Radiant heat
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Radiant heat is heat in the form of light, normally in the infrared part of the electromagnetic spectrum. Unlike conduction and convection, radiation does not require a medium to pass through.
One example is sunlight being absorbed by the surface of the earth. However, the reflection of light from the surface tends to produce light of longer wavelength called infrared (IR) radiation. IR radiation is also radiated from a hot surface like a hot piece of metal.
Mathematics
For a black body, the heat current radiated off it, 
may be described as:
where
is the area of the body
is it's emissivity
is the Stefan-Boltzmann constant
is the temperature of the body
