P6: Waves

• Waves transfer energy from one place to another but they don't transfer any matter
• When light waves pass from a phone screen to your eye or sound waves pass from the speakers to your ear, only energy is being transferred
• Our brain is able to build up images and tunes from the light and sounds that it receives
• Displacement distance graph


  The distance is how far the wave has traveled from the starting point, while the displacement is how far from the equilibrium point the wave has oscillated
• Amplitude


  The maximum displacement of the wave
• Wavelength


  The distance of one entire oscillation
• Crest


  The very top of a wave
• Trough


  The opposite of the crest
• Displacement time graph


  The length of one complete oscillation is the time period, instead of the wavelength
• Calculating frequency


  Time period = 1 / Frequency
• Transverse waves


  • Oscillations are perpendicular to the direction of energy transfer
  • Examples: electromagnetic waves, ripples and waves in water, waves of strings
• Longitudinal waves


  • Oscillations are parallel to the direction of energy transfer
  • Examples: sound waves, seismic p waves
• Waves aren't always reflected whenever a wave arrives at a boundary between two materials or mediums
• What can happen when a wave arrives at a boundary

  • The wave might be absorbed by the material
  • The wave could be transmitted and pass through the material
  • The wave could be reflected off the surface
• Which of the three things happens depends on the wavelength of the wave involved and the properties of the two materials
• Ray diagram


  Used to show reflection taking place
• Ray diagram


  • Start at the boundary between the two materials
  • Draw the incoming ray of light
  • Draw the normal (perpendicular line)
  • Measure the angle of incidence
  • The angle of incidence is always equal to the angle of reflection
  • Draw the reflected ray
• Point of incidence


  The point where the incoming ray touches the boundary
• Specular reflection


  Reflection off a perfectly smooth surface, where the normals are all in the same direction and the light is reflected in the same direction
• Diffuse/scattered reflection


  Reflection off a rough surface, where the normals point in different directions and the light is reflected in all different directions
• In both specular and diffuse reflection, the angle of incidence is always equal to the angle of reflection
• The surface of a rough material isn't flat, so the normals will be pointing in different directions, causing the reflected rays to get scattered
• Refraction of light waves


  When waves change direction as they pass from one medium to another, like from air into glass
• Refraction


  • Waves travel at different speeds in different materials or mediums
  • The higher the density of the material, the slower the wave will travel through it
• Refraction of light waves

  1. Wave travels from a less dense medium (e.g. air) to a more dense medium (e.g. glass)

  2. Wave slows down as it passes into the more dense medium

  3. If wave hits the boundary at an angle, it will be refracted (change direction)

  4. Wave bends towards the normal (perpendicular line) when passing into a more dense medium
• Drawing ray diagrams for refraction

  1. Draw normal (perpendicular line) at point of incidence

  2. Draw refracted ray (about halfway towards normal)

  3. Draw emergent ray (bends away from normal when passing from more dense to less dense medium)

  4. Add angle of incidence and angle of refraction
• Frequency of wave stays the same, only wavelength changes when wave speed changes
• Different wavelengths of light are refracted by different amounts when passing through a triangular prism, causing them to spread out like a rainbow
• Electromagnetic (EM) waves


  Transverse waves that oscillate perpendicular to the direction of energy transfer
• EM waves travel at 3x10^8 m/s in a vacuum
• EM waves travel at different speeds in different mediums, which can lead to refraction
• Types of EM waves

  • Radio waves
  • Microwaves
  • Infrared
  • Visible light
  • Ultraviolet
  • X-rays
  • Gamma rays
• Wavelength and frequency


  Inversely related - as one increases, the other decreases
• Wavelengths range from multiple kilometers for radio waves to less than 0.001 nm for gamma rays
• Visible light


  The only part of the EM spectrum that human eyes can detect
• Colors of the visible spectrum

  • Red
  • Orange
  • Yellow
  • Green
  • Blue
  • Indigo
  • Violet
• The order of the EM spectrum can be remembered as: radio, microwaves, infrared, visible, ultraviolet, X-rays, gamma rays
• Ultraviolet, X-rays, and gamma rays are ionizing and can cause damage to cells
• Uses of EM waves

  Microwaves and radio waves are used in communication
• Sources of EM waves

  1. Radioactive decay emits gamma rays

  2. Electron energy level changes emit visible light, UV, X-rays

  3. Molecular bond vibrations emit infrared