The Electromagnetic Spectrum by Year 12 Physics Students

Ultraviolet light

By Ahlaq Hyder 12V

What is ultraviolet (UV) light?

https://www.youtube.com/watch?v=u8xx-cfzAeA

Ultraviolet (UV) light is a type of electromagnetic radiation and is found in the electromagnetic spectrum between the longer wavelength visible light and the shorter wavelength X-rays. UV light has an approximate wavelength of 1 x E-8 and 4 x E-7 metres [1].

[1] CGP AS-Physics revision guide: Edexcel; Page 44; ISBN: 9781847621290

UV light has frequencies ranging from about 7.5 x E14 Hz to 3 x E16 Hz [4].

[4] Edexcel AS Physics Student Book; ISBN: 978140596382

Unlike X-rays, UV radiation has a low power of penetration, hence why the only direct effect it has on humans is the skin – but this can lead to further complications. It is absorbed by matter and only has slight ionisation [1].

Typically, UV waves are invisible to the human eye; however some humans who suffer from aphakia – a condition where people lack a lens, can see some wavelengths and can see near UV as a whitish-blue/violet [7].

[7] Newspaper article; Let the light shine in; The Guardian; 29 May 2002

Or under certain artificial conditions, the human eye can see ultraviolet rays of about 3.1 x E-7 metres [2].

[2] Color and Light in Nature (2nd edition); Page 231; ISBN: 9780521775045

Near ultraviolet is visible to many different insects and birds, such as bumblebees [3].

[3] NASA – Mission: Science http://missionscience.nasa.gov/ems/10_ultravioletwaves.html

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http://pixgood.com/ultraviolet-light-wavelength.html

UV light can be further divided up in the following sub-divisions in order of decreasing wavelength and increasing energy. The classifications are mostly used in earth sciences.

Ultraviolet A 400 – 315 nm 3.10 – 3.94 eV long wave, black light, not absorbed by the ozone layer

Ultraviolet B 315 – 280 nm 3.94 – 4.43 eV medium wave, mostly absorbed by the ozone layer

Ultraviolet C 280 – 100 nm 4.43 – 12.4 eV short wave, germicidal (kills germs), completely absorbed by the ozone layer and atmosphere

Near Ultraviolet 400 – 300 nm 3.10 – 4.13 eV visible to birds, insects and fish

Middle Ultraviolet 300 – 200 nm 4.13 – 6.20 eV

Far Ultraviolet 200 – 122 nm 6.20 – 10.16 eV

Hydrogen Lyman-alpha 122 – 121 nm 10.16 – 10.25 eV spectral line at 121.6 nm, 10.20 eV. Ionizing radiation at shorter wavelengths

Vacuum Ultraviolet 200 – 10 nm 6.20 – 124 eV strongly absorbed by atmospheric oxygen, though 150–200 nm wavelengths can propagate through nitrogen

Extreme Ultraviolet 121 – 10 nm 10.25 – 124 eV entirely ionizing radiation by some definitions; completely absorbed by the atmosphere

http://en.wikipedia.org/wiki/Ultraviolet

Discovery

Johann Ritter discovered UV light in 1801.

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Johann Wilhelm Ritter (16 December 1776 – 23 January 1810) was a German chemist, physicist and philosopher. He was born in Samitz (Zamienice) near Haynau (Chojnów) in Silesia (then part of Prussia, since 1945 in Poland), and died in Munich.

http://en.wikipedia.org/wiki/Johann_Wilhelm_Ritter

He continued the work of William Herschel, who discovered infrared light. After hearing about Herschel’s discovery that a form of light beyond visible light had been discovered, Ritter went on to conduct experiments to determine if visible light existed beyond the violet end of the spectrum. He experimented with silver chloride, which turned black when exposed to sunlight, Ritter decided to measure the rate at which silver chloride reacted when exposed to different colours of the spectrum. To do this, he directed sunlight through a glass prism and placed silver chloride on each colour. He then noticed that silver chloride on showed little change in the red part of the spectrum but increasingly darkened towards the violet end of the spectrum. This proved that blue light exposure did cause silver chloride to turn blacker much more efficiently than exposure to red light.

Ritter then placed silver chloride in the area beyond violet end of the spectrum – no sunlight was visible. He saw that silver chloride displayed an intense reaction beyond the violet end of the spectrum – no light could be seen. This proved that there was an invisible form of light beyond the violet end of the spectrum – he called this ‘chemical rays’ which later became to be known as ultraviolet, which literally translates into beyond light [5].

