What is radiation in heat transfer?

Radiation in heat transfer refers to the mechanism by which heat is transferred from a warmer object to a cooler object through electromagnetic energy, without the need for a medium such as air or water. This mode of heat transfer occurs through the emission of infrared radiation, which can travel through the vacuum of space. For instance, the heat from the sun reaches the Earth through radiation.

This type of heat transfer is unique because, unlike conduction or convection, it does not require matter to transfer energy. Thus, even in a vacuum where there are no particles to facilitate the transfer of heat, radiant energy can still travel and perform this function.

The other options relate to different concepts of heat transfer: physical contact refers to conduction, while conduction itself involves heat transfer through solid materials, and fluids might typically involve convection but do not define radiation. Hence, the correct answer encompasses the essence of radiation as a distinct and crucial method of heat transfer.