Electromagnetic radiation/Ultraviolet (UV)

'''Electromagnetic radiation; 9. Ultraviolet (UV)'''

Ultraviolet radiation was discovered by Johann Wilhelm Ritter in 1801.

There are several terms used.


 * Near ultraviolet: 400-300 nm
 * Middle ultraviolet: 300-200 nm
 * Extreme or far ultraviolet: 200-100 nm
 * Vacuum ultraviolet is UV with wavelengths shorter than about 150-200 nm. Such wavelengths are strongly absorbed by air, especially oxygen, so are best used in a vacuum.  However, it is possible to use pure nitrogen rather than a vacuum.

UVA, UVB, UVC are terms used to assess how dangerous the UV is to human skin.


 * UVA: 320-400 nm: Not absorbed by the ozone layer in the atmosphere; fairly harmless.
 * UVB: 280-320 nm: Partly absorbed by ozone layer; more dangerous, especially where there is a "hole" in the ozone layer so more radiation can penetrate to ground level.
 * UVC: 100-280 nm: Totally absorbed by ozone layer and atmospheric oxygen; most dangerous.

Short-wave UV, X-ray and gamma-ray radiation are sometimes lumped together as ionising radiation, because the photons are energetic enough to knock electrons out of atoms, ionising them. Such radiation is dangerous to living things, because the ionisation can disrupt biochemical processes and even cause cancer. Alpha and beta particles can also be called ionising radiation.