Absorption, Reflection and Transmission of Heat Radiation

Heat can be transferred by three distinct modes: conduction, convection and radiation. Conduction and convection both require a material medium -- heat cannot flow by these routes through a vacuum. Radiation is fundamentally different: it does not require any material medium at all, and can carry heat straight through empty space. The most familiar everyday example is the transfer of heat and light from the Sun to the Earth across roughly 150 million kilometres of vacuum.

The term "radiation" is used in two closely related senses: it names this mode of heat transfer, and it also names the continuous emission of electromagnetic energy from the surface of any body simply because of its own thermal energy -- this emitted energy is called radiant energy, carried in the form of electromagnetic waves. Because electromagnetic waves travel at the speed of light, radiation is the fastest of the three modes of heat transfer, and it works equally well through vacuum or through any material medium that happens to be transparent to it -- physical contact between the two bodies exchanging heat is never required.

When radiation falls on a body that is not transparent to it (e.g. the floor, or a person's hand), some of it is absorbed and the body heats up as a result. Electromagnetic radiation emitted by bodies that are hotter than their surroundings is specifically called thermal radiation. …