Radiation heat transfer is a process where heat waves are emitted that may be absorbed, reflected, or transmitted through a colder body. Unlike conduction and convection, radiation does not need a medium for transmission. Radiation heat transfer occurs via electromagnetic waves, known as thermal radiation, that arises due to the temperature of a body. Any material with a temperature above absolute zero gives off some radiant energy, and most energy of this type is in the infrared region of the electromagnetic spectrum, although some of it is in the visible region. The total radiant heat energy emitted by a surface is proportional to the fourth power of its absolute temperature, according to the Stefan–Boltzmann law. Radiation heat transfer is essential to life as we know it, and the transfer of heat from the sun’s surface to planet Earth is a prominent example. The heating of the Earth by the Sun is an example of transfer of energy by radiation. Radiation heat transfer is characteristically different from conduction and convection in that it does not require a medium and, in fact, it reaches maximum efficiency in a vacuum. Radiation waves may travel in unusual patterns compared to conduction heat flow, and radiation allows waves to travel from a heated body through a cold nonabsorbing or partially absorbing medium and reach a warmer body again.