Ultraviolet Radiation Definition

Chemistry Glossary Definition of Ultraviolet Radiation

Ultraviolet light is invisible, but black lights or UV-lamps also emit some visible violet light.
Ultraviolet light is invisible, but black lights or UV-lamps also emit some visible violet light. Cultura RM Exclusive/Matt Lincoln / Getty Images

Ultraviolet Radiation Definition

Ultraviolet radiation is electromagnetic radiation or light having a wavelength greater than 100 nm but less than 400 nm. It is also known as UV radiation, ultraviolet light, or simply UV. Ultraviolet radiation has a wavelength longer than that of x-rays but shorter than that of visible light. Although ultraviolet light is energetic enough to break some chemical bonds, it is not (usually) considered a form of ionizing radiation.

The energy absorbed by molecules can provide the activation energy to start chemical reactions and may cause some materials to fluoresce or phosphoresce.

The word "ultraviolet" means "beyond violet". Ultraviolet radiation was discovered by the German physicist Johann Wilhelm Ritter in 1801. Ritter noticed invisible light beyond the violet portion of the visible spectrum darkened silver chloride treated paper more quickly than violet light. He called the invisible light "oxidizing rays", referring to the chemical activity of the radiation. Most people used the phrase "chemical rays" until the end of the 19th century, when "heat rays" became known as infrared radiation and "chemical rays" became ultraviolet radiation.

Sources of Ultraviolet Radiation

About 10 percent of the light output of the Sun is UV radiation. When sunlight enters the Earth's atmosphere, the light is about 50% infrared radiation, 40% visible light, and 10% ultraviolet radiation.

However, the atmosphere blocks about 77% of solar UV light, mostly in shorter wavelengths. Light reaching the Earth's surface is about 53% infrared, 44% visible, and 3% UV.

Ultraviolet light is produced by black lights, mercury-vapor lamps, and tanning lamps. Any sufficiently hot body emits ultraviolet light (black-body radiation).

Thus, stars hotter than the Sun emit more UV light.

Categories of Ultraviolet Light

Ultraviolet light is broken into several ranges, as described by ISO standard ISO-21348:

NameAbbreviationWavelength (nm)Photon Energy (eV)Other Names
Ultraviolet AUVA315-4003.10–3.94long-wave, black light (not absorbed by ozone)
Ultraviolet BUVB280-3153.94–4.43medium-wave (mostly absorbed by ozone)
Ultraviolet CUVC100-2804.43–12.4short-wave (completely absorbed by ozone)
Near ultravioletNUV300-4003.10–4.13visible to fish, insects, birds, some mammals
Middle ultravioletMUV200-3004.13–6.20 
Far ultravioletFUV122-2006.20–12.4 
Hydrogen Lyman-alphaH Lyman-α121-12210.16–10.25spectral line of hydrogen at 121.6 nm; ionizing at shorter wavelengths
Vacuum ultravioletVUV10-2006.20–124absorbed by oxygen, yet 150-200 nm can travel through nitrogen
Extreme ultravioletEUV10-12110.25–124actually is ionizing radiation, although absorbed by the atmosphere

Seeing UV Light

Most people cannot see ultraviolet light, however, this is not necessarily because the human retina can't detect it. The lens of the eye filters UVB and higher frequencies, plus most people lack the color receptor to see the light. Children and young adults are more likely to perceive UV than older adults, but people missing a lens (aphakia) or who have had a lens replaced (as for cataract surgery) may see some UV wavelengths.

People who can see UV report it as a blue-white or violet-white color.

Insects, birds, and some mammals see near-UV light. Birds have true UV vision, as they have a fourth color receptor to perceive it. Reindeer are an example of a mammal that sees UV light. They use it to see polar bears against snow. Other mammals use ultraviolet to see urine trails to track prey.