Infrared

Infrared is a form of electromagnetic radiation with a wavelength longer than that of visible red light but shorter than radio waves (microwave). It is invisible to the eye, but the skin typically senses it as heat.

Anything with a temperature above absolute zero emits infrared radiation. For this reason, infrared-sensing instruments are used to measure temperature at a distance, to see in darkness, and to detect heat loss from buildings.

History

In 1800, the astronomer William Herschel conducted an experiment in which he separated sunlight using a prism (a crystal that disperses light into its component colours) and then placed thermometers at different coloured locations. He discovered that beyond the red end of the spectrum—where no visible light appears—the thermometer showed a higher temperature. This experiment is recognized as the first evidence of infrared radiation.

Categories

Infrared is commonly divided into categories based on wavelength: near-infrared, mid-infrared, and far-infrared. The divisions are not strict and vary by scientific field—astronomy, engineering, or medicine.

Applications

Infrared is used in television remote controls, fiber-optic communication lines, thermal imaging cameras, military night-vision equipment, and handheld satellites for monitoring plants and soil moisture. Astronomers use it to observe obscured regions, since infrared radiation passes through dust more readily than visible light.

Earth's Atmosphere

Certain atmospheric gases—such as water vapour, carbon dioxide, and methane—absorb and re-emit infrared radiation from the earth's surface. This process underlies the greenhouse effect and is why climate scientists emphasize measuring infrared radiation.