Waves

• Wave


  An oscillation which transfers energy through a medium, without any overall movement of that medium
• Types of Waves

  • Transverse wave
  • Longitudinal wave
• Transverse wave

  • Vibration of the wave is perpendicular to its motion
  • e.g. light and all electromagnetic waves, some seismic waves
• Longitudinal wave

  • Vibration of the wave is parallel to its motion
  • e.g. sound, seismic waves
• Properties of a Wave

  • Wavelength
  • Amplitude
  • Frequency
  • Period
• Wavelength


  The distance between two identical points on the wave. Unit: metre
• Amplitude


  The maximum displacement of a point from its mean position
• Frequency, f

  The number of complete waves passing per second, or, The number of complete oscillations per second. Unit: hertz(Hz)
• Period, T


  The time for one complete wave/oscillation. Unit: second(s)
• Speed = frequency * wavelength
• Interference

  Happens when two or more waves meet, and the resultant displacement is equal to the algebraic sum of the individual displacements
• Constructive interference occurs when two waves combine to give a wave of larger amplitude
• Destructive interference occurs when two waves combine to give a wave of smaller amplitude
• Coherent sources

  • Sources that have the same frequency and are in phase with each other
• If coherent waves of equal amplitude have travelled different paths and the difference between their paths is half a wavelength, or an odd number of half wavelengths, then they will be half a wavelength out of phase and the result will be complete destructive interference
• If the path difference is a whole number of wavelengths the waves will arrive in phase and the result will be constructive interference
• An interference pattern is a pattern of alternate regions of constructive and destructive interference
• Polarisation


  The confining of the vibrations of transverse waves to one plane only
• Light from ordinary sources is not polarised because the light vibrations occur in all directions perpendicular to the direction in which the light is travelling
• Two Methods for Polarising Light

  • Using Polaroid Filters
  • By Reflection
• Polaroid Filters

  • Reduce intensity (brightness) by confining vibrations to one plane only (polarising)
• Reflection

  • Light that reflects off a plane surface is polarised vertically or horizontally (depending on the orientation of the plane)
• Applications of Polarisation

  • Polaroid sunglasses reduce intensity (brightness)
  • Polaroid sunglasses remove glare from a surface by reversing polarisation by reflection
  • Waves carrying TV signals can be polarised
• When a piece of plastic is stressed by forces exerted on it, it polarises light passing through the stressed area of the plastic (stress polarisation)
• Adjusting a telescope

  1. Adjust leveling screws to ensure table is level

  2. Adjust the eyepiece until the cross-threads are in focus

  3. View a distant object with the telescope and adjust the telescope until the distant object is in focus

  4. Illuminate the slit and view it through the telescope, move the slit relative to the collimator so that the slit is in focus

  5. Adjust the width of the slit so that its image in the telescope is suitably bright
• Monochromatic light

  Light of only one wavelength (or frequency)
• Monochromatic light sources

  • Light from a sodium vapour lamp
  • Light from a laser
• Diffraction

  The sideways spreading out of a wave when it passes through a gap or passes by an obstacle
• Diffraction grating

  A piece of transparent material on which a very large number of parallel lines are engraved. The spaces between the lines behave as slits and allow the light to pass through.
• Grating constant (d) or grating spacing (d)

  The distance between 2 adjacent slits (ie. the width of one line and one slit) on the diffraction grating
• Grating constant (d)

  Inversely proportional to the number of lines per metre on the grating
• Interference demonstrates light's wave nature
• A diffraction grating cannot diffract X-rays because the wavelength of X-rays is much shorter than the spacing between the lines on the grating
• Deriving the diffraction grating formula

  Constructive interference occurs at values of θ where the path difference of light from adjacent slits is an integer multiple of the wavelength
• If the number of lines per mm on a diffraction grating increases

  The distance between the bright images on the screen increases
• Diffraction grating formula


  nλ = d Sin
• If d gets smaller

  Sin θ must get bigger
• If d gets smaller

  The bright images get further apart
• Monochromatic light

  Wavelength 6.2 x 10-7 m
• The largest value of Sin θ is Sin-1