P6 - Waves

• Transverse waves - oscillate perpendicular to the direction of energy transfer
• Longitudinal waves - oscillate parallel to the direction of energy transfer
• Frequency - the number of waves passing a fixed point per second. Measured in Hertz
• Amplitude - wave height
• Wavelength - the distance from peak-peak or trough-trough
• Period - the time taken for one complete oscillation in seconds
• Amplitude can indicate how much energy a wave is carrying. Higher energy = more amplitude
• Measuring water waves Required Practical:

  1. Time one wave travelling the length of a water tank
  2. Calculate wave speed
  3. Count number of waves passing a fixed point in a second to find the frequency
  4. Estimate wavelength by using a ruler to measure the waves
  5. Use a stroboscope to make the same measurements and compare the results
• Speed and wavelength are directly proportional
• As waves are transmitted from one medium to another:

  • Wavelength changes
  • Frequency does not change
• When waves reach a boundary between two mediums, they can be:

  • Reflected
  • Retracted
  • Absorbed
  • Transmitted
• Ray diagrams show waves reaching a medium boundary
• Angle of incidence = Angle of reflection
• When light travels into a denser medium, it bends towards the normal
• When light travels into a lighter medium, it bends away from the normal
• Refractive index - the way in which a material affects refraction
• When a light wave enters a slower medium at an angle:

  • The first part of the wave slows down
  • The rest continues at higher speed
  • This causes the wave to change direction towards the normal
• Reflection and refraction Requred Practical:

  1. Place a block on paper with a light ray pointing at it
  2. Draw around the block and mark the position of the light at the start, end, and entering/leaving the block
  3. Remove the block, connect the lines, and draw a normal
  4. Measure angles of incidence and refraction
  5. Repeat for a range of incidence angles and transparent block materials
• Sound waves:

  • Higher amplitude = higher volume
  • Higher frequency = higher pitch
• Human hearing - 20Hz to 20 kHz
• Sound is due to vibration of particle in the medium. Sound waves are converted into vibrations
• Ultrasound:

  • Frequency greater than 20kHz
  • Reflected off medium boundaries so we can use the time taken to find the distance away the boundary is
  • Used to detect defects without cutting into products
  • Used in medical scanning
• Echo sounding (sonar) - Sending ultrasound pulses underwater and timing their return to find distances using the known speed of sound in water
• P-Waves:

  • Longitudinal
  • Travel at speed of sound
  • 2x as fast as S-Waves
  • Different speeds through solids and liquids
• S-Waves:

  • Transverse
  • Can't travel through liquid
• Types of seismic waves:

  • P-Waves (primary)
  • S-Waves (secondary)
• S-Wave shadow zone:

  • S-waves cannot travel through the liquid core
  • This causes a shadow zone opposite the epicentre
  • This provides evidence for the size of earth's liquid core
• P-Wave shadow zones:

  • P-Waves can travel through the core but are refracted
  • This results in shadow zones either side of the S-Wave shadow zone
  • This gives evidence fir the size and shape of the core
• Electromagnetic waves:

  • Transverse
  • All travel at the same speed in air or a vacuum
  • Extends from low frequency, low energy -> high frequency, high energy
  • EM wave wavelength affects how it is absorbed, transmitted, reflected, or refracted
• Radio waves:

  • TV, radio
  • Low energy so not harmful
• Microwaves:

  • Satellite comms, cooking food
  • Travel in straight lines through atmosphere, so are good for satellite signals
• Infrared waves:

  • Electric heaters, thermal cameras
  • Toasters etc glow red hot and this transmits infrared energy that is absorbed by the food and converted into thermal energy
• Visible light:

  • Fibre optic communications
  • Travels down optical fibres without being lost through the sides
• Ultraviolet waves:

  • Energy efficient bulbs, tanning
  • In bulbs, UV waves are produced by the gas in the bulb and absorbed into the bulbs coating, which fluoresces
• X-rays:

  • Medical imaging
  • They penetrative soft tissue but not bone. A photographic plate will show shadows where bones are
• Gamma rays:

  • Sterilising, tumour treatment
  • Highest energy of EM waves and destroy bacteria and tumours
• EM waves:

  • Radio
  • Micro
  • Infrared
  • Visible light
  • UV
  • X-ray
  • Gamma rays
• Infrarer radiation Required Practical:

  1. Take four painted boiling tubes: matt black, shiny black, white, silver
  2. Pour hot water in each
  3. Measure and record the start temperature of each
  4. Measure each temp every minute for 10 mins
  5. The tube that cools fastest emits infrared energy quickest
• Radio signals:

  • Radio waves can be caused by an alternating current
  • Frequency of the radio wave matches frequency of the electrical oscillation
  • Radio signal is produced
  • Radio waves are absorbed by a conductor and create an AC current with the same frequency as the radio wave
  • Radio signal is recieved
  • When this oscillation is induced in an electrical circuit it creates and electrical signal which matches the wave
• Changes in atoms and nuclei can generate or absorb waves:

  • Electrons moving between energy levels due to heat or electricity can generate waves
  • Changes in the nucleus e.g an unstable nucleus can generate waves