7. The properties, uses and dangers of ultraviolet radiation

The sources and properties of ultraviolet radiation

Ultraviolet light is produced when a gas is subjected to a high voltage discharge.

The atoms of the gas are excited to a high electronic energy state - electrons are promoted to a higher energy level (shell).

The electrons of the excited atoms drop down to lower more stable electronic energy levels by losing energy in the form ultraviolet FM radiation.

When ultraviolet light is absorbed, some of the wave energy is converted into heat, BUT uv radiation can cause ionisation - the process of knocking off outer electrons of atoms to create positive ions (see Part 10 ionisation for more details) - so uv light is an ionising radiation.

Ultraviolet light can electronically excite atoms or molecules so that they give off visible light photons as the electrons fall back down to lower more stable energy levels - this is called fluorescence (an example of luminescence).

The re-emitted radiation is of longer wavelength and lower frequency of the EM radiation absorbed.

The absorption of uv light and emission of visible light by molecules causes very bright 'fluorescent' light colours to appear - the molecules are made to fluoresce by the uv light.

Ultraviolet light is emitted by very hot objects with temperatures of over 4000^oC e.g. the Sun, but this is not usually considered a practical source of uv radiation.

 

The uses of ultraviolet radiation

Small doses of ultraviolet rays are good for us - they are absorbed by the skin and the energy helps in the synthesis of vitamin D.

Vitamin D helps regulate the amount of calcium and phosphate in the body. These nutrients are needed to keep bones, teeth and muscles healthy. A lack of vitamin D can lead to bone deformities such as rickets in children, and bone pain caused by a condition called osteomalacia in adults.

Producing decorative bright fluorescent colours with fluorescent materials (mechanism explained above).

 

Fluorescent lights use uv radiation to make materials emit light - they are much more energy efficient than filament bulbs for large scale multi-hour lighting e.g. in an office or classroom.

The uv radiation is created by a high voltage discharge in a low pressure gas, the excited electrons lose energy in the form of uv radiation.

The uv radiation strikes a fluorescent phosphorus coating on the inside of the glass light tube where it is absorbed and re-emitted as visible light.

These kind of lights are energy efficient and useful when lights are used for long periods of time e.g. shops, factories and classrooms!

Very little, if any, uv light is emitted from the outer surface of fluorescent lights.

 

Fluorescent materials (often organic molecules), absorb higher energy ultraviolet radiation and become 'excited'.

On relaxing to their normal lowest energy state, the molecules re-emit radiation as visible light, and that's what we call fluorescence.

You can mark objects with a security pen with ink that is invisible in visible light.

When uv light is shone on the ink markings they become visible due to the ink fluorescing.

This can be used to identify stolen property.

A similar technique is used to detect forgeries of bank notes and passports - the genuine bank notes or passports are printed with special markings that only show up when illuminated with uv light.

 

People give themselves an artificial sun-tan with UV lamps in tanning salons or you can just sit out in the Sun which radiates ultraviolet radiation.

These are a life-style choices - definitely not any of mine - why take a risk?

Your skin naturally produces the dark pigment melanin, and more so when exposed to extra uv light.

Melanin absorbs uv radiation to protect skin cells from damage, but over exposure to uv can cause skin damage - this happens particularly to pale coloured people who are exposed to a lot of bright sunlight, hence a lot of uv radiation.

You must increase your risk of skin damage, but it is a personal decision as to whether you feel the risk of cancer is real enough for you to avoid the salon or sitting out in the Sun without enough sun-blocker!

Since uv radiation can damage and kill cells, some water treatment plants sterilise the water by exposing it to uv radiation which kills harmful bacteria.

 

The dangers of ultraviolet radiation

UV photons are of shorter wavelength/higher frequency than visible light and so carry more energy and are the first of three types of EM radiation in our sequence that can cause ionisation and cause biological harm to cells - life!

UV photons have sufficient energy to collide with molecules knocking off electrons - a process we call ionisation.

Strong uv light can damage your eyes and possibly cause blindness - over exposure is not recommended!

People with very 'light sensitive' eyes wear shaded glasses to reduce the intensity of uv (or visible) light hitting the retina at the back of the eye.

Ultraviolet light can penetrate the skin and be absorbed by the cells causing damage.

You can also suffer from tissue damage (uv burn) or even radiation sickness.

This is why in bright sunlight you should use sunscreens (sun-blockers), which absorb the harmful uv radiation.

Ultraviolet light causes premature aging of skin - wrinkles and darker pigmentation spots.

If the cell damage involves the DNA then cancerous cells can multiply from the genetic mutation, which can lead to skin cancer.

The melanin in your skin is an effective absorbent of uv light and this dark pigment can dissipate most of the incoming uv radiation.

This is why fair-skinned people should be most cautious out in bright sunlight and use sun-blocker appropriately.

 

You should know that the ozone layer in the upper atmosphere partially protects us from potentially harmful uv radiation.

UV radiation from the Sun is absorbed by oxygen molecules (O₂) to form ozone (O₃) in the upper atmosphere.

Ozone molecules are very good absorbers of potentially damaging uv radiation. Therefore the ozone layer of the Earth's atmosphere protects us from the harmful effects of uv radiation - skin cell damage - burns and genetic damage leading to skin cancer.

For more details see Ozone, effect of CFC's, free radicals notes.

See section 10 for more general comments on the dangers of ionising radiation