Sujeethan Gnanathas 12B
Ultraviolet (UV) light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, that is, in the range between 400 nm and 10 nm, corresponding to photon energies from 3 eV to 124 eV. It is so-named because the spectrum consists of electromagnetic waves with frequencies higher than those that humans identify as the colour violet. These frequencies are invisible to most humans except those with aphakia (absence of the lens of the eye).
Though these waves are invisible to the human eye, some insects, like bumblebees, can see them.
The discovery of UV radiation was associated with the observation that silver salts darkened when exposed to sunlight. In 1801, the German physicist Johann Wilhelm Ritter made the hallmark observation that invisible rays just beyond the violet end of the visible spectrum darkened silver chloride-soaked paper more quickly than violet light itself. He called them “oxidizing rays” to emphasize chemical reactivity and to distinguish them from “heat rays”, discovered the previous year at the other end of the visible spectrum. The simpler term “chemical rays” was adopted shortly thereafter, and it remained popular throughout the 19th century. The terms chemical and heat rays were eventually dropped in favour of ultraviolet and infrared radiation, respectively.
The name means “beyond violet” (from Latin ultra, “beyond”), violet being the colour of the shortest wavelengths of visible light. UV light has a shorter wavelength than violet light.
Johann Wilhelm Ritter (16 December 1776 – 23 January 1810) was a German chemist, physicist and philosopher.
The discovery of the ultraviolet radiation below 200 nm, named vacuum ultraviolet because it is strongly absorbed by air, was made in 1893 by the German physicist Victor Schumann.
Victor Schumann (1841 – September 1, 1913) was a physicist and spectroscopist who in 1893 discovered the vacuum ultraviolet.
Scientists have divided the ultraviolet part of the spectrum into three regions: the near ultraviolet, the far ultraviolet, and the extreme ultraviolet. The three regions are distinguished by how energetic the ultraviolet radiation is, and by the “wavelength” of the ultraviolet light, which is related to energy.
The Sun emits ultraviolet radiation at all wavelengths, including the extreme ultraviolet where it crosses into X-rays at 10 nm. Extremely hot stars emit proportionally more UV radiation than the Sun. For example, the star R136a1 has a thermal energy of 4.57 eV, which falls in the near-UV range
UV light is also emitted by electric arcs and specialized lights such as mercury lamps and black lights.
The above picture on the left shows two black light fluorescent tubes. The top is a F15T8/BLB 18 inch, 15 watt tube, used in a standard plug-in fluorescent fixture. The bottom is an F8T5/BLB 12 inch, 8 watt tube, used in a portable battery-powered black light sold as a pet urine detector. The picture on the right shows that electric arcs produce UV light, and arc welders must wear eye protection to prevent welder’s flash.
UV radiation can cause chemical reactions, and causes many substances to glow or fluoresce.
The above picture shows a collection of mineral samples brilliantly fluorescing at various wavelengths as seen while being irradiated by UV light.
Light-emitting diodes (LEDs) can be manufactured to emit light in the ultraviolet range, although practical LED arrays are very limited below 365 nm. LED efficiency at 365 nm is about 5–8%, whereas efficiency at 395 nm is closer to 20%, and power outputs at these longer UV wavelengths are also better. Such LED arrays are beginning to be used for UV curing applications, and are already successful in digital print applications and inert UV curing environments. UV gas lasers, laser diodes and UV solid-state lasers can also be manufactured to emit light in the ultraviolet range. The nitrogen gas laser uses electronic excitation of nitrogen molecules to emit a beam that is mostly UV. The strongest lines are at 337.1 nm wavelength in the ultraviolet. Other lines have been reported at 357.6 nm, also ultraviolet.
A 380 nanometre UV LED makes some common household items fluoresce.
A large fraction of UV, including all that reaches the surface of the Earth, is classified as non-ionizing radiation. The higher energies of the ultraviolet spectrum from wavelengths about 120 nm to 10 nm (‘extreme’ ultraviolet) are ionizing, but, due to this effect, these wavelengths are absorbed by nitrogen and even more strongly by dioxygen, and thus have an extremely short path length through air.
The above picture shows the levels of ozone at various altitudes and blocking of different bands of ultraviolet radiation. In essence, all UVC is blocked by dioxygen (from 100–200 nm) or by ozone (200–280 nm) in the atmosphere. The ozone layer then blocks most UVB. Meanwhile, UVA is hardly affected by ozone and most of it reaches the ground.
Despite being classed as non-ionizing the entire spectrum of ultraviolet radiation has some of the biological features of ionizing radiation: It does far more damage to many molecules in biological systems than is accounted for by simple heating effects (an example is sunburn).
The reddening of the skin due to the action of sunlight depends both on the amount of sunlight and on the sensitivity of the skin (“erythemal action spectrum”) over the UV spectrum.
Sun exposure means an exposure to UV light which can be a benefit to the organism as well as harmful. UVB exposure causes the skin to produce vitamin D.
Ultraviolet radiation has other medical applications, in the treatment of skin conditions such as psoriasis and vitiligo. But an overexposure to UVB radiation can cause sunburn and some forms of skin cancer. The International Agency for Research on Cancer of the World Health Organization has classified all categories and wavelengths of ultraviolet radiation as a Group 1 carcinogen. This is the highest-level designation for carcinogens. High intensities of UVB light are hazardous to the eyes, and may lead to cataracts and other eye problems.
Ultraviolet photons harm the DNA molecules of living organisms in different ways. In one common damage event, adjacent thymine bases bond with each other, instead of across the “ladder”. This “thymine dimer” makes a bulge, and the distorted DNA molecule does not function properly.