SP4- waves

• Waves transfer energy and information but do not transfer matter
• Wave frequency


  The number of waves passing a point each second measured in hertz (Hz). A frequency of 1 hertz means 1 wave passing per second. For sound, the wave frequency determines the pitch (high or low) and for light it determines colour
• Wave period


  The length of time it takes one wave to pass a given point
• Wave wavelength


  The distance from a point on one wave to a point in the same position on the next wave, measured in metres
• Wave amplitude


  The maximum distance of a point on the wave away from its rest position measured in metres. The greater the amplitude of a sound wave, the louder the sound
• Wave velocity


  The speed of the wave in the direction it is travelling. Waves travel at different speeds in different materials
• Longitudinal waves


  • The particles in the material the sound is travelling through move back and forth along the same direction (parallel) that sound is travelling. Particles in a longitudinal wave move along the same direction as the wave
• Transverse waves


  • The particles of water move in the direction at right angles (perpendicular) to the direction the wave is travelling. Particles in a transverse wave move across the direction the wave is travelling
• Longitudinal waves


  • Sound and seismic P waves
• Transverse waves


  • Water surface, electromagnetic waves and seismic S waves
• Equation relating wave speed, frequency and wavelength

  wave speed (m/s) = frequency (hertz) × wavelength (metre)
• Equation relating wave speed, distance and time

  wave speed = distance (metres) / time (seconds)
• Measuring velocity of sound in air (method 1 - using an echo)

  1. 1 - measure the distance from the source of the sound to the reflecting surface (wall)

  2. 2 - measure the time interval, with a stopwatch, between the original sound being produced and the echo being heard

  3. 3 - use speed = distance/time to calculate the speed of sound in air
• Measuring velocity of sound in air (method 2 - microphones and oscilloscopes)

  1. 1 - set up microphones one in front of the other at different distances in a straight line from a sound speaker

  2. 2 - set the frequency of the sound from loudspeaker to a known, audible value

  3. 3 - display the two waveforms on an oscilloscope. Measure the distance between the microphones

  4. 4 - move the microphones apart so that the waveforms move apart by 1 wavelength

  5. 5 - calculate the speed of sound using equation wave speed(m/s) = frequency(Hz) × wavelength(m)
• Measuring velocity of waves on the surface of water


  Set the power supply to vibrate the paddle at a known frequency. Use a strobe light to 'freeze' the water waves so that you can measure the wavelength. Use the equation wave speed(m/s) = frequency(Hz) × wavelength(m) to calculate the speed or velocity of the water waves on the surface of the ripple tank
• Refraction


  Sound waves and light waves change speed when they pass across the boundary between two substances with different densities, such as air and glass. This causes them to change direction and this effect is called refraction
• Refraction of waves


  • The change in direction is called refraction and happens at the interface (boundary) between the two media. A line at right angles to the interface is called the normal line.
  • When light goes from a low to a higher density medium, light will be refracted towards the normal line. When light goes from a higher to a lower density medium, light is refracted away from the normal line. Light travelling along the normal line will not change direction
• An object on the bottom of a swimming pool looks closer than it really is because light reflected by it changes direction when it leaves the water
• Change in wave speed

  Causes a change in direction
• None of the properties of a wave are changed by reflection. The wavelength, frequency, period and speed are same before and after reflection. The only change is the direction in which the wave is travelling
• Filters let through different colours of light and absorb all the other colours. E.g. a green filter will transmit green light and absorb all the other wave length
• The colour of an object appears is based on how to atoms at its surface respond to the light being shone on them. A material will appear green because its atoms reflect the green wavelength and absorb all of the others
• Echos can be heard when sound is reflected by a hard surface. Some materials absorb sound well and some transmit it will. Sound is refracted when it goes into different materials
• Sound waves travel at different speeds in different materials

  Wave velocity = frequency multiplied by wavelength, so if velocity changes, either frequency or wavelength (or both) must change
• Parts of the human ear, in the order in which they transmit vibrations

  1. Sound enters ear canal

  2. Ear drum's thin membrane vibrates

  3. Tiny bones amplify vibrations

  4. Vibrations passed to liquid in cochlea

  5. Hairs in cochlea create impulses

  6. Impulses travel along auditory nerve to brain
• Functions of parts of the ear

  • The part of the membrane that vibrates depends on the frequency of the sound waves in the liquid inside the cochlea, as different thicknesses of the membrane vibrate best at different frequencies. Thousands of hair cells along the membrane detect vibrations. Each hair cell is connected to a neuron that sends impulses to the brain. The brain interprets signals from different neurons as different pitches of sound
• Sound waves can travel through solids causing vibrations in the solid. Within the ear, sound waves cause the ear drum and other parts to vibrate which causes the sensation of sound. The conversion of sound waves to vibrations of solids works over a limited frequency range. This restricts the limits of human hearing
• Calculating depth of water from time and wave velocity

  distance = speed x time
• Ultrasound


  Sounds with frequencies greater than 20 000 Hz
• Using ultrasound in sonar


  Sonar uses pulses of ultrasound to find the depth of water beneath the hip. The sonar equipment measures time between sending the sound and detecting its echo. This time is used to calculate the depth of the water using the equation distance = speed x time
• Uses of ultrasound in body scanning

  • Ultrasound is sent into the patient's body. Some of the ultrasound is reflected at each boundary between different tissues or organs.
  • Ultrasound may be used instead of x-rays for certain scans, such as scan of unborn babies. Compared to x-ray photographs, ultrasound scans:
  • Do not damage living cells
  • Produce images of soft tissue
• Using ultrasound in foetal scanning

  Ultrasound waves are sent into the woman's body and some sound is reflected each time it meets a layer of different tissue with a different density to the one i has just passed through.The scanner detects the echoes and a computer uses the information to make a picture
• Infrasound


  Sound with frequencies less than 20 Hz
• Uses of infrasound


  • Detect volcanic eruptions

  Track the passage of meteors through the atmosphere

  Determine the structure of rocks beneath the earth's crust
• Earthquakes produce shockwaves called seismic waves. Some seismic waves are infrasound waves
• P waves and S waves

  • P waves are longitudinal and can travel faster than S waves. They can travel through solids and liquids. S waves are transverse waves and travel slower than P waves. S waves can only travel through solids
• Information about time the waves arrive in different places and the speed of the wave in different types of rocks to model the paths the waves have taken through earth