Topic 4 - Waves

• In the sea, buoys stay still despite waves passing by them
• Amplitude


  Distance from equilibrium line to the maximum displacement (crest or trough)
• Period


  The time taken for a whole wave to completely pass a single point
• If passing into a more optically denser medium

  • The wave will be refracted at the boundary and will change direction to bend towards the normal
  • Speed decreases
  • Wavelength decreases
  • Energy of a wave is constant, and energy is directly linked to frequency of a wave. So if frequency is constant and speed decreases, wavelength must also decrease
• Increase frequency

  Velocity increases
• Measuring velocity of ripples on water surface

  1. Use a stroboscope with the same frequency as the water waves, measure distance between the 'fixed' ripples and use v = fλ

  2. Move a pencil along the paper at the same speed as a wavefront, measure the time taken to draw this line and the length of the line, then use speed = distance/time
• Longitudinal waves

  • Sound waves, seismic P waves
  • Has compressions and rarefactions
  • Vibrations are in the same direction as the direction of travel
• Types of Waves

  • Transverse waves
  • Longitudinal waves
• The smoother the surface, the stronger the reflected wave is
• The angle of incidence = angle of reflection
• Transmission

  1. Waves will pass through a transparent material

  2. The more transparent, the more light will pass through the material

  3. It can still refract, but the process of passing through the material and still emerging is transmission
• Wavefront

  The plane in which the wave travels (i.e. the direction of the wave)
• Wavelength


  Distance between the same points on two consecutive waves
• Waves transfer energy without transferring matter
• Frequency

  The number of waves that pass a single point per second
• Transverse waves

  • Light, or any electromagnetic wave, seismic S waves, water waves
  • Has peaks and troughs
  • Vibrations are at right angles to the direction of travel
• Wavelength increases

  Velocity increases
• Measuring velocity of sound in air

  1. Make a noise at ~50m from a solid wall, record time for the echo to be heard, then use speed = distance/time

  2. Have two microphones connected to a datalogger at a large distance apart, record the time difference between a sound passing from one to the other, then use speed = distance/time
• Waves will reflect off a flat surface
• Waves pass from one medium to another
• Rough surfaces scatter the light in all directions, so appear matt and not reflective
• Light will reflect if the object is opaque and is not absorbed by the material
• Absorption

  1. If the frequency of light matches the energy levels of the electrons

  2. The light will be absorbed by the electrons and not reemitted

  3. They will be absorbed, and then reemitted over time as heat

  4. So that particular frequency has been absorbed
• Effect of Wavelength

  • Different substances may absorb, transmit, refract or reflect waves depending on their wavelength
  • Glass transmits/refracts visible light
  • Reflects UV
• Ultrasound (Physics only)

  • This is a sound wave with a higher frequency than 20 000Hz
  • Uses: Sonar, Foetal Scanning
  • Sonar: Pulse of ultrasound is sent below a ship to calculate depth, detect shoals of fish, or measure seabed depth
  • Foetal Scanning: Non-invasive method to create an image of the foetus for developmental checks
  • Ultrasound waves partially reflect at each surface boundary to create an image
• Infrasound (Physics Only)


  • Infrasound is a sound wave with a frequency lower than 20Hz – also known as seismic waves
  • Uses: Explore Earth’s core
  • P waves are longitudinal and can pass through solids and liquids
  • S waves are transverse, only passing through solids
  • On the opposite side of the Earth to an earthquake, only P waves are detected, suggesting the core of the Earth is liquid – hence no S waves can penetrate it
• The Ear (Physics only)

  1. Outer ear collects the sound and channels it down the ear canal

  2. As it travels down, it still is a pressure air wave

  3. The sound waves hit the eardrum

  4. Tightly stretched membrane which vibrates as the incoming pressure waves reach it

  5. The eardrum vibrates at the same frequency as the sound wave

  6. The small bones connected to this also vibrate at the same frequency (stirrup bone)

  7. Vibrations of the bones transmitted to the fluid in the inner ear (the cochlea)

  8. Compression waves are thus transferred to the fluid

  9. The small bones act as an amplifier of the sound waves the eardrum receives

  10. As the fluid moves due to the compression waves, the small hairs that line the cochlea move too

  11. Each hair is sensitive to different sound frequencies, so some move more than others for certain frequencies

  12. The hairs each come from a nerve cell

  13. When a certain frequency is received, the hair attuned to that specific frequency moves a lot, releasing an electrical impulse to the brain, which interprets this to a sound

  14. The higher the frequency, the more energy the wave has – which would damage cells in the ear more quickly, and would not be able to work effectively long-term

  15. This, and the fact that we have evolved not needing to hear very high or low frequencies, means the ear only works for a limited frequency range