation is absorbed by the metals and converted into internal energy. However, nonmetals required more thickness to absorb the radiation and convert them into internal energy. Similarly, a glass window allows to pass the radiation from its surface but can not absorb the radiation and convent into internal energy.
Good radiation absorbers have low surface reflectivity and high internal absorption. This prevents the radiation to pass through the body and it can convert easily into internal energy. For example, when small pieces of metals are deposited into the subsurface of a body. This will act like a low reflectivity and high absorption surface.
Importance of thermal radiation in applications
Thermal radiation contributes to the transfer of energy in combustion chambers, furnaces, fires, and nuclear explosions. Thermal radiation governs the temperature distribution of the sun and solar emission. It is used to study solar energy utilization. There are some devices that use high temperatures for achieving good efficiency. Thermal radiation is used to calculate the thermal effect in racked nozzles, power plants, large engines, and heat exchangers.
It is used as compared to convection and conduction where a physical medium must be necessary. By using, thermal radiation, radiative energy can be passed through a vacuum. When there is no medium present, thermal radiation is the best mode of heat transfer. For example, heat leakages in the thermos bottle, etc.
One of the best applications of thermal radiation is the use of radiation from the sun as an energy source. This radiation travels through space from the sun and is received at earth in solar collectors. These collectors convert thermal radiation into internal energy.
Examples of thermal radiation
These are the substances or materials that emit thermal radiation.
- Solar radiation
- Heat radiators
- Microwave oven
- Incandescent lamp
- Emission of gamma rays
What are three examples of thermal radiation?
These are the examples of thermal radiation:
- Incandescent lamp
- Heat radiators
- Solar radiation
What are the types of thermal radiation?
Thermal radiation includes three types of regions in the electromagnetic spectrum.
- A small portion of the ultraviolet region
- Complete visible region
- Complete infrared region
Is thermal radiation a light?
Yes, it can be in the light region. Because the visible region can be seen through a naked eye.
Is thermal radiation infrared?
Yes, it is infrared. This is because infrared is fall in the thermal radiation regions.
How does thermal energy transfer by radiation?
Unlike convection and conduction, thermal radiations are transferred through waves without any medium.
What’s the wavelength range of thermal radiation?
The wavelength range of thermal radiation is from 0.1 micrometers to 100 micrometers.
How is thermal radiation different from light radiation?
The only difference is in the intensity of radiation.
Do laptops emit radiation or thermal heat?
Yes, they emit thermal radiation. This radiation is not too effective because they have very low intensities.
How does thermal radiation emit from a heated body?
Radiation always travels from hot body to cold body. Thermal radiation is emitted by a heated body in the form of waves.
What are the instruments used in measuring thermal radiation?
Radiometers are used to measure thermal radiations.
Is thermal radiation electromagnetic in nature?
Yes, they are electromagnetic in nature.
What is the difference between thermal conduction and radiation?
Conduction required a medium to transfer from source to object while radiation does not require any medium to travel. Even radiation can travel through space.
- Thermal Radiation Heat Transfer, Fourth Edition By Robert Siegel
- Thermal radiation (sciencedirect.com)
- Thermal radiation explanation (wikipedia.org)
- Thermal radiation characteristics (vedantu.com)