Thermal Radiation

Here is yet another "little" fact that we will need soon: Thermodynamics says that any body with a temperature above absolute zero will radiate its energy away.

Recall that heat is just the internal energy of motion and vibration of the molecules of the substance. And also recall that whenever a body with electric charge vibrates it radiates energy as electromagnetic radiation. These two facts explain the radiation process.

Your body is radiating at a rate of about 60 Watts, the same as a 60W light bulb. Most of that radiation is in the infrared region.

The higher the temperature of a body the faster it radiates energy away.

Technical note: for a perfect radiating body the rate of energy radiation is a universal constant times the fourth power of the absolute temperature.

Whether the radiation is mostly as infrared or visible light or X-rays etc. depends on the temperature of the body. For example, if we heat up a steel bar it starts to glow "red hot." Increasing the temperature shifts the radiation spectrum and it glows "white hot." Raise the temperature further and the radiation becomes blue.