Infrared refers to the wavelengths of the electromagnetic spectrum that lie between visible light and microwaves. Infrared radiation is perceived by us as heat, although we can’t see it. The origin of infrared heat is at the atomic level, where the heat generated by the movement of subatomic particles is changed into electromagnetic radiation in the infrared range. Infrared heat should be distinguished from thermal convection and thermal conduction, because it can travel through a vacuum.
To observe how infrared heat works, the heating and cooling of the earth is probably the most helpful example. Apart from driving all weather events and patterns, as well as ocean currents, infrared heat from the sun is what warms the Earth during the day, and it is what the ground gives off at night after the sun sets. It is interesting to note that while the sun does emit a huge amount of infrared heat, only about half of the heat we feel on the ground is from direct infrared radiation from the sun. The other half comes from energy from visible light that is absorbed by objects on earth, and then emitted later as infrared heat.
Man-made infrared heat is used in a variety of applications. In industrial contexts, the heating and welding of plastic is accomplished by infrared heaters, as is the curing of certain coatings, and certain steps in glass production. Many of us have seen infrared heat lamps in restaurants, which are used to keep food warm before it is served. These are specially designed light bulbs that are made to give off as much infrared energy as possible, with less emphasis on producing light.
Infrared heat can also be deliberately produced in such a way as to have a medically therapeutic effect. Far infrared radiation is sometimes used to provide pain relief to arthritis patients and others who suffer from chronic pain. It is also employed to de-ice aircraft wings, and in infrared saunas.
Not all infrared wavelengths are felt by us as heat. Light that is just beyond the red side of the visible spectrum is not hot at all, and this is the type of infrared light that is used by remote controls and some computing devices to transmit data. This part of the infrared range is called near infrared, because it is near the visible spectrum. Mid and far infrared waves are those that we notice as caloric heat.