Topic 3 - Waves and Electromagnetic Spectrum

• waves


  transfer energy and information in the direction theyre travelling

  amplitude - displacement from rest position to crest/trough

  wavelength - length of full cycle of wave (crest to crest)

  frequency - number of complete cycles of wave passing through a certain point per second

  period - number of seconds it takes for one full cycle
• transverse waves


  perpendicular (sideway) vibrations to the direction the wave travels

  most waves transverse including:

  • all electromagnetic waves e.g. light
  • s-waves
  • ripples and waves in water
• longitudinal waves


  parallel vibrations to the direction the wave travels

  • sound waves
  • p-wave

  squash up and stretch out the arrangement of particles they pass through - making compressions and rarefactions

  compressions = high pressure, many particles

  rarefactions = low pressure, fewer particles
• wave speed


  tell you how quickly wave moves though space

  two different ways to calculate:

  • v=x/t (distance/time)
  • v=f$\lambda$ (frequency X wavelength) - called the wave equation
• oscilloscope - measure the speed of sound (practical)


  attach signal generator to speaker - can generate sounds with specific frequency - can use two mics and oscilloscope to find wavelength

  1. set up oscilloscope so detected waves at each mic are shown as separate waves
  2. start with both mics next to speaker - slowly move one away until two waves are aligned on display (but have moved exactly one wavelength apart)
  3. measure distance between mics to find one wavelength
  4. use formula to find speed of soundwaves passing through air

  (frequency is whatever you set the generator to in the first place)
• strobe light - measure speed of ripples (practical)


  attach signal generator to dipper of ripple tank - can create waves at set frequency

  1. dim lights, turn on strobe light - see wave pattern made by shadows of wave crests on screen below tank
  2. alter frequency of strobe light until wave pattern appears to stop moving (frequency and strobe lights equal)
  3. distance between each shadow line is equal to one wavelength - measure distance between lines that are 10 wavelengths apart - find average wavelength
  4. use formula to calculate wave speed
• peak frequency - speed of waves in solids (practical)


  measuring frequency of sound waves produced when you hit object

  1. measure and record the length of a metal rod
  2. set up apparatus - making sure rod is secure at its centre
  3. tap rod with hammer - write down peak frequency displayed by computer
  4. repeat 3 times to get average wave frequency
  5. calculate speed of wave (wavelength = twice the length of rod)
• waves - absorbed, transmitted, reflected at boundaries


  • absorbed by second material - wave transfers energy to materials energy stores (often to thermal - leads to heating)
  • transmitted through second material - wave carries on travelling through new material (often leads to refraction) - can be used in communications, lenses of glasses and cameras
  • reflected - incoming ray 'sent back' away from second material - how echoes created
• refraction 


  waves travel at different speeds in materials with different densities - when wave crosses boundary between materials it changes speed

  hits boundary at angle - change of speed = change of directio

  along normal - change speed (not refracted)

  greater change in speed = more a wave bends

  bends towards normal = slows down / away = speeds up

  frequency of wave stays the same when crossing boundary
• what does a ray diagram show

  path that a wave travels
• refracted light ray - ray diagram


  1. draw boundary between two materials and the normal
  2. draw incident ray that meets normal at boundary
  3. angle between ray and normal = angle of incidence
  4. draw refracted ray on other side - if second material optically denser=refracted ray bends to normal / angle of refraction smaller than angle of incidence
  5. less optically dense = angle of refraction larger than angle of incidence
• refraction - wave front diagrams


  when one part of wave crosses a boundary into denser material - that part travels slower than rest of the wavefront - wave bends
• sound waves

  caused by vibrating objects

  vibrations passed through surrounding medium as series of compressions and rarefactions

  type of longitudinal wave

  through solid - does so by causing particles in the solid to vibrate

  not all frequencies of sound can be transferred through an object
• what determines which frequencies an object can transmit


  size

  shape

  structure
• sound waves (more)


  different speeds in different media - faster in liquids than gases - faster in solids than liquids

  frequency of sound doesnt change when passes through one medium to another - wavelength does (speeds = longer / slows = shorter)

  can refract as they enter different media

  reflected by hard, flat surfaces (echoes = reflected sound waves)

  sound cant travel in space - mostly a vacuum (no particles to move/vibrate)
• eardrum


  sound waves that reach your ear drum cause it to vibrate

  vibrations passed on to ossicles (tiny bones) through semi-circular canals and cochlea

  chochlea turns vibrations into electrical signals which get sent to brain

  brain interprets signals as sounds of different pitches and volumes - depending on frequency and density (higher frequncy = higher pitch)

  human hearing limited by size/shape of eardrum / structure of all the parts within the ear that vibrate to transmit the sound waves
• ultrasound


