Topic 6 – Waves

• Waves


  Transfer energy not matter (objects don't move)
• Types of waves


  • Transverse
  • Longitudinal
• Transverse waves


  • Vibrations are perpendicular
• Longitudinal waves


  • Vibrations are parallel
• Types of waves


  • EM Waves (e.g. Light)
  • Ripples on water
  • S waves
  • Sound
  • P waves

  
• Measuring speed of sound

  1. Set a Freq. on speaker

  2. Start with microphones in-line together

  3. Both waves appear sync on oscilloscope

  4. Move one mic back, waves will de-sync

  5. Keep moving and stop when re-sync

  6. The distance between two mics = 1 WL

  7. Speed = Freq. x WL
• Measuring speed of water waves

  1. Set Freq. on signal generator (dipper)

  2. Use a lamp to create shadows of waves

  3. Alter freq. of lamp until shadow appear still

  4. Now, Freq. of lamp = Freq. waves

  5. Measure distance between shadows = 1 WL

  6. (Or calculate average distance)

  7. Speed = Freq. x WL

  
• Wave speed


  v = f λ
• Measuring wave speed on a string

  1. Adjust Freq. on signal generator

  2. Measure distance for no. Of half WLs (e.g 4 half WLs = 2m)

  3. Get mean half WL (2m / 4 = 0.5m)

  4. Then double to get full WL (0.5m x 2 = 1m)

  5. Speed = Freq. x WL
• What happens when a wave hits a boundary (change in medium)

  • Absorption
  • Reflection
  • Transmission (-> Refraction)

  
• Normal


  A perpendicular line (90o) to surface at point of incidence
• Types of reflection


  • Specular reflection (Smooth surface, e.g. Mirror)
  • Diffuse reflection (Rough surface, e.g. Paper)
• In both specular and diffuse reflection, the angle of incidence (AoI) equals the angle of reflection (AoR)
• Investigating reflection in different materials

  1. Draw line on piece of paper with pencil

  2. Place object on the line

  3. Shine light on object using ray-box and draw normal at incidence

  4. Measure AoI and AoR using protractor and width of light
• Smooth surfaces = thin + bright + visible line, Rough surfaces = wide + dim + no clear line
• Refraction


  Change in direction of a wave when density of medium changes
• During Refraction, Frequency NEVER changes
• Low to High Density

  Waves come closer, Wavelength decreases, Speed decreases, Bends TOWARD normal
• High to Low Density


  Waves move further apart, Wavelength increases, Speed increases, Bends AWAY from normal
• If wave comes at 90o angle to boundary it will not refract (only change speed)

  
• Investigating Refraction in different Materials

  1. Place block on material on paper and draw around it

  2. Shine light using ray box

  3. Trace over incident and emerging rays

  4. Remove block, draw Normal and connect incident and emerging rays

  5. Use a protractor to calculate AoI and AoR
• Different densities will produce different Angles of Refraction (AoR)
• Similarities between Electromagnetic Waves

  • Can travel in a vacuum (space)
  • Same speed
  • All are TRANSVERSE waves
• Differences between Electromagnetic Waves


  • Frequency and Wavelength
  • Gamma has most energy, Radio has least energy
• How Radio Waves are produced and detected

  1. An A.C causes charges (e-) in the transmitter to oscillate

  2. Radio waves are created

  3. Freq. of Radio waves = Freq. of A.C

  4. Radio waves are absorbed by e- in receiver, which start oscillating, creating an A.C

  5. Freq. of new A.C = Freq. of Radio waves
• Types of Radio Waves

  • SHORT (10-100m)
  • LONG (1-10km)
• Short and Long Radio Waves

  • Transmitter and Receiver don't have to be in line of sight
  • Reflected by Ionosphere
  • Diffracts (bends) across planet
• Microwaves


  • Waves can pass watery layer atmosphere
  • Waves absorbed by water in food
  • Satellites use different Freq. to ovens to stop absorption by watery layer atmosphere
• Uses of Microwaves


  • Satellites
  • Microwave oven
• Uses of Infrared Radiation

  • Food
  • Electric Heaters
  • Thermal Monitors (E.g. Thermoscope)
• Infrared Radiation


  Ovens release IR which is absorbed by food = (heat^)
• Visible Light


  Data is transmitted as light, which reflects inside a tube that has glass walls
• Uses of Ultraviolet


  • Security Pens
  • Fluorescent Lamps
• Ultraviolet


  • Mark your property with security ink It will glow under UV otherwise invisible
  • UV → Phosphorous layer → Visible light Energy efficient
• Uses of X-rays and Gamma rays

  • Check for broken bones
  • Medical tracer
  • Cancer Treatment
• 1. rays and Gamma rays

  • X-rays pass through flesh but absorbed by dense material e.g. bones
  • Gamma emitting source is injected into patients and followed. Gamma can easily leave the body and detected from outside
  • High dose of either type can be targeted to kill cancer cells and avoid healthy cells
• PROTECT YOURSELF WHEN USING X-rays and Gamma rays. Lead aprons, Lead Screen or leave the room to avoid too much exposure
• Radiation dose


  Measured in Sieverts (Sv), measure of risk of harm to body exposed from radiation
• Types of images


  • Virtual Image
  • Real Image

  
• Virtual Image


  • Same side, can't project onto screen