P12- Wave properties

• Waves


  Vibrations that transfer energy without transferring matter
• Waves


  • Can be used to transfer energy
  • Can be used to transfer information
• Types of waves


  • Transverse waves
  • Longitudinal waves
  • Sound waves
  • Water waves
  • Waves on springs and ropes
  • Seismic waves
  • Light waves
  • Radio waves
  • Microwaves

  
• Transverse waves


  Waves where the oscillations are perpendicular to the direction of energy transfer

  
• Longitudinal waves


  Waves where the oscillations are parallel to the direction of energy transfer

  
• Mechanical waves need a medium (a substance) to travel through
• Electromagnetic waves can travel through a vacuum
• Observing transverse waves


  1. Tie a ribbon to the middle of a rope

  2. Move one end of the rope up and down

  3. Observe the ribbon moving up and down but not along the rope
• Observing longitudinal waves


  1. Push and pull the end of a slinky

  2. Observe areas of compression and rarefaction moving along the slinky

  
• When waves travel through a substance, the substance itself doesn't travel
• Amplitude


  The maximum displacement of a point on the wave from its undisturbed position
• Wavelength


  The distance from a point on the wave to the equivalent point on the adjacent wave
• Frequency


  The number of waves passing a fixed point every second

  
• Period


  The time taken for each wave to pass a fixed point
• Measuring the speed of sound in air

  1. Two people stand apart in a field

  2. One person bangs cymbals together

  3. The other person times the interval between seeing the impact and hearing the sound

  4. Calculate the speed of sound using the equation speed = distance/time taken

  
• Mechanical waves can be transverse or longitudinal

  
• Transverse wave


  • Ripples on the surface of water

  
• Longitudinal wave


  • Sound waves in air

  
• When a sound wave passes through air, the air particles at a point of compression are pushed closer together

  
• Waves on a rope with a knot in the middle are transverse waves
• The energy of waves transfers along the rope, but the knot only moves up and down
• Demonstrating longitudinal and transverse waves using a slinky

  1. Move one end of the slinky up and down to show transverse waves

  2. Push and pull the end of the slinky to show longitudinal waves

  
• When waves travel across a pond, a small ball floating on the surface moves in a circular motion
• Electromagnetic waves are transverse, and sound waves are longitudinal

  
• Mechanical waves need a medium to travel through
• Arrange the plate so that the waves cross a boundary between the deep and shallow water


  The water over the plate needs to be very shallow

  
• Waves at a non-zero angle to a boundary

  • The waves change their speed and direction when they cross the boundary

  
• Waves crossing from deep to shallow water


  They change direction towards the boundary
• Waves perpendicular to a boundary (at normal incidence)


  • The waves cross the boundary without changing direction, but their speed changes

  
• Waves crossing a boundary

  Their speed changes
• Safety: Mop up any water spillages
• Explaining refraction

  Each tiny section of the wavefront creates a wavelet that travels forward, the wavelets move forward together to recreate the wavefront

  
• Refraction


  • When plane waves cross a boundary at a non-zero angle, each wavefront experiences a change in speed and direction

  
• Waves crossing a boundary from deep to shallow water

  The refracted wavefronts are closer together and at a smaller angle to the boundary than the incident wavefronts

  
• The refracted waves and the incident waves have the same frequency, but they travel at different speeds, so they have different wavelengths</b>

  
• Reflection


  When a wave is directed at a substance, some or all of the wave may be reflected at the surface

  
• Absorption


  Of the waves that go into a substance, some or all of them may be absorbed by the substance, heating it
• Transmission


  Waves that are not absorbed by the substance they are travelling through are transmitted by it
• Plane waves reflecting from a straight barrier

  The angle of each reflected wavefront to the barrier is the same as the angle of each incident wavefront to the barrier

  
• Plane waves passing from deep to shallow water at a non-zero angle to a straight boundary

  Draw some refracted wavefronts, indicating their direction