  electrical devices can be made that produce electrical oscillations of any frequency - can easily be turned into mechanical vibrations to produce sound waves of 20,000 Hz (above range of human hearing)
• ultrasound - partially reflected


  when wave passes from one medium to another - some of the wave is reflected off boundary , some transmitted and refracted - partial reflection

  you can point a pulse (short burst) of ultrasound at an object - wherever there are boundaries, some ultrasound gets reflected back

  time it takes for reflections to reach a detector can be used to measure how far away the boundary is
• ultrasound - medical imaging (prenatal scanning of foetus)


  can pass through body but whenever they reach a boundary between two different media (fluid of womb and skin of foetus) - some wave is reflected back and detected

  exact timing and distribution of these echoes - processed by computer to produce video images of foetus

  ultrasound imagining is completely safe
• ultrasound - industrial imaging (finding flaws in materials)


  can find flaws in objects like pipes, or materials like wood or metal

  waves entering material will usually be reflected by far side of the material

  if flaw inside object (crack) - waves will be reflected sooner
• ultrasound - other uses


  echo sounding - type of sonar used by boats and submarines to find out distance to sea bed / locate objects in deep water
• infrasound


  so low in frequency that humans cant hear them (below 20Hz)

  some animals communicate using infrasound (elephants/whales) - by detecting infrasound, scientists able to track these animals

  natural events (erupting volcanoes, avalanches, earthquakes) produce infrasound in local area - scientists can monitor infrasound to predict events (volcano eruption)
• seismic waves


  earthquakes produce these at a range of frequencies which travel out through earth - detect them using seismometers

  seismologists work out time it takes for the waves to reach seismometer / note which parts of earth dont recieve waves at all

  when seismic waves reach a boundary between different layers of material inside earth - some absorbed / refracted

  most times if refracted - change speed gradually (curved path)

  when properties change suddenly - wave speed changes abruptly (kink)
• p-waves and s-waves

  main two seismic waves

  by observing how seismic waves are absorbed and refracted - work out where properties of earth change dramatically
• p-waves (inside earth)


  longitudinal

  travel through solids and liquids

  travel faster than s-waves
• s-waves (inside earth)


  transverse

  only travel through solids

  slower than p-waves
• law of reflection (used for all reflected waves)

  angle of incidence = angle of refelction
• what is the angle of incidence


  angle between incoming wave and normal
• what is the angle of reflection


  angle between reflected wave and normal
• what is the normal


  imaginary line thats perpendicular to the surface at the point of incidence

  usually shown as dotted line
• total internal reflection


  wave hitting surface can experience this (reflected back into material)

  can only happen when wave travels through dense material (glass/water) towards less dense sunstance (air) and the angle of incidence is larger than the critical angle

  every boundary has its own different critical angle
• reflection - specular/diffuse

  waves are reflected by different boundaries in different ways

  specular = waves are reflected in single direction by smooth surface - get clear reflection

  diffuse = waves reflected by rough surface - waves reflected in different directions - happens because normal is different for each incident ray so each ray has different angle of incidence - when light is reflected by something rough = surface looks matt and you dont get a clear reflection
• investigating refraction - practical (conditions)


  best to do in dim room - clearly see ray of light

  ray must be thin so you can easily see the middle when tracing it and measuring angles from it

  to do this, use ray box - enclosed box that contains light bulb - a thin slit is cut into one of the sides allowing a thin ray of light out of box that you can use for your experiment
• investigating refraction - practical


  1. put glass block on piece of paper and trace around it - use ray box to shine ray of light at middle of one side
  2. trace incident ray and emergant ray on other side of block - remove block - join up incident ray and emergent ray to show path of refracted ray through block
  3. draw normal at the point where light ray entered block - use protractor to measure angle of incidence and angle of refraction
  4. do same for point where ray emerges from the block
  5. repeat three times - keeping angle of incidence as ray enters same
  6. calculate average for each of the angles
• investigating refraction - conclusion (practical)


  should see that the ray of light bends towards the normal as it enters the block (angle of refraction is less than angle of incidence) - because air has one of the lowest optical densities so light ray will almost slow down when it enters block

  should see ray of light bends away from normal as it leaves block - ray speeds up as it leaves block and travels through air

  remember - all EM waves can be refracted - this experiment uses visible light so that you can actually see ray being refracted as it travels through block
• colour


  colour is about differences in absorbtion, transmission, and reflection of different wavelengths by different materials
• opaque objects


  do not transmit light

  absorb some wavelengths and reflect others

  colour of object depends on which wavelengths of light are reflected
• white objects


  reflect all wavelengths of light equally
• black objects


  absorb all wavelengths of visible light
• transparent and translucent objects

  transmit light

  not all light that hits surface of object is absorbed or reflected