Early in
nineteenth century a scientist called William
Herschel discovered that if a sensitive thermometer was held in the various colours of the
spectrum, the reading on the thermometer went up. This shows that all colours
of light produce some heat. When Herschel held the thermometer just beyond the red end
of spectrum the reading on the thermometer suddenly shot up. His discovery
suggested that there were invisible rays with longer wavelengths than that of
visible light. These produced more heat
than the rays of visible light. These rays are called infra-red rays. All bodies above absolute zero give off these rays.
To detect
infra-red rays (radiant-heat) we use a thermocouple. These rays can be refracted
just as light rays. Other invisible rays exist whose wavelength is just a little shorter than that of violet
light. They are called Ultraviolet rays and have
several uses. They kill germs in operating rooms, they enable the skin to
manufacture vitamin D, and they cause certain chemicals to shine brightly even
in the dark.
Other waves
which have wavelengths longer than those of infra-red are radio waves, X-rays
and gamma rays. These have much shorter wavelengths than that of ultraviolet.
All these radiations, including light, form what is called “electromagnetic spectrum”. Their
sources and properties are discussed in details below.
The electromagnetic spectrum is the range of all
possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an
object is the characteristic distribution of electromagnetic radiation emitted
or absorbed by that particular object.
The electromagnetic spectrum extends from low
frequencies used for modern radio to gamma radiation at the short-wavelength end, covering
wavelengths from thousands of kilometres down to a fraction of the size of an atom. The long wavelength limit is the size of the universe itself,
while it is thought that the short wavelength limit is in the vicinity of the Planck length, although
in principle the spectrum is infinite and continuous.
The electromagnetic spectrum and sources of each type of radiation
INFRARED RADIATION
The infrared part of the electromagnetic spectrum covers the range
from roughly 300 GHz (1 mm) to 400 THz (750 nm). It can be
divided into three parts:
1.
Far-infrared, from 300 GHz (1 mm) to 30 THz (10 μm). The
lower part of this range may also be called microwaves. This radiation is
typically absorbed by so-called rotational modes in gas-phase molecules, by
molecular motions in liquids, and by phonons in solids.
The water in the Earth's atmosphere absorbs so strongly in this range that it
renders the atmosphere effectively opaque. However, there are certain wavelength ranges
("windows") within the opaque range which allow partial transmission,
and can be used for astronomy. The wavelength range from approximately 200 μm
up to a few mm is often referred to as "sub-millimetre" in
astronomy, reserving
far infrared for wavelengths below 200 μm.
2.
Mid-infrared, from 30 to 120 THz (10 to 2.5 μm). Hot objects (black-body radiators) can radiate strongly in this range. It is absorbed by
molecular vibrations, where the different atoms in a molecule
vibrate around their equilibrium positions. This range is sometimes called the fingerprint
region since the mid-infrared absorption spectrum of a compound is very
specific for that compound.
3.
Near-infrared, from 120 to 400
THz (2,500 to 750 nm). Physical processes that are relevant for this range
are similar to those for visible light.
Our bodies
detect infrared radiation (IR) by
its heating effect on the skin. It is sometimes called ‘radiant heat’ or ‘heat
radiation’.
Anything
which is hot but not glowing, i.e. below 500°C, emits IR alone. At about 500°C
a body becomes red-hot and emits red light as well as IR – the heating element of an electric fire, a toaster or a grill are
example. At about 1500°C things such as lamp filaments are white-hot and
radiate IR and white light, i.e. All the colours of the visible spectrum.
Infrared is
also detected by special temperature sensitive photographic films which allow
pictures to be taken in the dark. Infrared sensors are used on satellites and
aircraft for weather forecasting, monitoring of land use, assessing heat loss from buildings and locating victims of
earthquakes.
Infrared
lamps are used to dry the paint on cars during manufacture and in the treatment
of muscular complaint. Remote control keypads for televisions contain a small
infrared transmitter for changing programs.
ULTRAVIOLET RADIATION
This is radiation whose wavelength is shorter than
the violet end of the visible spectrum, and longer than that of an X-ray.
