Heat transfer by radiation can best be explained by using the sun as an example of the heat source. The sun is approximately 93 million miles from earth’s surface, yet we can feel its intensity. Heat transferred by radiation travels through space without heating the space and is absorbed by the first solid object that it encounters. Radiation is the only type of heat transfer that can travel through a vacuum, such as space, because it is not dependent on matter as a medium of heat transfer. Convection and conduction require some form of matter, like air or water, to be the transmitting medium. The earth does not experience the total heat of the sun because heat transferred by radiation diminishes by the inverse of the square of the distance traveled. In practical terms, this means that every time the distance is doubled, the heat intensity decreases by a factor of 4. If you hold your hand close to a light bulb, for example, you feel the heat’s intensity, but if you move your hand twice the distance away, you feel only one-fourth of the heat intensity, Figure 1. Keep in mind that, because of the inverse-square-of-the-distance rule, radiant heat does not transfer the actual temperature or heat quantity value. If it did, the earth would be as hot as the sun.
Electric heaters that glow red hot are practical examples of radiant heat. The red-hot electric heater coil radiates heat into the room. It does not heat the air, but it warms the solid objects that the heat rays encounter. Any heater that glows has the same effect.
Copied from REFRIGERATION & AIR CONDITIONING TECHNOLOGY by WILLIAM C. WHITMAN, WILLIAM M. JOHNSON, JOHN A. TOMCZYK and EUGENE SILBERSTEIN