Introduction
All the objects at non-zero temperature emit radiation in the form of light, it is well described by Max Planck and Stefan Boltzmann. Thermal emission is a process of emitting radiation above zero temperature. Some of the objects emit a considerable amount of electromagnetic radiation of the thermal nature known as thermal radiation or heat radiation. Thermal radiations are emitted by objects above zero temperature. All the objects that emit thermal radiation follow an idealized blackbody radiation curve. The thermal radiations indicate that some of the thermal energy has been converted into electromagnetic radiations.
What is Heat Radiation?
Sometimes, it can also be asked what is thermal radiation, and the answer to both questions will be the same. Another name for heat radiation is thermal radiation. Now, heat radiations refer to the radiations emitted by any objects at nonzero temperatures.
Electromagnetic waves represent the propagation of the collection of discrete packets of energy (known as the photons) due to the change in the electromagnetic configuration of molecules or atoms. The thermal radiation definition says that thermal radiation is the electromagnetic wave emitted by the bodies. Only at absolute temperature ( technically which can not be achieved), the electromagnetic radiations will vanish. From the heat radiation definition, we can understand that thermal radiations are electromagnetic waves.
Thermal radiation is usually emitted from light bulbs and from the sun. Even if the temperature is not sufficiently high enough to generate the visible thermal radiation, there may be some strong infrared radiation that can be felt on the skin. Thermal radiation is visible due to sufficiently high temperatures, it is also known as incandescence.
The thermal radiations for most of the solids and liquids are assumed to be a surface phenomenon since the radiations emitted by the interior molecules are strongly absorbed by the other molecules and will not be reacting with the surroundings. The good emitters are good absorbers. Whenever we discuss thermal radiations we must know about the black bodies. The black bodies are the materials that can absorb all the radiations striking onto it.
Let us Have a Look at Some Heat Radiation Examples:
The transmission of electromagnetic waves through the oven is also known as the microwave oven.
The heat energy emitted by the radiator.
The solar radiations.
The light energy radiated by incandescent lamps.
The emission of gamma rays.
These are a few thermal radiation examples. We witness the thermal radiations in our lives most frequently.
Thermal radiation is relevant in many areas of science. Let us have a look at a few examples:
Many sources of light are based on the emission of thermal radiation. The best example is the incandescent lamp.
Thermal radiation is a challenge when some object needs to be thermally insulated. For example, when an object is suspended in a vacuum chamber, such that thermal conduction is minimized, can exchange thermal energy with its surroundings through radiation.
Thermal infrared emission is utilized for infrared temperature measurements and thermal imaging processes.
As the photon energy of visible light is well above the thermal energy at room temperature, thermal emission is usually not an issue in optics, including photodetection. This is different in radio technology, for example, where sensitive detectors can easily register thermal radiation.
Characteristics of Thermal Radiations:
Thermal radiation is the transfer of thermal energy without any contact between the bodies.
Thermal radiation is a surface phenomenon, unlike heat transfer by conduction or convection, the thermal radiations will not require any contact between the source and the receiver.
Every matter having nonzero temperature or temperature more than 0k emits thermal radiations.
Thermal radiations have a wavelength ranging from 0.1micrometer to 100micrometers.
While the transfer of any energy via conduction requires a medium to process, whereas for thermal radiations it does not require any medium for conduction. Radiation is the only mode of transfer that can be carried out even in a vacuum.
Did You Know:
The ice cubes in our drinks get melted after some time. Did you know what's the reason for the melting? The reason is the ice cube isn’t cooling our drink, the drink itself heating the ice cube. The transfer of heat from the drink to the ice cube is the reason why our drinks or any beverages get cold by adding ice cubes.
Another fun fact of thermal energy is that people often get confused that thermal energy and heat are the same. Whereas the thermal energy isn’t the same as the heat. Heat is a sense, it is something that is transferred between two bodies. While the thermal energy is stored or internal.
FAQs on Thermal Radiation
1. What is meant by thermal radiation?
Thermal radiation is the transfer of energy through electromagnetic waves that are emitted by any matter with a temperature above absolute zero (0 Kelvin). This process does not require any medium to occur. It happens due to the thermal motion of atoms and molecules within the object, which converts some of its internal energy into electromagnetic energy. It is also commonly referred to as heat radiation.
2. What are some common examples of thermal radiation in daily life?
We encounter thermal radiation in many situations. Here are a few key examples:
- The Sun: The warmth we feel from the sun is thermal radiation that has travelled through the vacuum of space.
- Campfire or Heater: The heat you feel when sitting near a fire or an electric heater is primarily due to thermal radiation.
- Incandescent Light Bulb: An old-fashioned light bulb glows and feels hot because its filament emits both visible light and infrared radiation.
- Human Body: All living beings emit thermal radiation, which is how thermal imaging cameras can detect people in the dark.
3. How is thermal radiation different from conduction and convection?
Thermal radiation is fundamentally different from the other two modes of heat transfer, conduction and convection. The primary distinction is the medium of transfer. While conduction requires direct contact between particles and convection requires the bulk movement of a fluid (liquid or gas), thermal radiation can travel through a complete vacuum as it propagates via electromagnetic waves. This is why heat from the sun can reach Earth.
4. Which scientific laws are fundamental to understanding thermal radiation?
Two main laws, as per the CBSE syllabus for 2025-26, govern thermal radiation:
- Stefan-Boltzmann Law: This law states that the total energy radiated per unit surface area of a black body is directly proportional to the fourth power of its absolute temperature (E ∝ T⁴). This means hotter objects radiate significantly more energy.
- Wien's Displacement Law: This law states that the wavelength at which the maximum radiation is emitted by a black body is inversely proportional to its absolute temperature. This explains why an object glows from red to orange to white as it gets hotter.
5. Why is a good emitter of thermal radiation also a good absorber?
This principle is explained by Kirchhoff's Law of Thermal Radiation. For an object to be in thermal equilibrium with its surroundings, it must radiate energy at the same rate it absorbs it. A surface that is highly effective at absorbing energy across all wavelengths (a good absorber) must also be highly effective at emitting energy across those same wavelengths to maintain this balance. An idealised blackbody, which absorbs all incident radiation, is therefore also a perfect emitter.
6. What is a 'blackbody' in the context of thermal radiation?
In physics, a blackbody is an idealized object that absorbs all electromagnetic radiation that falls on it, regardless of frequency or angle of incidence. It does not reflect or transmit any radiation, making it a perfect absorber. Paradoxically, a blackbody is also a perfect emitter of thermal radiation. Its emission spectrum depends only on its temperature, not its composition, and serves as a theoretical standard for studying radiation.
7. Is all thermal radiation visible to the human eye?
No, most thermal radiation is not visible. At everyday temperatures, like room temperature, objects primarily emit radiation in the infrared spectrum, which is invisible to the human eye but can be felt as heat. For thermal radiation to become visible (a phenomenon called incandescence), an object must be heated to a very high temperature, typically over 525°C (around 800K), at which point it starts to glow red, then orange, and eventually white-hot as the temperature increases further.
