Physics 5054 O level, notes

1. ELECTROMAGNETIC
SPECTRUM
Physics, Grade 9,
Anwar Mohamed

2. WHAT ARE ELECTROMAGNETIC WAVES?
• In terms of classical wave theory, a very hot object such as any star or sun produces a range
of electromagnetic waves. (E.g. The Sun produces ultraviolet light in addition to visible
light) Electromagnetic waves are often collectively referred to as electromagnetic radiation.
• Electromagnetic radiation is produced by acceleration or sudden movement of electrons.
The electron in motion constitutes the electric current that generates the magnetic field in
the wave.
• Electromagnetic waves are transverse waves, consisting of electric and magnetic waves at
right angles to each other and perpendicular to the direction of wave propagation.
• Electromagnetic waves can travel through vacuum and do not need a medium to traverse.
They can travel through solids, liquids and gases.
• All electromagnetic waves have the same speed in vacuum of 3 x 108 m s-1 (commonly
referred to as the ‘speed of light’). But the speed slows down in other material media.

3. THE ELECTROMAGNETIC SPECTRUM

4. • The spectrum of the radiation emitted by a hot body is continuous because
there are many different kind of oscillators in any real lump of matter, so that in
practice radiation exists at all frequencies.
• The main components of the electromagnetic spectrum are gamma-rays, x-rays,
ultra-violet, visible light, infra-red, microwaves and radio-waves.
• Gamma-rays and X-rays are the highest-energy electromagnetic waves and
able to penetrate matter easily. They have the shortest wavelengths and highest
frequencies. Gamma rays may be emitted by radioactive substances, while X-rays
may be produced by X-ray tubes.
• Ultra-violet(UV) radiation is the next highest-energy. It may be produced by
very hot bodies, such as the sun. The atmosphere filters most of the UV radiation
from the sun. UV radiation also causes sun-burned skins.

5. • The visible light is the only electromagnetic radiation that is visible to our eyes. It
occupies a surprisingly small portion in the electromagnetic spectrum. Sun and
other white-hot bodies emit white light, which is made up of violet, indigo, blue,
green, yellow, orange and red colours. (in order of reducing frequencies).
• Infra-red radiation (IR) may be produced by any hot bodies similar to white light,
but with longer wavelengths. IR radiation is readily absorbed by matter and raises
their temperature.
• Microwave radiation is generated by special electronic devices known as klystron
valves. Microwave receivers can be used to detect microwaves. They are reflected by
metals and partly absorbed by non-metals.
• Radio waves have the longest wavelengths, varying from a few centimetres to
thousands of metres. These may be generated by oscillations in special electronic
circuits and by electrical sparks and are detected by metal aerials.

6. REMEMBER THIS : RONALD MCDONALD IS VERY UGLY EXCEPT GARY

7. APPLICATION OF RADIO WAVES
• Radio waves can be used to carry messages and simple television pictures at
very large distances. Radio-waves are grouped into bands; each band has
particular nature and special set of uses.
• In order to reach greater distances than line of sight, some radio waves can
be made to travel towards the sky. (sky waves) The waves are then refracted
by the ionosphere (layers of air stretching from 80 – 550 km above the Earth)
and returned later to Earth. These wave signals are subsequently intercepted
by receiving ground stations.

8. APPLICATION OF MICROWAVES
• Usage of microwaves in satellite television and telephone
• Microwaves are used to carry telephone calls and television channels as
digital signals from ground station to satellites and vice versa. They are used
for communications because they pass through the atmosphere without
significant attenuation and they can be directed in beams with much less
diffraction than radio-waves.
• The micro-wave frequencies are of the order of 10 GHz, sufficiently high to
carry much more information than a copper cable or radio-waves. They allow
thousands of telephone calls and several television channels to be
transmitted at the same time.

9. APPLICATION OF INFRA-RED WAVES
• Usage of infra-red waves in household appliances, television controllers and intruder
alarms
• Infra-red radiation is emitted or absorbed by every object. We feel warm under the sun as
we absorb more IR radiation than we emit (gain internal energy). We feel cool in an air-
conditioned room as we emit more IR radiation than we absorb (lose internal energy).
• IR radiation is commonly used in wireless remote control units for air-conditioners,
television, sound systems and similar devices. The remote unit emits coded IR that is
detected by the appliances.
• IR radiation can be detected using electronic sensors, as in an intruder alarm system.
Modern IR detectors are solid state detectors which act rather like TV cameras, but in the IR
range.
• (Interesting tip: Your smartphones’ camera is able to detect infra-red. You can check this
out by pointing a wireless remote control at the camera. The infra-red radiation will register
as a red light)

10. APPLICATION OF LIGHT
• Usage of light in optical fibres, in medical uses and telecommunications
• Endoscopes are used to get the images of hollow organs by sending a beam
of light through optical fibres. In an endoscope there are two bundles of very
narrow optical fibres. The illumination bundle carries the light to the object
being studied, and the image bundle carries back reflected light to provide
the image.
• Optical fibres are increasingly being used to replaced coaxial cables, the
messages being converted into pulses of light rather than as radio waves.
Since the frequency of light (~107bits s-1) is much higher than the radio
waves (~102 bits s-1), optical fibres are capable of handling vast amounts of
information.

11. APPLICATION OF ULTRA-VIOLET
• Usage of ultra-violet in sunbeds, fluorescent tubes and sterilization
• UV radiation is energetic enough to ionize atoms and so can cause harm to living
tissue. Under excessive exposure, it will cause sunburn and skin cancers.
• It is used benefically in the sterilization of water, by killing off harmful living
organisms present in the water for potable uses.
• Paper money are coated with invisible chemicals that only become reflective(visible)
when UV radiation is incident on the paper. The main aim is to distinguish the real
money from its counterfeits easily.
• Florescent tubes contain mercury vapour and their inner surfaces are coated with
florescent poweders. Fluorescent materials absorb “invisible” ultraviolet radiation
emitted by excited mercury atoms and re-radiate as “visible” light.

12. APPLICATION OF X-RAYS
• Usage of x-rays in hospital use and engineering applications
• X-ray photographs are used for medical diagnosis Soft X-rays penetrate flesh
more readily than they do bone, which makes X-ray ‘shadow’ photographs
possible.
• In industry, X-ray photographs are used to check for flaws in welded metal
joints.

13. APPLICATION OF GAMMA RAYS
• Usage of gamma rays in medical treatment
• Gamma rays are ionizing radiation which can kill living cells. They are used to
treat malignant tumours in radiotherapy. For treatment deep within the
body, high energy photons are sent to reach the target tumour without
affecting the surrounding tissue. Though x-rays are also ionising radiation,
because of the lower energy compared to gamma rays, they may fail to reach
sufficiently deep into the body and may cause instead damage to the
surrounding tissues that absorbed them.
• Gamma rays can also act as surgical knifes in surgery.