Ultraviolet (UV) light is a type of electromagnetic radiation, as are radio waves, infrared radiation, X-rays and gamma-rays.

UV light, which comes from the sun, is invisible to the human eye. It makes black-light posters glow, and is responsible for summer tans - and sunburns. However, too much exposure to UV radiation is damaging to living tissue.

Electromagnetic radiation is transmitted in waves or particles at different wavelengths and frequencies. This broad range of wavelengths is known as the electromagnetic (EM) spectrum. The spectrum is generally divided into seven regions in order of decreasing wavelength and increasing energy and frequency. The common designations are radio waves, microwaves, infrared (IR), visible light, ultraviolet (UV), X-rays and gamma-rays.

Ultraviolet (UV) light falls in the range of the EM spectrum between visible light and X-rays. It has frequencies of about 8 × 1014 to 3 × 1016 cycles per second, or hertz (Hz), and wavelengths of about 380 nanometers (1.5 × 10−5 inches) to about 10 nm (4 × 10−7 inches).

UV is generally divided into three sub-bands:

  • UVA, or near UV (315–400 nm);
  • UVB, or middle UV (280–315 nm);
  • UVC, or far UV (180–280 nm).

Radiations with wavelengths from 10 nm to 180 nm are sometimes referred to as vacuum or extreme UV. These wavelengths are blocked by air, and they only propagate in a vacuum.

Most of the natural UV light people encounter comes from the sun. However, only about 10 percent of sunlight is UV, and only about one-third of this penetrates the atmosphere to reach the ground.

Of the solar UV energy that reaches the equator, 95 percent is UVA and 5 percent is UVB. No measurable UVC from solar radiation reaches the Earth's surface, because ozone, molecular oxygen and water vapor in the upper atmosphere completely absorb the shortest UV wavelengths.

Still broad-spectrum ultraviolet radiation UVA and UVB is the strongest and most damaging to living things.