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9.4 Information Systems 3. Different waves have different properties
Syllabus reference (October 2002 version) |
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| 3. Eletromagnetic waves have different properties which are utilised in a range of communication systems through air and space | Students learn to:
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Students: |
Extract from Senior Science Stage 6 Syllabus (Amended October 2002). Board of
Studies, NSW.
[Edit: 28 Aug 08]
Prior learning: ScienceStages 4–5 syllabus: Outcome 4.6 (content 4.6.5:
light energy), Outcome 5.6 (content 5.6.1: the wave model).
identify that where information systems cannot be physically linked the information may be transmitted in wave form through the atmosphere or space
- Many information systems rely on physical links, such as cables and optic fibres. In some instances, such links may not be physically possible, say for geographic reasons, or may not be economically viable.
- Communication can still be achieved by using electromagnetic waves, which carry the information to be transmitted in the form of a code. A transmitting antenna produces waves that travel through air or space at great speed and over a range of distances. This may be several metres or many thousands of kilometres. A receiving antenna tuned to particular frequencies will detect the signal and relay this to the receiving communication device.
identify
the properties of energy from the electromagnetic spectrum that make it useful
in communication technologies including its:
– speed of travel
– ability to travel in a straight line
– ability to be reflected
Speed of travel
- Electromagnetic waves transmit energy from one place to another at the speed of 300 million metres per second. The very high speed of electromagnetic radiation (EMR) means that the time between sending, receiving and decoding a signal is almost instantaneous for Earth-based systems.
Ability to travel in a straight line
- Electromagnetic waves travel in a straight line unless there is a change in the medium through which they are travelling. This makes using them predictable.
Ability to be reflected
- Reflection is the change in direction of a wave due to its bouncing off a boundary between two media. This effect is used by shortwave AM radio frequencies. Signals are deliberately bounced off the ionosphere.
- Scattering occurs when shorter wavelengths strike an object and are reflected in many directions, e.g. microwaves can be scattered by rain. Scattering weakens the signal.
Other properties
- Refraction is the change in direction of a wave when it moves into a new medium. This causes a change in the wave velocity.
- Absorption occurs when waves pass through a medium, e.g. when white light is shone onto a black surface, most of the light is absorbed. In general, the shorter the wavelength, the more easily absorbed it is (by solid objects and liquids).
- Wave intensity is a measure of the energy carried by a wave. The stronger the signal (energy content), the further away from the transmitter it can be detected and used. The power used by local radio stations is less than that used by stations transmitting nationally.
The electromagnetic spectrum and the properties of its components 
Goddard Space Flight Center's web site, NASA, USA.
plan choose equipment or resources for, and perform a first-hand investigation to compare the quality of reception of AM and FM radio waves
- You could plan an investigation as a series of simple tests using a radio. One example would be to listen to the clarity of communication as you walk a pre-planned path around and through buildings. You will need to consider what characteristics of AM and FM radio you will compare and how you will measure or rate these, e.g. clarity of signal at different times of the day. Consider different conditions to which you could test the radio’s reception, such as through different materials, under different weather conditions or the effect of other nearby sources of electromagnetic radiation (EMR).
- When choosing equipment or resources, use a single radio with both AM and FM reception to help reduce some of the variables to be considered in your investigation. Choose an AM station and an FM station that sound clear under optimal conditions.
- Perform the investigation several times under different conditions to gain the most reliable results.
- Your comparison of the data obtained could be recorded in a table. You could measure:
-the strength of the signal (Did it fade under certain conditions and then return?)
-the clarity of signal (Was there interference?) and
-the quality of the sound.
describe the individual properties of visible light, radio waves (AM, FM, TV waves) and microwaves and relate these to their use in communication systems
- The properties of the types of the electromagnetic radiation (EMR) used in communication
technologies affect their use.
Type of EMR
(Properties)Use in communication systems
Visible light (Frequency range:
1014 – 1015 hertz
Wavelengths: approximately 10-4 metre)
The frequencies of light are the highest available for practical communication systems. Light has the highest information carrying capacity of all the communication systems currently in use. Light can be refracted and reflected to achieve total internal reflection.Optical fibres are used to transmit light pulses generated by an electrical signal. The use of the fibre ensures privacy and an energy efficient way of sending information.
Microwaves (Frequency range:
109 – 1012 Hertz
Wavelengths: approximately 0.01 metre)
Microwaves are easily absorbed and scattered by particles in the atmosphere. Thus they need directional aerials for transmission and reception to ensure sufficient signal strength.
Special equipment is needed to generate microwaves.
TV(Frequency range:
107 – 108 Hertz
Wavelengths: approximately 0.1 – 1 m)
Signals have to be able to carry both sound and visual information and so a wider bandwidth is required for each TV station than for the radio stations.For best reception, external aerials are designed precisely to match the wavelengths used by the TV stations. They also need to be orientated carefully to receive the signal at the optimum angle.
FM radio (Frequency range:
approximately 106 Hertz
Wavelengths: approximately 1 m)
The audio signal changes the frequency of the carrier wave producing an FM signal. FM waves are affected less by electrical interference and produce a higher quality transmission of sound.
AM radio (Frequency range:
540 – 1600 kilohertz
Wavelengths: approximately 100 m – 1 km)
Easy to do with existing technology. Historically, practical considerations were important. The size of components for transmitters and receivers dictated the wavelengths that were selected for use.This part of the electromagnetic spectrum is not greatly affected by the atmosphere (i.e. not absorbed or scattered).
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