Waves and wave motion

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Waves and wave motion
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Can you think of some examples of waves?
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Waves

• Sound
• Light
• Water
• Radio
• Seismic.

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What is the function of a wave?

• A wave is a means of transferring energy from one
  place to another

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TYPES OF WAVES

• Waves are classified into different types
  according to their natures

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Wave Types
Mechanical Electromagnetic
e.g. Water waves, waves on a rope, sound e.g. Radio, X-Rays, Light
Must have a substance to travel through Cannot travel in a vacuum Does not need a substance to travel through but can travel through various substances Can travel in a vacuum
Vibrations passed on from molecule to molecule Travel at very fast speed in a vacuum 3 108 m/s
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Travelling Waves

• Waves that travel from one place to another
• e.g. Waves on rope
• e.g Waves on water
• e.g. compression waves on spring

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Compression and Rarefraction
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Longitudinal wave
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Types Of Waves
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TRANSVERSE
A wave where the direction of the vibration is
perpendicular to the direction in which the wave
travels
LONGITUDINAL
A wave where the direction of the vibration is
parallel to the Direction in which the wave
travels.
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Transverse Waves

• Transverse waves
• A transverse wave is a one in which the direction
  of vibration is perpendicular to the direction of
  propagation.
• Examples
• Light waves.
• Radio waves.
• Waves on a rope.
• Water waves

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Longitudinal Waves

• Longitudinal Waves
• A longitudinal wave is one in which the direction
  of vibration is parallel to the direction of
  propagation.
• Examples
• Sound waves in a solid, liquid or gas.
• Compression waves on a spring.
• Seismic waves.

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Wave..........

• The wavelength of a wave is the distance from
  one point on the wave to the corresponding point
  on the next cycle.

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• The frequency of a wave is a measure of the
  number of oscillations (vibrations) of the wave
  per second.
• The periodic time of a wave (T) is the time taken
  for one complete cycle.

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Symbols and units
Variable Symbol Unit Symbol for Unit
Frequency f Hertz Hz
Wavelength ? (lamda) metre m
Velocity v (or c for light) metre/second m/s
Time T second s
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The relationship between frequency, velocity and
wavelength

• c f ?
• or
• v f ?

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Properties of waves

• 1. Reflection Reflection is the bouncing of a
  wave off an object.

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Properties of waves

• 2 Refraction is the bending of a wave as it
  travels from one medium to another. Note that
  when a wave travels from one medium to another
  its frequency does not change.

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Properties of waves

• 3. Diffraction is the spreading of waves around a
  slit or an obstacle.
• This effect is only significantly noticeable if
  the slit width is approximately the same size as
  the wavelength of the waves.

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Properties of waves

• 4 Interference occurs when waves from two sources
  meet to produce a wave of different amplitude.

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Interference of Waves
When two or more waves propagating in the same
medium meet at the same point, interference is
said to occur.
A stable interference pattern can be observed
when two water waves of same frequency meet one
another in a ripple tank.


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Two types of interference
Constructive Interference Destructive
Interference
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Constructive Interference

• Constructive Interference occurs when waves from
  two coherent sources meet to produce a wave of
  greater amplitude.
• (Constructive interference occurs when the crests
  of one wave are over the crests of another wave).

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Destructive Interference

• Destructive Interference occurs when waves from
  two coherent sources meet to produce a wave of
  lower amplitude.
• (Destructive interference occurs when the crests
  of one wave are over the troughs of the second
  wave. This will happen if one wave is half a
  wavelength out of phase with respect to the
  other).

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5. Polarisation

• Only TRANSVERSE waves can be polarised.
• To polarise a wave means to make it vibrate in
  one plane only
• horizontally plane polarised or
• vertically plane polarised

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Polarised sunglasses
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The frequency of a wave is a measure of the
number of oscillations (vibrations) of the wave
per second. Another way of defining frequency of
a wave is to say it is the number of waves that
pass a fixed point per second. T 1/f
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• The relationship between periodic time and
  frequencyT 1/f
• or
• f 1/T

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Standing waves
A standing wave/stationary wave is the result of
two waves of the same wavelength, frequency, and
amplitude travelling in opposite
directions through the same medium.
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• From the diagram we can see that
• The distance between two consecutives nodes is
  ?/2
• The distance between two consecutive antinodes is
  ?/2
• The distance between an anti-node and the next
  node is ?/4

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The Doppler Effect

• The Doppler effect is the apparent change in
  frequency of a wave due to the motion of the
  source or the observer.
• The observed frequency is higher when the source
  and observer are getting closer.
• The observed frequency is lower when the source
  and observer are getting farther away.

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• The distance between successive crests is the
  constant and so the number of crests passing a
  point in one second is the frequency of the wave.
• These waves will pass an observer in equal
  intervals of time.
• This means that the wavelength, and the
  frequency, will be constant.

Consider a source S emitting a wave with crests
1, 2, 3 as shown.
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In this case the source is moving to the right
while emitting the waves.

• The result is that
• Ahead of the moving source the crests are closer
  together than crests from the stationary source
  would be. This means that the wavelength is
  smaller and the frequency is greater.
• Behind the moving source, the crests are further
  apart than crests from the stationery source
  would be. This means the wavelength is greater
  and so the frequency is less.

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Formula f apparent frequency f actual
frequency c speed of the wave u speed of the
moving source
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Example of the Doppler Effect
The noise from a racing car as it approaches and
then moves away from an observer is an example
of the Doppler effect. This is NOT an
APPLICATION of the Doppler effect.
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Applications of the Doppler Effect

• Police speed traps
• Measuring the red shift of galaxies in astronomy

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Doppler shift gives radial velocity