William Herschel first detected infrared light in the eighteenth century. Its nature and properties gradually became known to the scientific world. Infrared light is a form of electromagnetic radiation, like X-rays, radio waves, microwaves and ordinary light that the human eye can detect. Infrared light possesses many properties in common with all other electromagnetic radiation plus special properties that are uniquely its own.
All electromagnetic radiation, including infrared light, originates when there is some alteration in the movement of electrons. For example, when an electron moves from a higher orbit or energy level to a lower one, the emission of electromagnetic radiation ensues.
Infrared light and other electromagnetic radiation consist of transverse waves. When the displacement or waviness of a wave lies at right angles to the direction in which the wave’s energy is traveling, the wave is a transverse wave, according to “Serway’s College Physics.”
The waves of infrared light have their own unique wavelengths. The shortest infrared wave lengths are about 0.7 microns, according to the Department of Astronomy and Astrophysics of the University of Chicago. But there is no general agreement on the upper limit. The longest infrared wavelengths are about 350 microns, according to Space Environment Technologies. According to RP Photonics, the upper limit is about 1000 microns. A micron is one millionth of a meter.
Infrared light, like all electromagnetic radiation, travels at the speed of 299,792,458 meters per second, according to "Serway’s College Physics."
Besides its wave properties, infrared light also exhibits properties that are characteristic of particles. The quantum theory provides a framework in which infrared light can exist both as a wave and as a particle at the same time, according to “The New Quantum Universe.”
Absorption and Reflection
Like the radiation of visible light, infrared radiation can be absorbed or reflected, depending on the nature of the substance that it strikes. Water vapor, carbon dioxide and ozone absorb infrared radiation effectively, according to the Oracle Education Foundation.
Heat is a transfer of energy. Infrared light is one of the means by which energy transfer is effected, according to “Serway’s College Physics.” For example, the rays emitted by the sun include infrared radiation. When this radiation strikes oxygen or nitrogen molecules in the air or the iron molecules in a metal sheet, it makes them vibrate or move faster. The molecules will then have more energy than before. In other words, infrared radiation causes materials to get hotter.
Infrared light exhibits the property of refraction. This means that the direction in which the light is moving suffers a slight change in direction when the radiation passes from one medium, such as outer space, into another medium of different density, such as the Earth's atmosphere.
If two infrared rays of the same wavelength meet each other, they will interfere with one another. Depending on how they join, they will annul or reinforce one another in varying degrees.