Infrared light is a form of electromagnetic radiation with a wavelength longer than that of visible light, meaning that infrared light is light beyond the red part of the rainbow that is invisible to our eyes. Measured from the nominal edge of visible red light at 0.7 micrometers, and extending conventionally to 300 micrometers. These wavelengths correspond to a frequency range of approximately 1 to 430 Thz, and include most of the thermal radiation emitted by objects near room temperature. Microscopically, IR light is typically emitted or absorbed by molecules when they change their movements.
Sunlight at zenith provides an irradiance of just over 1 kilowatt per square meter at sea level. Of this energy, 527 watts is infrared radiation, 445 watts is visible, and 32 watts is ultraviolet radiation.
All objects produce infrared light and the warmer they are the more they produce, which allows machines to detect sudden changes in temperature by detecting the level of infrared radiation in the area
Far infrared waves are thermal, meaning that we experience this type of infrared radiation in our daily lives in the form of heat. For example, just fire and the heating of food in some fast food restaurants show the usage of far infrared waves everyday. On the other hand, shorter, near infrared waves are not even hot at all, to the extent that you cannot feel them. These shorter wavelengths are used by your TV's remote control.
Though infrared light is invisible to humans, snakes in the pit viper family, like rattlesnakes, actually make use infrared light to detect warm blooded animals. Snakes with 2 sensory pits are even thought to have some depth perception in the infrared! (Source: NASA's Infrared Processing and Analysis Center)
Besides people and animals, the Earth, the Sun, and far away things like stars and galaxies also emit infrared let. For a view from Earth orbit, whether we are looking out into space or down at Earth, we can use instruments with infrared features on board satellites to observe planets.
Infrared light can be split into three categories:
Near-infrared (near-IR) - Closest to visible light, near-IR has wavelengths that range from 0.7 to 1.3microns, or 700 billionths to 1,300 billionths of a meter.
Mid-infrared (mid-IR) - Mid-IR has wavelengths ranging from 1.3 to 3 microns. Both near-IR and mid-IR are used by a variety of electronic devices, including remote controls.
Thermal-infrared (thermal-IR) - Occupying the largest part of the infrared spectrum, thermal-IR has wavelengths ranging from 3 microns to over 30 microns