Ultraviolet
(UV) rays have shorter wavelengths
than light. They cause sun-tan and produce vitamins in the skin but can
penetrate deeper, causing skin cancer. Dark skin is able to absorb more UV so
reducing the amount reaching deeper tissues. Exposure to the harmful UV rays
present in sunlight can be reduced by wearing protective clothing such as a hat
or using sunscreen lotion.
Ultraviolet
causes fluorescent paints and clothes washed in some detergents to fluoresce.
They glow by re-radiating as light the energy they absorb as UV. This may be
used to verify ‘invisible’
signatures on bank documents.
A UV lamp
used for scientific or medical purposes contains mercury vapour and this emits
UV when an electric current passes through it. Fluorescent tube also contains
mercury vapour and their inner surfaces are coated with special powder called phosphors which radiate
light.
Being very energetic, UV can break chemical bonds,
making molecules unusually reactive or ionizing them, in general changing their
mutual behaviour. Sunburn, for
example, is caused by the disruptive effects of UV radiation on skin cells, which is the main cause of skin cancer, if the radiation irreparably damages the complex DNA molecules
in the cells (UV radiation is a proven mutagen). The Sun
emits a large amount of UV radiation, which could quickly turn Earth into a
barren desert. However, most of it is absorbed by the atmosphere's ozone layer before reaching the surface.
RADIO WAVES
Radio waves have the longest wavelength in the electromagnetic spectrum. They are
radiated from aerials and used to carry sound, pictures and other information
over long distances.
1.
Long, medium and short waves (wavelength of 2 km to 10m)
These
diffract round obstacles so can be received when hills etc. Are in their way. They
are also reflected by layers of electrically charged particles in the upper
atmosphere (the ionosphere), which makes long-distance radio reception
possible.
2.
VHF(Very High
Frequency) and UHF (Ultrahigh Frequency) waves (wavelength of 10m to 10cm)
These
shorter wavelength radio waves need a clear, straight-line path
to the receiver. They are not reflected by the ionosphere. They are used for
local radio and for television.
3.
Microwaves (wavelengths of a few centimetres)
These are
used for international telecommunications and television relay via
geostationary satellites and for mobile phone networks via microwave aerial
towers and low-orbit satellites. The microwave signals are transmitted through
the ionosphere by dish aerials, amplified by the satellite and sent back to
dish aerials in another part of the world.
Microwaves
are also used for radar detection of ships and aircraft, and in police
speed traps.
Microwaves
can be used for cooking since they cause water molecules in the moisture of the food to vibrate vigorously at the
frequency of the microwaves. As a result, heating occurs inside the food which
cooks itself.
Living
cells can be damaged or killed by the heat produced when microwaves are
absorbed by water in the cells. There is some debate at present as to whether
their use in mobile phones is harmful; hands-free mode, where separate
earphones are used, may be safer.
The super high frequency (SHF) and extremely high frequency (EHF) of microwaves come next up the frequency scale. Microwaves are waves which are
typically short enough to employ tubular metal waveguides of
reasonable diameter. Microwave energy is produced with klystron and magnetron tubes, and with solid state diodes such as Gunn and IMPATT devices. Microwaves are absorbed by molecules that
have a dipole moment in liquids. In a microwave oven, this effect is used to heat food. Low-intensity
microwave radiation is used in Wi-Fi, although
this is at intensity levels unable to cause thermal heating.
Volumetric heating, as used by microwaves, transfers
energy through the material electromagnetically, not as a thermal heat flux.
The benefit of this is a more uniform heating and reduced heating time;
microwaves can heat material in less than 1% of the time of conventional
heating methods.
When active, the average microwave oven is powerful
enough to cause interference at close range with poorly shielded
electromagnetic fields such as those found in mobile medical devices and cheap
consumer electronics.
X-RAYS
These are
produced when high-speed electrons are stopped by a metal target in an x-rays
have smaller wavelengths than UV.
They are
absorbed to some extent by living cells but can some solid objects and affect a
photographic film. With materials like bones, teeth and metal which they do not
pass through easily, shadow pictures can be taken, like that of someone
shaving. In industry, x-ray photography is used to inspect welded joints.
