waves

• progressive wave (moving wave)


  carries energy from one place to another without transferring any material
• a wave is caused by something making particles or fields oscillate at a source
• a wave transfers energy away from the source.

  • so the source of the wave loses energy
• waves can be reflected, refracted and diffracted
• reflection

  the wave is bounced back when it hits a boundary
• refraction

  • the wave changes direction as it enters a different medium
  • the change in direction is a result of the wave slowing down or speeding up
• diffraction

  the wave spreads out as it passes through a gap or round an obstacle
• displacement

  • measured in metres
  • how far a point has moved from its undisturbed position
• amplitude(A)

  • measured in metres
  • the maximum magnitude of displacement
• wavelength (lambda)

  • measured in metres
  • the length of one whole wave oscillation or wave cycle
  • example: the distance between two crests(or troughs) of a wave
• period (T)

  • measured in seconds
  • time taken for one whole wave cycle
• frequency(f)

  • measured in hertz
  • the number of whole wave cycles(oscillations) per second passing a given point
• phase

  • measured in degrees/radians/fractions of a cycle

  a measurement of the position of a certain point along the wave cycle
• phase difference

  • measured in degrees/radians/fractions of a cycle
  • the amount by which one wave lags behind another wave
• frequency and period

  $f=$$1/T$
• wave speed (c) (ms-1)

  $c=$$fλ$
• all EM waves travel at a speed of c=3.00e8 ms-1 in a vacuum
• two types of waves

  • transverse
  • longitudinal
• transverse waves

  • the displacement of the particles or field is at right angles to the direction of energy transfer
  • travel as vibrations through magnetic and electric fields
  • examples: ripples on water, waves on a string, s-waves
• longitudinal waves

  • the displacement of the particles or fields is along the direction of energy transfer.
  • example: sound
• longitudinal waves

  made up od compressions and rarefactions
• polarised wave

  a wave that oscillates in one direction only
• polarising filter

  it only transmits vibrations in one direction
• polarisation can only happen for transverse waves
• uses of polarisation

  • glare reduction
  • polarising sunglasses
  • improving TV and radio signals
• superposition of waves

  when two or more waves pass through each other

  • when they cross their displacements combine
  • then each wave continues on its way
• principle of superposition

  when 2 or more wave cross, the resultant displacement equals the vector sum of the individual displacements
• the superposition of two or more waves can result in interference
• constructive interference

  when two waves meet, if their displacements are in the same diirection, the displacements combine to give a bigger displacement
• destructive interference

  if a wave with a positive displacement meets a wave with a negative displacement, they will undergo destructive interference and cancel each other out.
• total destructive interference

  if two waves with equal and opposite displacements meet, they will canel each other out completely
• when waves are superposed, points in phase will interfere constructively with each other and points in antiphase will interfere destructively
• points in phase have the same displacement and velocity
• waves dont have to have the same amplitude to be in phase

  • but they do need to have the same frequency and wavelength
• stationary waves (standing)

  • it is the superposition of two progressive waves with the same frequency (or wavelength) and amplitude, moving in opposite directions.
  • no energy is transmitted by a stationary wave
• nodes

  points on the wave where amplitude of the vibration is zero
• antinodes

  points of maximum amplitude
• a stationary wave is only formed at a resonant frequency
• factors affecting resonant frequency

  • length
  • mass per unit length
  • tension
• longer the string = the lower the resonant frequency

  because c=fλ, so if λ increases, f decreases for fixed c