Waves

• Waves transfer energy and information
• Waves


  Oscillations or vibrations about a fixed point

  
• Waves


  • Ripples cause particles of water to oscillate up and down
  • Sound waves cause particles of air to vibrate back and forth
• Waves transfer energy without transferring matter
• Objects floating on water provide evidence that waves only transfer energy and not matter
• Transverse waves


  • The motion of the wave is perpendicular to the direction of energy transfer
  • They transfer energy, but not the particles of the medium
  • They can move in solids and on the surfaces of liquids but not inside liquids or gases
• Crest (Peak)


  The highest point on a wave above the equilibrium, or rest, position
• Trough


  The lowest point on a wave below the equilibrium, or rest, position
• Amplitude


  The distance from the undisturbed position to the peak or trough of a wave
• Wavelength


  The distance from one point on the wave to the same point on the next wave
• Frequency


  The number of waves passing a point in a second
• Wave speed


  The distance travelled by a wave each second
• Wavefront


  A useful way of picturing waves from above: each wavefront is used to represent a single wave

  
• Determining wave properties in a ripple tank

  1. Measure the length of the screen to determine wavelength

  2. Time how long it takes for a given number of waves to pass a point to determine frequency

  3. Use the equation wave speed = frequency x wavelength to determine wave speed

  
• Transverse and longitudinal waves both obey the wave equation: v = f x λ
• Longitudinal waves


  • The motion of the wave is parallel to the direction of energy transfer
  • They can travel through solids, liquids and gases
• cm/s


  Unit of speed
• kHz


  Unit of frequency, 1 kHz = 1000 Hz
• Transverse waves


  • Waves where the points along its length vibrate at 90 degrees to the direction of energy transfer
  • Energy transfer is perpendicular to wave motion
  • They transfer energy, but not the particles of the medium
  • They can move in solids and on the surfaces of liquids but not inside liquids or gases
  • Some can move in solids, liquids, gases and a vacuum

  
• Peak/Crest


  The highest point above the rest position
• Trough


  The lowest point below the rest position
• Examples of transverse waves


  • Ripples on the surface of water
  • Vibrations on a guitar string
  • S-waves (seismic waves)
  • Electromagnetic waves (radio, light, X-rays)

  
• Transverse waves are drawn as a single continuous line, usually with a central line showing the undisturbed position</b>
• Longitudinal waves


  • Waves where the points along its length vibrate parallel to the direction of energy transfer
  • Energy transfer is in the same direction as the wave motion
  • They transfer energy, but not the particles of the medium
  • They can move in solids, liquids and gases
  • They cannot move in a vacuum

  
• Compressions


  Points on the wave that are close together
• Rarefactions


  Points on the wave that are spaced apart
• Examples of longitudinal waves

  • Sound waves
  • P-waves (seismic waves)
  • Pressure waves in liquids and gases
• Longitudinal waves are usually drawn as several lines to show the wave moving parallel to the direction of energy transfer
• Differences between transverse and longitudinal waves

  • Direction of vibration
  • Direction of energy transfer
  • Ability to travel in a vacuum

  
• Reflection


  • Occurs when a wave hits a boundary and does not pass through, but stays in the original medium
  • The angle of incidence = the angle of reflection
• Refraction


  • Occurs when a wave passes a boundary between two different transparent media and undergoes a change in speed
  • Also causes a change in wavelength (but not frequency)
  • And a change in direction
• Diffraction


  • Occurs when waves pass through a narrow gap and spread out
• The extent of diffraction depends on the width of the gap compared to the wavelength of the waves
• Diffraction is most prominent when the gap width is approximately equal to the wavelength
• Diffraction can also occur when waves pass an edge
• Reflection, refraction and diffraction can be demonstrated using a ripple tank
• Reflection can be shown by the waves hitting a plane (straight) surface
• Refraction can be shown by placing a glass block in the tank, causing the waves to slow down when travelling over the block
• Diffraction can be shown by placing small barriers and obstacles in the tank, causing the waves to spread out
• The amount of diffraction depends on the size of the gap compared to the wavelength of the water wave