Invisible light rays of very short wavelength beyond the violet end of the sun’s spectrum. Ultraviolet-C (UVC) (wavelength <290 nm [nanometre – see APPENDIX 6: MEASUREMENTS IN MEDICINE]) is entirely absorbed by the earth’s atmosphere and would otherwise be lethally damaging. Ultraviolet-B (UVB – 290– 320 nm) intensity increases with altitude: it is greatest in midsummer and at midday and penetrates cirrhus cloud. UVB causes sunburn and also tanning. Ultraviolet-A (UVA – 320– 400 nm) penetrates deeper into our skins but does not cause sunburn; it is implicated in many photochemical reactions and PHOTODERMATOSES and in CARCINOGENESIS. UVR helps the skin to synthesise vitamin D.
Ultraviolet lamps produce UVR and are used to tan skin but, because of the risk of producing skin cancer (see SKIN, DISEASES OF), the lamps must be used with great caution.... ultraviolet rays (uvr)
The term applied to a group of infective agents which are so small that they are able to pass through the pores of collodion ?lters. They are responsible for some of the most devastating diseases affecting humans: for example, INFLUENZA, POLIOMYELITIS, SMALLPOX and YELLOW FEVER. The virus of in?uenza measures 80 nanometres, whereas the STAPHYLOCOCCUS measures 1,000 nanometres (1 nanometre = one thousand-millionth of a metre).
A single virus particle, known as a virion, comprises an inner core of NUCLEIC ACID which is surrounded by one or two protective coverings (capsid) made of protein. Sometimes the capsid is enclosed by another layer called the viral envelope (also a protein structure). The envelope often disintegrates when the virus invades a cell. Viruses enter cells and then indulge in a complex and variable process of replication using some of the cells’ own structure. Viruses may stay in a host’s nucleus, being reactivated months or years later. There are more than a score of large families of viruses, from papoviruses, which cause WARTS, through HERPES viruses (cold sores, CHICKENPOX, SHINGLES) and orthomyxoviruses (in?uenza), to corona viruses (common cold) and retroviruses (AIDS/HIV). Viral diseases are more dif?cult to treat than those caused by bacteria: ANTIBIOTICS are ine?ective but INTERFERON, a group of natural substances, shows promise. IMMUNISATION is the most e?ective way of combating viral infections; smallpox, poliomyelitis, MUMPS, MEASLES and RUBELLA are examples of viral diseases which have been successfully combated. Research is progressing to ?nd a vaccine against HIV.... virus
The ability to see different parts of the colour spectrum. Light perceived by the human eye consists of electromagnetic radiation (energy waves) with a spectrum of different wavelengths
between about 400 and 700 nanometres. Different wavelengths produce sensations of violet, indigo, blue, green, yellow, orange, and red when they fall on the retina and stimulate nerve signals, which are processed in the brain.
As light falls on the retina, it strikes light-sensitive cells called rods and cones. The rods can detect all visible light, but only the cones can distinguish colour. There are 3 types of cones: red-sensitive, blue-sensitive, and green-sensitive. Each responds more strongly to a particular part of the light spectrum. Because the cones are most concentrated in a central area of the retina called the fovea, colour vision is most accurate for objects viewed directly and is poor at the edges of vision. When light hits a cone, it causes the cone to emit an electrical signal, which passes to the brain via the optic nerve. Colour perception requires a minimum level of light, below which everything is seen as shades of grey. (See also colour vision deficiency; eye; perception; vision.)... colour vision
n. a unit of length equal to one ten millionth of a millimetre (10?10 m). It is not a recommended *SI unit but is sometimes used to express wavelengths and interatomic distances: the *nanometre (1 nm = 10 Å) is now the preferred unit. Symbol Å.... angstrom
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