A third mode of heat transfer from a warmer to a colder body is the thermal radiation.
Unlike the other two processes, conduction and convection, irradiation allows heat transfer, or radiant heat.
Heat waves from the sun travel a great distance in a vacuum until they reach Earth and transfer heat to it from the sun.
In addition to heat radiated by the sun, several other everyday examples are related to thermal radiation.
The heat of a bonfire or a fireplace reaches a person through the radiation. Foods bake in conventional ovens thanks to the heat radiated by the flame. Ordinary lamps, in addition to emitting visible light, radiate considerable amounts of infrared heat. In poultry farms, chicks are kept warm by lamps that stay on day and night.
When sunlight passes through a glass prism, it is separated into lights of different colors, the colors of the rainbow. This event, called the scattering of white light.
In 1800, the English astronomer William Herschel (1792-1871) made an important discovery. He placed a thermometer in the regions illuminated by different colored lights and noticed that the red light warms the thermometer more than the violet light. In other words, Red light carries more heat than violet light. When Herschel placed the thermometer in the area next to the red where no illumination was seen, he was surprised to find that the thermometer indicated that heat was coming.
Herschel concluded that some kind of “invisible light” came to this region and called it infrared.
From the discovery of infrared it became known that a body does not necessarily have to emit visible light in order to emit heat by radiation. By carelessly approaching the hand of a connected iron, even without touching, a person can get burned by the heat radiated by it. Despite emitting infrared heat, the iron does not emit light. There are now special devices that make it possible to "see" infrared. Such devices, infrared sensors, are employed, for example, by police forces in night observation activity in dark places.