Light is a form of electromagnetic radiation. The electromagnetic waves that comprise electromagnetic radiation consist of oscillations in electric and magnetic fields, just as water waves consist of oscillations of the water in the ocean.
Certain properties describe all types of waves. One is the wavelength, which is the distance between two adjacent peaks in the wave. The frequency is the number of peaks that move past a stationary observer in one second. In the case of water waves at the beach, the frequency would be the number of incoming waves that hit a person in one second, and the wavelength would be the distance between two waves. A higher frequency corresponds to a shorter wavelength and vice versa.
The different colors of light that our eyes can detect correspond to different wavelengths—or frequencies—of light. Red light has a longer wavelength than violet light. Orange, yellow, green, and blue are in between. Infrared light, ultraviolet light, radio waves, microwaves, and gamma rays are all forms of electromagnetic radiation, but they differ in wavelength and frequency.
Infrared light has slightly longer wavelengths than red light. Our eyes can not detect infrared light, but we can feel it as heat. Infrared astronomy uses the wavelength range from about 1 micrometer to a few hundred micrometers. Wavelengths near 1,000 micrometers (1 millimeter) are considered radio waves and studied by radio astronomers using different techniques than infrared astronomers.
Infrared astronomers divide the infrared spectrum into near-, mid-, and far-infrared. The exact boundaries between these regions are indistinguishable, but near-infrared is generally considered to be from one to five micrometers. Wavelengths of 5-20 micrometers are considered mid-infrared. Wavelengths longer than about 20 mircrometers are far-infrared.