Phase difference

• Phase difference


  The difference in the point of the cycle between one or multiple waves
• Calculating phase difference


  1. A and C = 1/2 λ = 180° = π rads

  2. C and G = 1 λ = 360° = 2π rads

  3. D and E = 1/4 λ = 90° = 0.5π rads
• Stationary waves


  • Time period (T) = 0.12 ms = 1.2 x 10^-4 s
  • Frequency (F) = 1/T = 8.3 kHz
  • Phase difference = 90° = 0.5π rads
• Principle of superposition


  The resultant displacement equals the vector sum of the individual displacements when two or more waves cross
• Constructive interference


  When waves add together to produce a larger amplitude
• Destructive interference


  When waves cancel each other out to produce a smaller amplitude
• Graphical superposition


  Add the individual displacements at each point along the x-axis and plot the resultant
• Electromagnetic radiation (light) is a transverse wave, and can be plane-polarised
• Passing EM radiation through a polarising filter will remove the components at 90° to that of the filter
• Diffraction


  When waves travel beyond a gap or barrier, they spread out beyond the gap/barrier
• Diffraction grating


  Has lots of very small gaps close together, where the wave crests overlap and constructively interfere
• Practice questions


  • What does the principle of superposition say?
  • Describe constructive interference.
  • What is total destructive interference?
  • What is the phase difference of two points on a wave?
  • Give three possible units for phase difference.
  • When are two points on a wave exactly out of phase?
  • What does it mean for two waves to be in phase?