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
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
Ultraviolet light is emitted by very hot
objects with temperatures of over 4000oC 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
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.
(often organic molecules), absorb higher energy ultraviolet radiation and
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
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
plants sterilise the water by exposing it to uv radiation which kills harmful
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
Strong uv light can damage your eyes
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
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
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 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
See section 10 for more
on the dangers of ionising radiation