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Light refers to a small band of frequencies visible to the human eye among the larger electromagnetic (EM) radiation scale. Most EM waves oscillate at a rate that humans are unable to detect visually. This might be compared with a dog whistle with a pitch that human ears cannot hear. In the same way, some animals can see EM frequencies that humans cannot. Bees, for example, see in the ultra-violet (UV) range to pick out patterns in flowers only visible with UV-equipped vision.
EM radiation is an electric field with magnetic properties that propagate from one point to another, or radiate outward. EM radiation is a wave with frequency and amplitude. Frequency refers to how many waves pass a stationary point per second, while amplitude measures the height of a wave. Visible light has a wavelength of 400 to 700 nanometers. To put this in perspective, a nanometer is one billionth of a meter (one billionth of 3.281 feet).
Light has different properties depending on its amplitude and wavelength. Longer waves, or lower frequencies, result in red light, while shorter waves, or higher frequencies, result in blue. Red is at one extreme end of the visible spectrum, while blue or violet light is at the other. Just beyond the blue/violet spectrum are ultra-short waves called ultra-violet. This just-visible and near-visible light is also called High Energy Ultra Violet (HEV) light.
At the extreme end of the blue spectrum, most of the radiation becomes invisible, resulting in dim violet light, also called black light. This wavelength has interesting properties in that certain pigments absorb the extra radiation that cannot be seen, causing these pigments to re-radiate the energy and glow. One example is a black light poster. Slightly shorter wavelengths produce black light used in criminal forensics to fluoresce body fluids such as urine and blood. Beyond UV radiation on the EM scale are x-rays and gamma rays. Cosmic rays, when included, fall here; though many scientists believe cosmic rays do not technically belong in the EM spectrum.
The opposite end of the visible spectrum goes beyond red to infrared. Infra is Latin for “below,” so infrared literally means "below red." Infrared light is used for night vision cameras and thermal imaging. In this wavelength, warm objects appear brighter than cold objects. Infrared is also used for short-range networking of computer peripherals with the Infrared Data Association (IrDA) specification. As wavelengths continue to grow longer, we reach microwaves, followed by radio waves, and finally, the broadcast spectrum.
Though light is often described as a wave, it has a dual nature according to quantum physics. Physics describes light as photons, or massless energy particles that can at times behave like a wave. Whether wave, particle, or vibrating “string,” as superstring theory suggests, in a vacuum all EM radiation moves at a constant speed of 186,282 miles per second, or 299,792,458 meters per second. A light year is therefore the distance that light can travel in a year. The nearest star, Alpha Centauri, is four light years away.