Infrared radiation, also known as thermal radiation, is part of the optical radiation and thus a part of the electromagnetic spectrum. It joins visible light in the direction of longer wavelengths. It is divided into three categories: close, middle and long-range infrared. Close-range infrared is the part of the infrared spectrum that is closest to visible light, long-range infrared is close to the frequency range of microwaves. Their wavelength range extends from 780 nanometers to 1 millimeter.

The most important natural source of infrared radiation is the sun. Infrared radiation accounts for 50 percent of the solar radiation reaching the ground. In addition, the earth heated by solar radiation emits infrared radiation.

We ourselves experience infrared radiation every day. The heat we feel in sunlight or in front of a fire, a radiator or a hot asphalt surface is infrared radiation. Even if our eyes cannot see it, the nerves in our skin perceive it as heat. We also use infrared radiation in remote controls, for example for televisions