Physics Ideas

Thermal radiation: properties, examples, applications

The thermal radiation is the energy transmitted by a body with its temperature and wavelengths of the infrared electromagnetic spectrum. All bodies, without exception, emit some infrared radiation, no matter how low the temperature.

It turns out that when they are in accelerated motion, electrically charged particles oscillate and, thanks to their kinetic energy, continuously emit electromagnetic waves.

The only way for a body not to emit thermal radiation is for its particles to be at complete rest. In this way, its temperature would be 0 on the Kelvin scale, but reducing the temperature of an object to that point is something that has not yet been achieved.

Thermal radiation properties

A notable property that distinguishes this heat transfer mechanism from others is that it does not require a material medium to produce it. Thus, the energy emitted by the Sun, for example, travels 150 million kilometers through space and continuously reaches the Earth.

There is a mathematical model for knowing the amount of thermal energy per unit of time an object radiates:

P = σ and 4

This equation is known as Stefan’s law and the following magnitudes appear in it:

– Thermal energy per unit of time P , which is known as power and whose unit in the International System of Units is watt or watt (W).

-The surface area of the heat-emitting object A , in square meters.

-A constant, called Stefan – Boltzman constant , denoted by σ and whose value is 5.66963 x10 -8 W / m 2 K 4 ,

-A – E (also called  emissivity ) of the object and , a dimensionless quantity (no units) whose value is between 0 and 1. It is related to the nature of the material: for example, a low emissivity glass has, as a very dark body has high emissivity.

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-And finally the temperature T in Kelvin.

Examples of thermal radiation

According to Stefan’s law, the rate at which an object radiates energy is proportional to the area, the emissivity, and the fourth power of the temperature.

As the rate of thermal energy emission depends on the fourth power of T, it is evident that small changes in temperature will have a huge effect on the emitted radiation. For example, if the temperature doubles, the radiation will increase 16 times.

A special case of Stefan’s law is the perfect radiator, a completely opaque object called a black body , whose emissivity is exactly 1. In this case, Stefan’s law looks like this:

P = σ 4

It turns out that Stefan’s law is a mathematical model that roughly describes the radiation emitted by any object, since it considers emissivity as a constant. Emissivity really depends on the wavelength of the emitted radiation, surface finish, and other factors.

When e is considered constant and Stefan’s law is applied as indicated at the beginning, the object is called a gray body .

The emissivity values ​​for some substances treated as gray bodies are:

-0.05 polished aluminum

-Black coal 0.95

-Human skin of any color 0.97

-Wood 0.91

-ice 0.92

-Water 0.91

-Covers between 0.015 and 0.025

-Steel between 0.06 and 0.25

The thermal radiation of the sun

A tangible example of an object that emits thermal radiation is the Sun. It is estimated that approximately 1370 J of energy in the form of electromagnetic radiation reaches the Earth from the Sun every second.

This value is known as the solar constant, and each planet has one, which depends on its average distance from the Sun.

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This radiation passes through each m 2 of atmospheric layers perpendicularly and is distributed at various wavelengths.

Almost everything comes in the form of visible light, but much of it comes as infrared radiation, which is precisely what we perceive as heat, and some as ultraviolet rays. It’s a lot of energy enough to meet the planet’s needs in order to capture it and use it conveniently.

In terms of wavelength, these are the bands within which the solar radiation reaching the Earth is found:

– Infrared , which we perceive as heat: 100 – 0.7 μm *

– Visible light , between 0.7 – 0.4 μm

– Ultraviolet , less than 0.4 µm

* 1 µm = 1 micrometer or one millionth of a meter.

Vienna law

The following image shows the distribution of radiation with respect to wavelength for various temperatures. The distribution obeys the Wien displacement law, according to which the wavelength of the maximum radiation λ max is inversely proportional to the temperature T in kelvin:

max = 2,898. 10 −3 m⋅K

The Sun has a surface temperature of approximately 5700 K and radiates mainly in shorter wavelengths, as we have seen. The curve closest to the Sun is the 5000 K curve, in blue, and, of course, it has the maximum in the visible light range. But it also emits a good part in infrared and ultraviolet.

Thermal radiation applications

Solar energy

The vast amount of energy that the sun radiates can be stored in devices called collectors and then conveniently transformed and used as electrical energy.

infrared cameras

These are cameras that, as the name implies, operate in the infrared region, rather than under visible light, like common cameras. They take advantage of the fact that all bodies emit thermal radiation to a greater or lesser degree, depending on their temperature.

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If temperatures are too high, measuring them with a mercury thermometer is not the best option. For this, pyrometers are preferred , through which the temperature of an object is deduced knowing its emissivity, thanks to the emission of an electromagnetic signal.


Starlight is modeled very well with the approach of the black body, as well as the entire universe. And, in turn, Wien’s law is often used in astronomy to determine the temperature of stars according to the wavelength of light they emit.

military industry

The missiles are directed at the target through infrared signals that seek to detect the hottest areas of the planes, such as engines, for example.

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