Physics - P1.5 - Waves

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P1.5.1 - The Nature of Waves

  • Waves - used to transfer energy and information, direction of travel = direction wave transfers energy
  • Different types of waves:
    • transverse wave - oscillation of particles is perpendicular to direction wave travels
    • longitutinal wave - oscillation of particles is parallel to waves travel direction, made up of compressions and rarefactions
    • electromagnetic waves - light and radio waves, travel through vacuum, no particles - waves are oscillations in electric and magnetic fields, oscillation is perpendicular to direction wave travels - tranverse waves
    • mechanical waves - waves on springs, and sound waves, travel through a medium - transverse or longitudinal
  • Sound waves are longitudinal waves
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P1.5.2 - Measuring Waves

  • Amplitude - height of wave crest, or depth of wave trough, from rest position, greater amplitude = more energy carried
  • Wavelength - transverse: distance from one crest, or trough, to the next, longitutinal: middle of one compression, or rarefaction, to the middle of the next
  • Frequency - transverse: number of wave crests passing a point in one second, longitudinal: number of compressions passing a point in one second, unit - hertz (Hz) - equivilant to per second (s)
  • Speed of wave - v = f x λ
    • v - wave speed - m/s
    • f - frequency - Hz
    • λ - wavelength - m
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P1.5.3 - Wave Properties: Reflection

  • Image seen in a mirror is due to reflection of light
  • Incidence ray - goes towards the mirror
  • Reflected ray - goes away from mirror
  • Normal - perpendicular to mirror, at the point incidence ray hits mirror
  • Angle of incidence - angle between incidence ray and normal
  • Angle of reflection - angle between reflected ray and normal
  • Any reflected ray - angle of incidence = angle of reflection
  • Image in a plane mirror is: same size, upright, same distance behind mirror, virtual
  • Real image - can be formed on a screen - rays of light that produce image actually pass through it
  • Virtual image - can't be formed on a screen - rays of light that produce image only appear to pass through it
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P1.5.4 - Wave Properties - Refraction

  • Waves change speed and wavelength when cross a boundary between different substances, frequency stays the same
  • Refraction - property of all waves, including light and sound
  • Change in speed of the waves causes change in direction
  • Light enters more dense substance - slows down, changes direction towards normal
  • Light enters less dense substance - speeds up, changes direction away from normal
  • If wave is travelling along normal it won't change direction
  • Different colours of light - different wavelength and refracted by slightly different amounts - when white light is shone onto a triangular prism, spectrum is produced - dispersion
  • Violet light refracted most, red light refracted least
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P1.5.5 - Wave Properties: Diffraction

  • Diffraction - property of all waves, including light and sound - spreading of waves when passing through a gap or around an obstacle
  • Effect is most noticable if wavelength of wave = size of gap or obstacle
  • TV signals carried by radio waves - people living in hilly areas may not receive signal as a hill is blocking it - radio waves passing the hill will be diffracted around the hill - don't diffract enough = poor radio or TV signal
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P1.5.6 - Sound

  • Casued by mechanical vibrations
  • Travels as a wave through solids, liquids and gases - fastest in solids and lowest in gases - can't travel through a vacuum
  • Longitudinal waves, direction of vibration = direction of travel
  • Range of frequences heard by humans - 20-20000Hz - decreases with age
  • Reflected to produce echoes - only hard, flat surfaces reflect sound, soft things absorb sound, an empty room will sound different once things are put in it
  • Refraction takes place at boundaries between layers of air at different temperatures
  • Sound waves can also be diffracted
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P1.5.7 - Musical Sounds

  • Pitch depends on frequency of sound waves - higher frequency = higher pitch
  • Loudness depends on amplitude of sound waves - greater amplitude = more energy carried by wave = louder sound
  • Differences in waveform can be shown on an oscilloscope
  • Tunng forks and signal generators produce 'pure' waveforms - quality of note depends on waveform
  • Different instruments = different waveforms - sound different
  • Vibrations created in an instrument when it's played produce sound waves
  • In some instruments, a column of air vibrates, others, a string vibrates, some instruments vibrate when struck
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