Physics Topic 4

• Waves


  Transfer energy without transferring matter
• Wavelength


  Distance between the same points on two consecutive waves
• Amplitude


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


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


  The time taken for a whole wave to completely pass a single point
• Wavefront


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


  Velocity increases
• Period


  Inversely proportional to frequency
• Smaller period


  Higher frequency, greater velocity
• Measuring velocity of sound in air

  1. Make a noise at ~50m from a solid wall, and 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, and record the time difference between a sound passing from one to the other – then use speed = distance/time
• Refraction


  • Waves pass from one medium to another
  • If passing into a more optically denser medium (from air to glass), 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
  • The light bends closer to the normal
• Reflection


  • Waves will reflect off a flat surface
  • The smoother the surface, the stronger the reflected wave is
  • Rough surfaces scatter the light in all directions, so appear matt and not reflective
  • The angle of incidence = angle of reflection
  • Light will reflect if the object is opaque and is not absorbed by the material, the electrons will absorb the light energy, then reemit it as a reflected wave
• Transmission


  • Waves will pass through a transparent material
  • The more transparent, the more light will pass through the material
  • It can still refract, but the process of passing through the material and still emerging is transmission
• Absorption


  • If the frequency of light matches the energy levels of the electrons, the light will be absorbed by the electrons and not reemitted, they will be absorbed, and then reemitted over time as heat, so that particular frequency has been absorbed
  • If a material appears green, only green light has been reflected, and the rest of the frequencies in visible light have been absorbed
• Different substances


  May absorb, transmit, refract or reflect waves depending on their wavelength
• The Ear


  • Outer ear collects the sound and channels it down the ear canal
  • As it travels down, it still is a pressure air wave
  • The sound waves hit the eardrum, a tightly stretched membrane which vibrates as the incoming pressure waves reach it, the eardrum vibrates at the same frequency as the sound wave, the small bones connected to this also vibrate at the same frequency (stirrup bone)
  • Vibrations of the bones transmitted to the fluid in the inner ear (the cochlea), compression waves are thus transferred to the fluid
  • The small bones act as an amplifier of the sound waves the eardrum receives
  • As the fluid moves due to the compression waves, the small hairs that line the cochlea move too, each hair is sensitive to different sound frequencies, so some move more than others for certain frequencies
  • 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
  • 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, 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
• Ultrasound


  • A sound wave with a higher frequency than 20 000Hz
  • Uses: Sonar - pulse of ultrasound is sent below a ship, and the time taken for it to reflect and reach the ship can be used to calculate the depth, this is used to work out whether there is a shoal of fish below the ship, or how far the seabed is below the ship
  • Foetal Scanning - non-invasive and not harmful, used to create an image of the foetus, allowing measurements to be made to check the foetus is developing normally, this works because ultrasound waves partially reflect at each surface boundary, this can be used to work out the distances and therefore an image of the foetus
• Infrasound


  • A sound wave with a frequency lower than 20Hz – also known as seismic waves, there are two: P and S waves
  • P waves are longitudinal, and can pass through solids and liquids
  • S waves are transverse, only passing through solids (these move slower too)
  • 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