Waves

• Waves


  Transfer energy from one place to another
• Types of waves


  • Mechanical waves
  • Sound waves
  • Electromagnetic waves
• Transverse waves


  • Vibrations of the particles are perpendicular to the direction of energy transfer
  • Have crests and troughs where particles have been displaced a maximum amount from their positions
• Longitudinal waves


  • Vibrations of the particles are parallel to the direction of energy transfer
  • Have compressions where particles are squashed together, and rarefactions where particles spread apart due to the vibrations
• Longitudinal waves


  • Sound waves
  • Seismic P-waves
• Amplitude


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


  The distance from a point on one wave to the equivalent point on the next wave (e.g. crest-to-crest or trough-to-trough)
• Frequency


  The number of waves passing a point per second
• Calculating period of a wave

  T=1/f
• Period


  The amount of time it takes a complete wave to pass a point
• Calculating wave speed


  v=f*λ
• Wave speed


  The speed at which energy is transferred through the medium
• Increasing wavelength (while keeping frequency constant)

  Increases wave velocity
• Different techniques are used to measure different types of waves
• Measuring Sound Waves


  1. Set up oscilloscope with microphones

  2. Turn on signal generator and note frequency

  3. Position microphones next to speaker

  4. Move one microphone away slowly

  5. When waves line up again, measure distance between microphones

  6. Calculate speed of sound using v=fλ
• Measuring the Speed of Water Waves

  1. Set up experiment in darkened room

  2. Switch on signal generator to produce waves

  3. Measure distance between wave shadows on screen

  4. Choose two points on screen and measure distance

  5. Use stopwatch to measure time for wave to travel between points

  6. Calculate wave speed using speed=distance/time
• Wavelength (λ)


  The distance between two consecutive wave crests or troughs
• Frequency (f)


  The number of wave cycles that pass a given point per unit of time
• Period (T)


  The time taken for one complete wave cycle
• Speed (v)


  Equals the product of wavelength (λ) and frequency (f)
• Electromagnetic waves


  Waves transferred by vibrations in magnetic and electric fields
• Electromagnetic waves can travel through a vacuum because they don't need particles to travel
• Electromagnetic waves


  Transverse waves that transfer energy from the source of the waves to an absorber
• Infrared waves


  • Emitted from the sun
  • Travel to the Earth and heat up the air
• Electromagnetic waves are transferred by vibrations of electric and magnetic fields, so they do not require a medium to travel through
• Types of electromagnetic waves

  • Visible light
  • Infrared
  • Ultraviolet
  • X-rays
  • Gamma rays
• All electromagnetic waves travel at the speed of light
• Humans can only see a very small portion of the electromagnetic spectrum, called visible light
• Refraction


  When a wave crosses a boundary between two materials, it may be refracted
• Refraction


  • Depends on how much the wave speeds up or slows down, which depends on the difference in density between the two materials
• The surface with the greatest infrared emission is the best emitter and absorber of infrared radiation
• Radio Waves


  Electromagnetic waves with wavelengths longer than 10 cm
• Transmitting Radio Waves


  1. Oscillating charges produce oscillating electric and magnetic fields

  2. Alternating current (ac) causes electrons in the transmitter to oscillate

  3. Electrons produce radio waves which are transmitted over a distance

  4. At the receiver, the radio waves cause electrons to oscillate

  5. The electrons produce an ac current in the receiver
• Frequency of radio waves

  Depends on the frequency of the alternating current
• Types of Radio Waves

  • Long-wave
  • Short-wave
  • TV and FM Radio Signals
• Long-wave Radio Waves


  • Wavelengths between 11 and 10 km
  • Diffract around large objects such as hills, tunnels and the curved surface on the Earth
  • Can transmit signals to a receiver that is not in the line of sight of the transmitter
• Short-wave Radio Waves


  • Wavelengths between 10 and 100 m
  • Do not diffract around large objects
  • Reflect off the ionosphere (an electrically charged layer of gas in the upper atmosphere)
  • Allow communication with transmitters or receivers that are not in the line of sight
  • Used for Bluetooth communication devices
• TV and FM Radio Signals

  • Use very short wavelengths
  • Do not diffract around large objects or reflect off the ionosphere
  • Transmitter and receiver need to be in the line of sight of one another
• There are a range of uses of ultraviolet, X-rays and gamma rays however these types of electromagnetic waves can be harmful
• Ultraviolet radiation (UV)


  Wavelengths of approximately 10−8 m