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Thermal Radiation-

The electromagnetic radiations emitted by a body by virtue of its temperature are called thermal radiation or radiant energy.

All bodies having temperature above 0 K emit thermal radiation continuously.

Examples-

These are electromagnetic waves.
The wavelength of thermal radiation ranges from 1 μm to 1000 μm.
They belong to infrared region of the electromagnetic spectrum. That is why, they are also called infrared radiations.
They travel along straight lines with the speed of light.
They do not require any medium for their propagation i.e. they can pass through vacuum too.
They do not heat the intervening medium through which they pass.
They can be reflected and refracted according to the laws of reflection and refraction of light.
Thermal radiations also exhibit the phenomenon of interference, diffraction and polarization as do the light radiations.

Thermal radiation and visible light are both forms of electromagnetic radiation.
The key difference between the thermal radiation and visible light lies in their wavelength range and the way they are perceived by human senses.

The following table lists some important differences between thermal radiation and visible light-

Basis	Thermal Radiation	Visible Light
Cause of Emission	Due to thermal motion of particles	Due to electronic transition in atoms
Wavelength Range	1 μm to 1000 μm (Mostly infrared-longer wavelengths)	400 nm -700 nm
Visibility	Not visible to human eye	Visible to human eye
Main Effect	Produces heating effect	Produces illumination
Dependence	Depends on the temperature of the object	Depends on the source and excitation
Emission at room temperature	Emitted by all objects above 0 K	Not emitted unless the object is luminous
Example	Heat felt from a hot utensil	Light from the sun or bulb

Thermal Radiation & Its Properties

Prevost’s Theory of Heat Exchange

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