Ultraviolet, X-rays and gamma rays

Ultraviolet runs from about 4 × 10⁻⁷ m (400 nm) down to 6 × 10⁻¹⁰ m (0.6 nm). Special lamps and very hot bodies emit it, and the sun is a major source. In atoms it comes from inner-shell electrons dropping to a lower level, and it is detected by photocells and photographic film.

Most of the sun's ultraviolet is absorbed by the ozone layer, at a height of about 40-50 km. Depletion of this layer by chlorofluorocarbons (CFCs) such as freon is a matter of international concern.

Large doses of UV are harmful. Exposure makes the skin produce more melanin, which causes tanning. Ordinary glass absorbs UV, so no one tans or burns through a glass window, and welders wear glass goggles or face masks against the UV of the welding arc.

Its short wavelength lets UV be focused into very narrow beams for precise work such as LASIK eye surgery. UV lamps kill germs in water purifiers.

X-rays lie beyond the UV, from about 10⁻⁸ m (10 nm) down to 10⁻¹³ m (10⁻⁴ nm). A common way to make them is to strike a metal target with high-energy electrons (an X-ray tube). They are detected by photographic film, Geiger tubes and ionisation chambers.

X-rays are used to diagnose illness and to treat some forms of cancer. They damage or destroy living tissue, so unnecessary or excessive exposure must be avoided.

Gamma rays occupy the top of the frequency range, with wavelengths from about 10⁻¹⁰ m to below 10⁻¹⁴ m. They come from nuclear reactions and radioactive nuclei, and are used in medicine to destroy cancer cells.

The regions overlap: UV is quoted down to 0.6 nm while X-rays are quoted from about 10 nm, and the X-ray range (down to 10⁻¹³ m) reaches well into the gamma range (from 10⁻¹⁰ m). The boundaries follow how the radiation is produced, not a sharp wavelength.