P12 - Wave properties

• Waves transfer energy from one place to another without transferring matter
• Mechanical waves


  • Require a medium to travel through
  • Examples include
  • sound waves
  • water waves
  • seismic waves
• When waves travel through a substance, the particles in the substance oscillate and pass energy on to neighbouring particles
• Transverse waves


  • The oscillations of a transverse wave are perpendicular to the direction in which the waves transfer energy
  • e.g. ripples on the surface of water
• Longitudinal waves


  • The oscillations of a longitudinal wave are parallel to the direvtion in which the wave transfers energy
  • Cause particles in a substance to be squashed close together and pulled apart, producing areas of rarefaction and compression in the substance
  • e.g. sound waves in air
• Amplitude


  • A
  • maximum displacement of a point on a wave from its undisturbed position
  • metres (m)
• Frequency


  • f
  • number of waves passing a fixed point per second
  • hertz (Hz)
• Period


  • T
  • time taken for one complete wave to pass a fixed point
  • seconds (s)
• Wavelength


  • λ
  • distance from one point on a wave to an equivalent point on another wave
  • metres (m)
• Wave speed


  • v
  • distance travelled by each wave per second, and the speed at which energy is transferred by the wave
  • m/s
• period = 1/frequency
• wave speed = frequency x wavelength
• When waves travel from one medium to another, their speed and wavelength change, but the frequency always stays the same
• Absorbtion


  the energy of the waves is transferred to the energy stores of the substance they travel into
• Reflection


  the waves bounce back at a boundary change
• Refraction


  the waves change speed and direction as they cross the boundary
• Transmission


  the waves carry on moving once they've crossed the boundary, but may be refracted
• The angle of incidence is always equal to the angle of reflection
• If a wave slows down as it crosses a boundary, the refracted ray will bend towards the normal
• If a wave speeds up as it crosses the boundary, the refracted ray will bend away from the normal
• If a wave travels along the normal (enters the boundary at a right angle), it will change speed but not direction
• Wave front diagrams


  • Wave front - an imaginary line at right angles to the direction the wave is moving
  • If a wave slows down as it crosses a boundary, the wave fronts become closer together
  • If a wave crosses a boundary at an angle, one end of the wave front changes speed before the other, so the wave changes direction
• Sound waves


  • mechanical - they need a medium to travel through
  • they cannot travel through a vacuum
  • longitudinal - the oscillations of the particles in the medium are parallel to the direction of energy transfer
  • When sound waves go from air into a solid, they cause vibrations of the same frequency in the solid
• Hearing

  • Sound waves cause solid parts of the ear to vibrate
  • The brain converts these vibrations into what we hear, but only over a limited frequency range
  • humans can hear 20Hz - 20KHz
• • higher frequency = higher pitch
  • greater amplitude = louder sound
• Ultrasound

  • Ultrasound waves have a frequency above the range of human hearing
  • Always partially reflected when they meet a boundary
  • The distance to a boundary can be found by timing how long it takes for an ultrasound reflection to come back to a detector
  • this can be used form medical and industrial imaging
• Echoes


  • A reflected sound wave
  • Echo sounding uses high-frequency sound waves to detect objects in deep water and to measure the depth of water
• Seismic waves


  • Seismic waves' paths curve, showing us that there is a gradual change in density of the mantle
  • S-waves are not detected on the opposite side of the Earth, suggesting that the outer core must be liquid
  • Changes in the path of P-waves suggest a sudden change in the density of the material they are passing through
  • this suggests that the innter core must be solid
• P-waves


  • Longitudinal
  • Travel through solids and liquids
• S-waves


  • Transverse
  • Only travel through solids