<ul>
<li>UVA, or near UV (315-400 nanometers)</li>
<li>UVB, or middle UV (280-315 nanometers)</li>
<li>UVC, or far UV (180-280 nanometers)</li>
</ul>
Radiations with wavelengths from 10nm to 180nm are sometimes referred to as vacuum or extreme UV.

UV is generally divided into three sub-bands

EM is the broad range of wavelengths. It is divided into seven regions and ordered in decreasing wavelength and increasing energy and frequency, namely radio waves, microwaves, infrared (IR), visible light, ultraviolet (UV), X-rays and gamma-rays.
Ultraviolet light falls is the range of the EM spectrum between visible light and X-rays.

The electromagnetic (EM) spectrum

There are also other celestial sources of UV radiation.
Very large young stars shine most of their light in ultraviolet wavelengths. But the Earth's atmosphere blocks much of the UV radiation, and observations are conducted using high-altitude balloons and special orbiting telescopes equipped to observe the UV region of the EM spectrum.

UV astronomy

UV radiation has enough energy to break chemical bonds. UV photons can cause ionization, meaning chemical bonds will break that otherwise would not.
It can be useful, for instance, in disinfecting a surface, but it can also damage materials and living tissues, particularly the skin and eyes that are affected by UVB and UVC radiation.

Ionization

Most natural UV light comes from the sun. Only 10 % of sunlight is UV, and only one-third of it gets through the atmosphere.
Of the solar UV energy that reaches the equator, 95% is UVA, and 5% is UVB. No measurable UVC from the sun gets to the Earth's surface, because ozone, molecular oxygen, and water vapour in the upper atmosphere absorb the shortest UV wavelengths.

UV effects

Substances like minerals, plants, fungi, microbes, and organic and inorganic chemicals, can absorb UV radiation, causing electrons to jump to a higher energy level. When the electrons return to a lower energy level, they emit a portion of the absorbed energy as visible light.
<ul>
<li>Fluorescence is used for signs, safety vests, and other applications in which high visibility is important.</li>
<li>In fluorescent tubes, an electric current is passed through mercury vapour, producing ultraviolet radiation along with a blue light.</li>
</ul>

Fluorescence

UV light comes from the sun and is transmitted in waves or particles at different wavelengths an frequencies.
It makes black-light posters glow, and is responsible for summer tans. But too much exposure to UV radiation damage living tissue.

Ultraviolet light is a type of electromagnetic radiation

Some skin conditions can be treated using UV light.
PUVA (psoralen ultraviolet light treatment) is used to treat lymphoma, eczema, psoriasis, and vitiligo. Patients take a drug or apply a lotion to make their skin sensitive to light. Then a UV light is shone on the skin, slowing down the growth of the skin cells.

UV light as a cancer treatment

A suntan is a reaction to exposure to UVB rays. When the body senses sun damage, it sends melanin, a pigment in the body, to absorb UV light and protect the body from more damage, resulting in a darkened skin.
Continued exposure to UV radiation can damage the body's DNA. When the body senses this destruction, it floods the area with blood and inflammation to help with the healing process. Sometimes the cells with DNA mutated by the sun's rays turn into problem cells, known as skin cancer.