[5] Coolcosmos webpage (NASA): http://coolcosmos.ipac.caltech.edu/cosmic_classroom/classroom_activities/ritter_bio.html

Sources of UV light

The natural source of UV light is from the Sun but artificial sources are “Black lights”, short wave UV lamps, gas-discharge lamps, UV LEDs, UV lasers, plasma and synchrotron sources of extreme UV

image

Two black light fluorescent tubes above left, showing use – The longer tube is a F15T8/BLB 18 inch, 15 watt tube, shown in the bottom image in a standard plug-in fluorescent fixture. The shorter is an F8T5/BLB 12 inch, 8 watt tube, used in a portable battery-powered black light sold as a pet urine detector. Above middle shows a commercial germicidal short wave lamp in butcher shop. Above right shows a 380 nanometre UV LED which makes some common household items fluoresce.

Environmental contexts

UV light is found naturally in sunlight and normally people cannot perceive it directly, since the lens of the human eye blocks mist radiation in the wavelength range of 300-400nm; shorter wavelengths are blocked by the cornea [6].

[6] Violet and blue light blocking intraocular lenses: photoprotection versus photoreception; British Journal of Ophthalmology 90 (6): 784-792

UVC rays are the most harmful to humans. UVB rays are the harmful rays that cause sunburn; UVB rays can cause an increase in the risk of DNA and other cellular damage in living organisms. Fortunately our atmosphere absorbs nearly all UVC rays whilst our atmosphere absorbs 95% of UVB rays. 98.7% of all UV radiation that reaches the earth is UVA [5].

image

The above image shows levels of ozone at various altitudes and blocking of different bands of ultraviolet radiation. In essence, all UVC is blocked by diatomic oxygen (from 100–200 nm) or by ozone (triatomic oxygen) (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. UVA makes up almost all of the ~ 25% of the Sun’s total UV that penetrates the Earth’s atmosphere.

Chemical process in the upper atmosphere can affect the amount of atmospheric ozone that shields us from the sun’s harmful UV radiation. Each year, a ‘hole’ of thinning ozone expands over the Antarctic region and southern parts of South America. These regions are now exposed too much higher levels of UV radiation [5].

If continuation of the depletion of the ozone continues, then more and more people are susceptible of getting UV related problems, such as cancer and sunburn. Not only humans, but studies show that UVB radiation penetrates to a depth of 30 metres in clear water, making it lethal to plankton and ultimately affecting other marine wildlife that rely on plankton and the rest of the food chains. In addition, marine scientists have suggested that a rise in UVB levels in the Southern Ocean between 1970 and 2003 was strongly linked to a simultaneous decline in fish, krill, and other marine life [8].

[8] Encyclopaedia Britannica webpage: http://www.britannica.com/EBchecked/topic/613529/ultraviolet-radiation

UV light can also degrade plastics and cause fading in paints and dyes. Many museums place black curtains over watercolour paintings and ancient textiles, for example. Since watercolours can have very low pigment levels, they need extra protection from UV light. Various forms of picture framing glass, including acrylics (plexiglass), laminates, and coatings, offer different degrees of UV (and visible light) protection.

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UV damaged polypropylene rope (left) and new rope (right)

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Effects of UV on finished surfaces in 0, 20 and 43 hours.

On the other hand, UV radiation also has positive effects in the human body. It stimulates the production if vitamin D in the skin. Vitamin D has several important functions, for example it helps regulate the amount of calcium and phosphate in the body [8]. A lack of vitamin D will result in vitamin D deficiency and this can lead to multiple problems including weak bones and bone deformities [9]. The vitamin also helps to regulate calcium metabolism for the nervous system, improve immunity, cell proliferation, insulin secretion, and blood pressure.

The amount of the brown pigment melanin in the skin increases after exposure to UV radiation at moderate levels depending on skin type; this is commonly known as a sun tan. Melanin is an excellent photoprotectant that absorbs both UVB and UVA radiation and dissipates the energy as harmless heat, protecting the skin against both direct and indirect DNA damage.

[9] Vitamin D; NHS: http://www.nhs.uk/conditions/vitamins-minerals/Pages/Vitamin-D.aspx

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Rickets is a condition usually seen in children where the bones do not mineralise properly. This can be due it lack of exposure to vitamin D.

http://flipper.diff.org/apptags/items/6938

Blocking UV light

Ultraviolet light absorbers are molecules used in organic materials (polymers, paints, etc.) to absorb UV light to reduce the UV degradation (photo-oxidation) of a material. The absorbers can themselves degrade over time, so monitoring of absorber levels in weathered materials is necessary.

