Topic 4

• Waves transfer energy and information but do not transfer matter
• Types of waves

  • Transverse
  • Longitudinal
• Transverse wave


  A wave for which the oscillations are perpendicular to the direction of energy transfer
• Longitudinal wave


  A wave for which the oscillations are parallel to the direction of energy transfer
• Examples of transverse waves

  • Electromagnetic waves
  • Seismic s-waves
• Examples of longitudinal waves

  • Sound waves
  • Seismic p-waves
• Parts of a longitudinal wave

  • Compressions
  • Rarefactions
• Parts of a transverse wave

  • Peaks
  • Troughs
• Wave amplitude

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

  The distance from a point on a wave to the same position on the adjacent wave, most commonly peak to peak or trough to trough
• Wave frequency


  The number of waves that pass a given point each second
• Unit of frequency

  Hertz, Hz
• A frequency of 200Hz means 200 waves pass a given point each second
• Wave speed


  The speed at which energy is transferred through a medium
• A wave transfers energy
• Wave velocity

  Wave velocity (measured in metres per second) is equal to the product of the wavelength and frequency of the wave
• Equation to calculate wave speed

  1. Wave Speed = Frequency x Wavelength

  2. Speed (m/s), Frequency (Hz), Wavelength (m)
• Wave period


  The length of time it takes for one full wave to pass through a point
• How do sound waves travel through a solid?

  The particles in the solid vibrate and transfer kinetic energy through the material
• How does the human ear work?

  1. The outer ear collects the sound which travels into the ear

  2. The sound waves cause the eardrum to vibrate at the same frequency

  3. This is amplified by three ossicles (small bones)

  4. This causes the hair in the cochlea to vibrate

  5. The cochlea converts the vibrations into electrical signals

  6. The signals are passed to brain through the auditory nerve

  7. The brain converts the electrical signals into sound
• Frequency range of human hearing

  20 Hz - 20kHz (1kHz = 1000 Hz)
• Ultrasound wave


  A sound wave with a frequency greater than 20,000 Hz
• What natural event causes seismic waves to be produced? What types are produced?

  1. Earthquakes

  2. They produce both P-waves and S-waves
• Difference between mediums P-waves and S-waves can travel through

  • P-waves travel through both solids and liquids
  • S-waves only travel through solids (not liquids)
• Significance of P and S waves
• Technique used to detect objects in deep water and measure water depth

  1. Echo sounding

  2. High frequency sound waves are emitted, reflected and detected

  3. Time difference between emission and detection, alongside wave speed, are used to calculate distances
• How does sonar work?

  1. When ultrasound waves are emitted they reflect off boundaries and their echoes are detected

  2. The speed of the ultrasound is known and also the time it takes to detect the echoes

  3. The equation distance=speed x time is used to find the distance travelled

  4. The distance travelled is halved to give the distance between emitter and boundary (as the wave had to travel there and back
• How does foetal scanning work?

  1. An ultrasound wave is sent into the patient's body. It passes through the body and reflects off the organs and tissue

  2. The device then uses the reflected ultrasound waves to produce an image of the foetus

  3. Ultrasound is safe and therefore does not damage cells
• Wave speed equation

  Wave Speed = Frequency x Wavelength
• Wave speed units

  Speed (m/s), Frequency (Hz), Wavelength (m)
• Reflection

  When a wave bounces off a surface
• Normal


  A vertical imaginary line which is perpendicular to the boundary
• Reflection of light

  Angle of incidence = Angle of reflection
• Refraction

  The change in speed of a wave as it reaches a boundary between two media, usually resulting in a change in direction (if it enters at an angle)
• Refraction of light

  1. Light changes speed and direction in the new medium

  2. If the new medium is more dense, the light will travel slower and bend towards the normal

  3. If the new medium is less dense, the light will travel faster and bend away from the normal
• When entering a denser material

  Light waves slow down and bend towards the normal
• When entering a less dense material

  Light waves speed up and bend away from the normal
• Measuring refraction

  The angle of incidence, i, and angle of refraction, r, can be measured and compared. All angles are measured relative to the normal.
• Absorption of different wavelengths in different mediums

  • Some materials behave differently depending on the wavelength
  • An example is glass which will transmit visible light, but reflect UV light
• Sound waves in solids

  The particles in the solid vibrate and transfer kinetic energy through the material