Topic 4 - Waves

• waves transfer energy without transferring matter

  • this is shown for example in the sea, where buoys stay still despite waves passing them, the waves are moving but not the particles
• wavelength


  the distance between the same points on two consecutive waves
• amplitude


  distance from the equilibrium line to maximum displacement (crest of trough)
• frequency

  the number of waved that pass a single point per second
• period


  time taken for a wave to completely pass a single point
• wavefront


  the plane in which the wave travels, (i.e. the direction of the wave)
• $v=$$fλ$

  velocity = frequency x wavelength

  wave speed (m/s) = frequency(Hz) x wavelength (m)
• a loud speaker does not cause a gust of wind, showing only the wave moves not the air around
• increase in frequency = increase in velocity
• wavelength increase = velocity increase
• the period is inversely proportional to frequency
• smaller period = higher frequency = greater velocity
• transverse waves

  • light, or any EM wave, seismic S waves, water waves
  • has peaks and troughs
  • vibrations are at right angled to the direction of travel
• longitudional waves

  • sound waves, seismic P waves
  • has compressions and rarefactions
  • vibrations are in the same direction as the direction of travel
• sound in air

  • make a noise at ~50m from a solid wall and record the time for an echo to be heard, use formula speed = distance/time
• sound in air

  • have two microphones connected to a datalogger at a large distance apart, and record the time difference between a sound passing from one to other, then use speed = distance/time
• ripples on water

  • use a stroboscope, which has the same frequency of water waved, then measure the difference between the fixed ripples, use v=fλ
  • or, mose a pencil along the paper at the same speed as a wavefront, and measure the time taken to draw this line, then use speed = distance/time
• refraction

  • waves pass from one medium to another
  • if passing into a denser medium, the wave will be refracted at the boundary and will change direction to bend towards the normal
  • speed decreases, wavelength decreases
  • the energy of a wave is constant, and energy is directly linked to the frequency of a wave, so if frequency is constant and speed decreases, wavelength must also decrease
  • the light bends close to the normal
• reflections

  • 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 non reflective
  • angle of incidence = angle of reflection
  • light will reflect if the object is opaque and it isn't absorbed by the material - the electrons will absorb the light energy, then reemit it as a wave
• waves will pass through a transparent material, the more transparent, the more light will pass. it can still refract but the process of passing through the material and still emerging is transmission
• if the frequency of light energy levels of the electrons, the light will be absorbed by the electrons and not reemitted. they will be absorbed and then remitted over time as heat, so this particular frequency has been absorbed.

  • if a material appears green, only green light has been reflected and the rest absorbed
• the ear

  • outer ear collects the sound and channels it down the ear
  • as it travels down it still is a pressure air wave
  • the sound waved hit the eardrum, tightly stretched eardrum which vibrates as the incoming pressure waves reach it, the eardrum vibrates at the same frequency of the sound wave, the small bones, (stirrup bone) connected will also vibrate at the same frequency
  • vibrations of the boned transmit to the fluid in the inner ear (cochlea), compression waved are then transferred to fluid
• the ear pt2

  • as the fluid moved due to compression waves the small hairs that line the cochlea move too
  • each hair is sensitive to different frequencies so some move more than other for certain frequencies, the hairs each come from a different nerve cell
  • when a certain frequency is received the hair tuned to the specific frequency moves a lot releasing an electrical impulse to the brain which then interprets the sound.
• the higher the frequency the more energy the wave has, which would damage cells in the ear quicker, and would not be able to work effectively long-term

  • this and the fact that we have evolved overtime not needing to hear very high or low frequencies, means the ear only works for a limited frequency range
• ultrasound, frequency higher than 20,000 Hz

  • sonar, pulse of ultrasound is sent below a ship and the time taken for it to reflect and reach the ship can calculate the depth
  • this is used to work out whether there is fish below the ship or how far the seabed is from 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
• • Infrasound is the opposite of ultrasound – it is a sound wave with a frequency lower than 20Hz – also known as seismic waves. There are two: P and S waves o This is used to explore the Earth’s core o P waves are longitudinal, and can pass through solids and liquids o S waves are transverse, only passing through solids (these move slower too) o 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