Wave Motion

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WAVES
Wave Motion
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Lesson Objectives

• By the end of this lesson, you will be able to
• State the different types of waves and the
  difference between them
• Describe the diffraction of light
• Explain how to work out the speed of wave
  propagation

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WAVES

• Waves transfer energy information but without
  transferring matter
• The energy is transferred by oscillations in the
  material which the wave is travelling though

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CLASSIFYING WAVES

• Mechanical Waves
• Waves that pass though a material are vibrations
  of that material
• eg. Sound waves, seismic waves, strings
• 2) Electromagnetic Waves
• Vibrating electrical or magnetic fields through
  space (no material needed)
• eg electromagnetic spectrum

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TYPES OF WAVES
1. Longitudinal Waves
Direction of Travel

• Oscillations occur parallel to direction of
  travel
• Sound waves a vibrating surface in contact
  with air.
• The surface pushes air molecules away which push
  adjacent air molecules

which push adjacent air molecules which push
adjacent air molecules which push adjacent air
molecules which push adjacent air molecules which
push adjacent air molecules which push adjacent
air molecules which push adjacent air molecules
which push adjacent air molecules which push
adjacent air molecules which push adjacent air
molecules which push adjacent air molecules which
push adjacent air molecules which push adjacent
air molecules which push adjacent air molecules
which push adjacent air molecules.
Compression (C) Expansion (E)
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TYPES OF WAVES
2. Transverse Wave
Direction of Travel

• Oscillations at right angles (90o) perpendicular
  to direction of travel
• Electromagnetic waves radiowaves, X-rays,
  microwaves, visible light etc
• Waves on a string or wire plucking a guitar
  string

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POLARISATION

• Can only polarise transverse waves

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SEISMIC WAVES

• When an earthquake occurs, a majority of the
  energy is transferred as wave energy and
  transmitted over long distances.
• Classified in 2 types
• P primary or push waves
• S secondary or shake/shear waves

LONGITUDINAL
TRANSVERSE
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Parts of a Wave
Wavelength
Peak
Amplitude
A
Trough
x
1 complete wave
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Key Terms

• Displacement (s) the distance from the
  equilibrium position
• Wavelength (l) the distance between identical
  points in a wave train
• Amplitude (A) maximum displacement of a
  particle (peak or trough max.)

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Key Terms

• Period (T) the time taken for 1 complete wave
  to pass a point
• Frequency (f) number of cycles per second.
  Measured in Hertz (Hz).

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Amplitude and Frequency
Low amplitude, low frequency
Low amplitude, high frequency
High amplitude, low frequency
High amplitude, high frequency
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WAVE SPEED

• We know the relationship between distance, speed
  and time.
• Speed Distance / Time
• Therefore

Wave Speed Wavelength / Period V ? / T But we
know that T 1/f So we can substitute.
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The Wave Equation
relates the speed of the wave to its frequency
and wavelength
Wave speed (v) frequency (f) x wavelength
(?) m/s Hz
m
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Some example wave equation questions

• The speed of sound is 330m/s (in air). When
  Cuthbert hears this sound his ear vibrates 660
  times a second. What was the wavelength of the
  sound?
• a) Purple light has a wavelength of around
  6x10-7m. If its frequency is 5x1014 Hz. What is
  the speed of light?
• b) Red light travels at the same speed. Work
  out its frequency if its wavelength is about
  4x10-7m.

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TSUNAMI
a) A typical tsunami has a wavelength of 300km
and a period of 30 mins. What speed does it
travel at? b) Brighton is 2800km from La Palma
how long do we have to evacuate before it hits?
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WAVE PROPERTIES
1. Reflection
Waves will bounce off a surface under certain
conditions eg the surface must be shiny for
electromagnetic waves
Reflective surface
Incident Ray
Reflected Ray
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WAVE PROPERTIES
Angle of Incidence
Normal
Angle of Reflection
Angle of Incidence Angle of Reflection
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WAVE PROPERTIES
2. Refraction
Waves cross a boundary causing a change in speed
and consequently wavelength Depends on the
refractive index of different substances
Normal
Glass Block
Air
Normal
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WAVE PROPERTIES
3. Diffraction

• Occurs when waves pass through a gap or around
  an object of roughly the same size or smaller
  than their wavelength.
• Large gap - the middle parts of the waves go
  straight through the gap, with a slight curving
  at the edges of the waves.
• Small gap - if the gap is smaller than the
  wavelength of the waves, the waves fan out in
  circles.

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INTERFERENCE
1. Constructive Interference - when the crests
(or troughs) of two waves coincide, they combine
to create an amplified wave.
The two waves are in phase with each other
there is zero phase difference between them.
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INTERFERENCE
2. Destructive Interference - where the crests of
one wave are aligned with the troughs of another,
they cancel each other out.
The waves are out of phase (or in antiphase) with
each other they are half a cycle different from
each other.
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Lesson Objectives

• By the end of this lesson, you will be able to
• State the different types of waves and the
  difference between them
• Describe the diffraction of light
• Explain how to work out the speed of wave
  propagation

GOOD WORK HUMANS!