X-ray
machines need to be shielded with lead since normal body cells can be killed by
high doses and made cancerous by lower doses.
GAMMA RAYS
After hard X-rays come gamma rays, which were discovered by Paul Villard in 1900. Gamma rays are more
penetrating and dangerous than x-rays. These are
the most energetic photons, having no
defined lower limit to their wavelength. They are useful to astronomers in the
study of high energy objects or regions, and find a use with physicists thanks
to their penetrative ability and their production from radioisotopes. Gamma rays are also used for the irradiation of food to kill harmful bacteria and seed for
sterilization, on surgical instruments and in medicine they are used in radiation cancer therapy to kill cancer cells and some kinds of diagnostic imaging such as PET scans. The wavelength of gamma rays can be measured with
high accuracy by means of Compton scattering.
Note that there are no precisely defined boundaries
between the bands of the electromagnetic spectrum. Radiations of some types
have a mixture of the properties of those in two regions of the spectrum. For
example, red light resembles infrared radiation in that it can resonate some chemical bonds.
VISIBLE RADIATION (LIGHT)
Above infrared in frequency comes visible light. This is the range in which the sun and stars similar to
it emit most of their radiation. It is probably not a coincidence that the human eye is sensitive to the wavelengths that the sun emits most strongly.
Visible light (and near-infrared light) is typically absorbed and emitted by
electrons in molecules and atoms that move from
one energy level to another. The light we see with our eyes is really a very
small portion of the electromagnetic spectrum. A rainbow shows the
optical (visible) part of the electromagnetic spectrum; infrared (if you could
see it) would be located just beyond the red side of the rainbow with ultraviolet appearing just beyond the violet end.
Electromagnetic radiation with a wavelength between 380 nm and
760 nm (790–400 terahertz) is detected by the human eye and perceived as
visible light. Other wavelengths, especially near infrared (longer
than 760 nm) and ultraviolet (shorter than 380 nm) are also sometimes
referred to as light, especially when the visibility to humans is not relevant.
If radiation having a frequency in the visible region
of the EM spectrum reflects off an object, say, a bowl of fruit, and then
strikes our eyes, this results in our visual perception of the scene. Our brain's visual system processes the
multitude of reflected frequencies into different shades and hues, and through
this not-entirely-understood psychophysical phenomenon, most people perceive a
bowl of fruit.
At most wavelengths, however, the information carried
by electromagnetic radiation is not directly detected by human senses. Natural
sources produce EM radiation across the spectrum, and our technology can also
manipulate a broad range of wavelengths. Optical fibre transmits light which, although not suitable for direct
viewing, can carry data that can be translated into sound or an image. The
coding used in such data is similar to that used with radio waves.
In summary, you can use the following technique to
master all the components of the electromagnetic spectrum, according to the
order of frequency and wavelength.
Roast Maize Is Very Unusual X-mas Gift |
Radio waves Microwaves Infrared Visible light Ultraviolet X-ray Gamma rays |
|
High frequency/ short wavelength |
The
mnemotechnique to remember the spectrum arrangement.
PROPERTIES OF ELECTROMAGNETIC RADIATION
Light is one member of a family of electromagnetic
radiation, which forms a continuous spectrum beyond both ends of the visible (light)
spectrum. While each type of radiation has a different source, all result from
electrons in atoms undergoing an energy change and all have
certain properties in common:
1.
All types of electromagnetic radiation travel through space at 300000km/s (
2.
They exhibit
interference, diffraction and polarization, which suggests they have a
transverse wave nature;
3.
They obey the
wave equation
4.
They carry energy
from one place to another and can be absorbed by matter to cause heating and
other effects. The higher the frequency and the smaller the
wavelength of the radiation, the greater is the energy
carried, i.e. gamma rays are more “energetic” than radio waves. This is shown
by the Photoelectric effect in which electrons are ejected from metal
surfaces when electromagnetic waves increase so does the speed (and energy)
with which electrons are emitted.