In sunscreen, ingredients that absorb UVA/UVB rays, such as avobenzone, oxybenzone and octyl methoxycinnamate, are known as absorbers or chemical “blockers”. They are contrasted with physical “blockers” of UV radiation such as titanium dioxide and zinc oxide.

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The above images demonstrate the effect of sunscreen. The man’s face has sunscreen on his right only. The left image is a regular photograph of the face; the right image is taken by reflected UV light. The side of the face with sunscreen is darker because the sunscreen absorbs the UV light.

Suspended nanoparticles in stained glass prevent UV light from causing chemical reactions that change image colours. A set of stained glass colour reference chips is planned to be used to calibrate the colour cameras for the 2019 ESA Mars rover mission, since they will remain unfaded by the high level of UV present at the surface of Mars.

Common soda lime glass is partially transparent to UVA but is opaque to shorter wavelengths, whereas fused quartz glass, depending on quality, can be transparent even to vacuum UV wavelengths. Ordinary window glass passes about 90% of the light above 350 nm, but blocks over 90% of the light below 300 nm. Wood’s glass is a nickel-bearing form of glass with a deep blue-purple colour that blocks most visible light and passes ultraviolet light.

UV in real life uses

UV has many uses in everyday life.

13.5 nm: Extreme ultraviolet lithography

30–200 nm: Photoionization, ultraviolet photoelectron spectroscopy, standard integrated circuit manufacture by photolithography

230–365 nm: UV-ID, label tracking, barcodes

230–400 nm: Optical sensors in various instruments

240–280 nm: Disinfection, decontamination of surfaces and water (DNA absorption has a peak at 260 nm)

200–400 nm: Forensic analysis, drug detection

270–360 nm: Protein analysis, DNA sequencing, drug discovery

280–400 nm: Medical imaging of cells

300–320 nm: Light therapy in medicine

300–365 nm: Curing of polymers and printer inks

300–400 nm: Solid-state lighting

350–370 nm: Bug zappers (flies are most attracted to light at 365 nm)

http://en.wikipedia.org/wiki/Ultraviolet_light_therapy

http://en.wikipedia.org/wiki/Light_therapy

One use is as a therapeutic agent for diseases such as psoriasis and jaundice.

Because of its bactericidal capabilities between wavelengths of 260nm to 280nm, UV radiation is useful as both a research tool and as a sterilising technique.

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A low-pressure mercury vapour discharge tube floods the inside of a hood with shortwave UV light when not in use, sterilizing microbiological contaminants from irradiated surfaces.

Florescent lamps exploit UV radiation to interact with materials known as ‘phosphors’ that emit visible light; compared with incandescent lamps, florescent lamps are a much more energy efficient form of artificial lighting [8].

Colourless fluorescent dyes that emit blue light under UV are added as optical brighteners to paper and fabrics. The blue light emitted by these agents counteracts yellow tints that may be present, and causes the colours and whites to appear whiter or more brightly coloured.

UV is also used in security services. The main use is to identify markings that show up in UV light – fingerprints to bodily fluids. This can then be used to track down suspects/criminals or to aid in solving a crime.

Since insects can see UV light, UV can be used to kill insects. UV lamps are used to attract insects, which when they touch get electrocuted by high voltage wires. This is very useful in places where malaria thrives, or where food is present.

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Entomologist using a UV light for collecting beetles in the Paraguayan Chaco.

UV light can also be used to detect forged bank notes. Currency in England has a special strip, which can be used to identify whether it is real or fake When a UV light is placed onto the strip, it will glow, proving that it is an original note. Other forms that consider these methods include passports [10].

[10] Uses of UV: http://www.darvill.clara.net/emag/emaguv.htm

http://en.wikipedia.org/wiki/Ultraviolet_photography

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Ultraviolet image on the left of the globular cluster NGC 1851 in the southern constellation Columba; Image on the right is an aurora at Jupiter’s north pole as seen in ultraviolet light by the Hubble Space Telescope.

Corona discharge on electrical apparatus can be detected by its ultraviolet emissions. Corona causes degradation of electrical insulation and emission of ozone and nitrogen oxide.

Some EPROM (erasable programmable read-only memory) modules are erased by exposure to UV radiation. These modules have a transparent (quartz) window on the top of the chip that allows the UV radiation in.

Ultraviolet radiation is also used in sunbeds, for when people want a tan. Although this is beneficial for the companies, it can also be quite dangerous. The human body is taking a direct source of UV radiation; this causes the skin to darken and thus, the tan. However, too much of this can lead to serious health complications such as sever sunburn, skin cancer and eye damage.

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