Waves and Wave Properties

• Transverse waves


  Waves where the oscillations are perpendicular (at right-angles) to the direction of energy transfer
• Longitudinal waves


  Waves where the oscillations are parallel to the direction of energy transfer
• All waves transfer energy from one place to another
• Transverse wave demonstration


  Move a rope or slinky spring up and down vertically - the wave then moves horizontally
• Longitudinal wave demonstration


  Move a slinky spring back and forward horizontally - the wave also moves horizontally
• The particles that make up a wave oscillate (vibrate) about a fixed point
• The energy moves along, but the matter remains
• If a stone is dropped into a pond, ripples travel outwards carrying the energy, The water does not travel outwards (otherwise it would leave a hole in the middle)
• Key Point


  Waves transfer energy not matter
• Properties of Waves


  • Frequency
  • Amplitude
  • Wavelength
  • Period
• Frequency


  The number of waves passing a fixed point per second, measured in hertz (Hz)
• Amplitude


  The maximum displacement that any particle achieves from its undisturbed position in metres (m)
• Wavelength


  The distance from one point on a wave to the equivalent point on the next wave in metres (m)
• Period


  The time taken for one complete oscillation in seconds (s)
• Observing waves (e.g. water waves)

  • Amplitude is seen as the wave height
  • The period is the time taken for one complete wave to pass a fixed point
• Amplitude


  Indicates the amount of energy a wave is carrying - the more energy, the higher the amplitude
• Wave Speed


  . The speed of a wave is the speed at which the energy is transferred (or the wave moves)

  . It is measured of how far the wave moves in one second and can be found with 'the wave equation':

  LEARN: wave speed (m/s) = frequency (Hz) x wavelength (m)

  . Ripples on the surface of water are slow enough that their speed can be measured by direct observation and timing with a stopwatch
• Measure the frequency, wavelength and speed of waves in a ripple tank

  1. Time how long it takes one wave to travel the length of the tank

  2. Calculate wave speed using speed = distance/time

  3. Count the number of waves passing a fixed point in a second to find the frequency

  4. Measure the peak-to-peak distance as the wave travels to estimate the wavelength

  5. Use a stroboscope to make the same measurements and compare the results
• Using a stroboscope


  • Can significantly improve the accuracy of measurements
  • By projecting a shadow of the waves onto a screen below the stroboscope, flash speed can be adjusted to make the waves appear stationary, making wavelength measurements much more accurate
  • For high frequencies that are difficult to count, this can be used with the wave speed measurement to calculate the frequency using frequency = wave speed/wavelength
• Water depth


  • The key control variable, it is important to ensure that the depth of the water is kept constant across the tank as, for a given frequency, the depth will affect the speed and wavelength