Because of
its electrical origin, its ability to travel in a vacuum (e.g. from the sun to
the earth) and its wave-like properties (i.e. 2 above), electromagnetic radiation
is regarded as a progressive transverse
wave. The wave is a combination of travelling electric and
magnetic fields. The fields vary in value and are directed at right angles to
each other and to the direction of travel of the wave.
Figure: 9.75 Electromagnetic wave.
DETECTION AND APPLICATIONS OF ELECTROMAGNETIC RADIATIONS
The means
of detection, the production and application of the components of
electromagnetic spectrum are summarised in the following table.
RADIATION |
PRODUCTION |
DETECTION |
APPLICATION |
Radio waves |
Oscillating
electrical circuit |
Antennae
and aerials |
Communications
(radio broadcasts, television and satellite communication, cellular
telephones, radar and navigation equipment). |
Microwaves |
Magnetrons
and klystrons (special vacuum tubes in microwave oven) |
Antennae
and aerials |
Communication
(mobile phones), radar cooking (microwave cookers), speed cameras. |
Infrared |
Thermal
vibration of atoms in hot bodies |
Heat
sensor, photographic film, semi-conductor devices. |
Medical
diagnosis (finding hot spots in the body) burglar alarms, military night
vision equipment, thermal imaging cameras, and remote controls for TV’s and
videos, green houses. |
Visible light |
Energy
level changes of electrons in atoms (anything that is hot enough to glow e.g.
the sun). |
Eye,
photographic film, photo-electric effect, semi-conductor devices
fluorescence. |
Plant
growth, sight, photography, optical fibres and laser beams (used in laser
printers, weapon aiming systems, compact disc players). |
Ultraviolet |
Energy
level changes of electrons in atoms (the sun, sparks and mercury vapour
lamps) |
Photographic
film, photoelectric effect, semi-conductor devices, fluorescence. |
Killing
bacteria, skin treatment, as a source of vitamin D, making ink that fluoresce
(for security marking like paper money), make clothes to glow, hardening some
types of dental fillings. |
x-rays |
Bombarding
metal with high energy electrons in x-ray tubes, i.e. energy level changes in
innermost shells) |
Photographic
films, ionisation detectors. |
Radiography
(identifying internal body structures like bones), cancer therapy,
crystallography (studying crystal structure), airport security checks. |
Gamma rays |
Energy
changes in the nuclei of radioactive atoms. |
Photographic
film, ionisation detectors, scintillation counters. |
Sterilising
food and medical instruments, killing cancer cells and other malignant
growths, controlling pests in grains, detecting flaws in metals. |
Table 9.7:
Production, detection and applications of electromagnetic spectrum
THE EFFECT OF RADIATION ON HUMAN POPULATION
Radiation
occurs when unstable nuclei of atoms decay and release particles. There are many
different types of radiation. When these particles touch various organic
materials such as tissue, damage may, and probably will, be done. Radiation can
cause burns, cancers, and death.
DANGERS OF ELECTROMAGNETIC WAVES
Radio waves:
large dose can cause cancer, leukaemia, and other disorders
Microwaves:
Prolonged exposure can cause eye defect (cataracts). Recent research indicates
that microwaves from mobile phones can affect parts of the brain.
Infrared:
can cause overheating
Visible
light: too much light can damage the retina in the eye.
Ultraviolet:
large doses can damage the retina in the eyes. Large doses can also cause
sunburn and skin cancer.
X-rays: Can
cause cell damage and cancers.
Gamma rays:
can cause cell damage, cancers and mutations in growing tissues.
CELLPHONES AND ELECTROMAGNETIC RADIATION
Cellphone radiation and
health concerns have been raised, especially following the enormous increase in
the use of wireless mobile telephony throughout the world. This is because
mobile phones use electromagnetic waves in the microwave range. These concerns
have induced a large body of research. Concerns about effects on health have
also been raised regarding other digital wireless systems, such as data
communication networks. The World Health Organisation has officially ruled out
adverse health effects from cellular base stations and wireless data networks,
and expects to make recommendations about mobile phones in 2007-08.
Cellphone users are
recommended to minimise radiation, by for example:
1. Use hands-free to decrease the radiation to the head.
2. Keep the mobile phone away from the body.
3. Do not telephone in a car without an external antenna.
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