Radio waves are also generated by vibrating or oscillating electrons (e.g. in a transmitting aerial), and are used in radio and television
broadcasting. Radio waves have wavelengths of hundreds of metres to tens of centimetres
and are classified into several categories. Long radio waves
are useful for communicating around the Earth, as they bend to follow the Earth’s surface
Marconi
Italian engineer Guglielmo Marconi is generally credited with inventing radio. In 1895
he transmitted a signal 2.4 kilometres in the grounds of his father’s property. He patented the first ‘wireless telegraphy’
system in 1896.
The behaviour of different types of radio waves
space satellite ionosphere reflects medium radio waves long radio waves diffract round the Earth short radio waves
(microwaves) pass through the ionosphere microwaves used for line-of-sight links Fig 3.3.12
(bending around objects like this is called diffraction). Short waves may also travel around the Earth, by reflecting from the ionosphere.
AM and FM
You are probably aware of the terms AM and FM when referring to radio stations, but what do they really mean?
Electromagnetic waves such as radio waves can carry information (e.g. sound or vision) as changes or fluctuations in either frequency or amplitude. Receivers detect these changes and convert them back to sound or vision or some other form.
This information first must be converted into a wave, in a process called modulation.
Amplitude modulation, or AM, is the process in which information is carried as changes in wave amplitude. Similarly, frequency modulation or FM is the process in which information is carried as changes in wave frequency.
Radio stations transmit sound using both AM and FM, while television stations transmit sound using FM, and vision using AM. Australia’s national broadcaster, the ABC, transmits AM carrier waves of frequency 774 kilohertz (1 kilohertz = 1000 hertz), which will be detected by a radio tuned to this frequency.
The higher frequencies of FM stations are less affected by interference, and provide a better quality sound than AM, though they have less range.
Waves in communication
Waves in communication
UNIT
UNIT
Sound wave (pressure wave) Microphone Oscillator Amplifier Amplified electrical signal Electrical signal Electric carrier wave added FM (frequency modulation) AM (amplitude modulation) Radio antenna ReceiverWaves cause electrons in radio antenna to vibrate.
Modulated waves transmitted by radio mast
Electrical signal created and amplified. Carrier wave removed from electrical signal (demodulation).
Speaker converts electrical signal to sound waves.
Fig 3.3.13
Modulation is one of many steps in the transmission of sound via radio waves.
Worksheet 3.3 Electromagnetic spectrum
3.3
UNIT
[
Questions]
Checkpoint
Two kinds of waves
1 State the names of the two kinds of waves. 2 Outline the main differences between them.
Light waves
3 Copy and complete the following statement:
A light wave is made up of changing ________ and ________ fields that are ________ to each other, and moves at ________ metres per second.
4 State whether the following statements are true or false. a All electromagnetic waves move at the same speed. b Each different colour of light has a different
wavelength.
c The visible spectrum contains the electromagnetic
spectrum.
d Waves transfer energy by moving particles along
with them.
Other types of electromagnetic waves
5 State one similarity and one difference between the
waves of the electromagnetic spectrum.
6 State the speed of light.
7 List the main types of waves in the electromagnetic
spectrum in order from shortest to longest wavelength.
AM and FM
8 AM and FM radio have modulated wavelengths. State the full name for ‘AM’ and ‘FM’.
9 Outline the purpose of modulating radio waves. 10 State an advantage of each type of carrier wave.
Think
11 Explain why it does not make sense to talk about the
wavelength of white light.
12 Identify which colour of light has the: a greatest wavelength
b highest frequency
13 State which colours the human eye is most sensitive to. 14 Infra-red cameras can help find a lost bushwalker.
Outline how this is possible.
15 Identify the radio wave that can penetrate the Earth’s
atmosphere.
16 State the name of the harmful rays that are released in
a nuclear explosion.
17 Is UV radiation good, bad or both? Justify your
answer.
18 Explain how a Geiger counter and gamma radiation
can be used to measure the thickness of an object.
>>
3.3
>>>
Fig 3.3.15 Fig 3.3.14
Analyse
19 State the wavelength of this wave.
20 A student shakes out
20 waves on a slinky in 10 seconds. Calculate the frequency of the waves.
21 The time between each wave
passing is called the period.
a Identify the period for the
waves in Question 20.
b If the wave frequency
increases, predict what
effect this will have on the period:
A The period will increase. B The period will stay the same. C The period will decrease.
D There is not enough information to answer the
question.
22 Identify which type of electromagnetic wave has a
wavelength of:
a 1 m b 1 km c 0.5 mm
d 1 millionth of a millimetre
23 Explain what you would expect to see if the following
polarising filters were placed in front of a light source as shown.
a a wavelength of 3 cm and amplitude
of 2 cm
b a wavelength of 10 cm and amplitude
of 1 cm.
25 Convert:
a 600 nanometres to metres
b 0.000 000 850 metres to nanometres 26 Construct a table with the following
headings and enter information about each type of electromagnetic wave.
Type of Typical Source How it is Use/s
electromagnetic wavelength detected
radiation (approx.)
Visible light 1 millionth The Sun, Cones in the eye, Sight, of a metre very hot objects photographic film photography
27 Construct a diagram of:
a a frequency-modulated carrier wave b an amplitude-modulated carrier wave
28 Calculate the frequency of carrier waves transmitted
by:
a 107.5 ZZZ FM
b 1278 2AW (an AM station)
Skills
24 Construct a diagram of a transverse wave that has:
[
Extension]
Investigate
1 Outline the contribution to science of one of the
following people by writing a brief biography of their life.
a Scottish physicist James Clerk Maxwell and his
work on electromagnetic wave theory
b the development of radio communications by the
American engineer Edwin Armstrong
c the first transmission of radio waves by Guglielmo
Marconi
d John Logie Baird’s contribution to the development
of television