Interference at a point

• When do waves interfere?
  • Waves from 2 or more identical sources pass through the same region of space
• What is the total disturbance?
  • Use the principle of superposition.
  • The total disturbance at a point is the sum of the disturbances from each source alone.
  • The result can be larger or smaller than the individual waves, depending on the relative phase.
• Constructive Interference
  • Crests coincide with crests.
  • Troughs coincide with troughs.
  • Waves are “in phase”.
  • The resulting disturbance has twice the amplitude of the individual waves…
  • …assuming two sources emitting waves of equal amplitude
• Destructive Interference
  • Crests coincide with troughs.
  • Waves are “180° out of phase”.
  • Zero total disturbance…
  • …again assuming two sources emitting waves of equal amplitude
• Constructive and destructive interference are just the extreme cases.
  • Smooth variation from one to the other
  • “Partially out of phase”

Interference Patterns

• What is an interference pattern?
  • Interference that is stable in time at each point in a region of space, creating an observable pattern of maxima (constructive interference, large amplitudes) and minima (destructive interference, small amplitudes)
• Require “coherent” sources
  • Same frequency
  • Constant phase relationship
• It’s not hard to make physically distinct sources of water waves or sound waves coherent.
• It’s very hard to do this with sources of visible light waves…
• …so take light from one source and create several different sources by sending the light on different paths before allowing it to interfere.
• This is most easily done with bright, monochromatic (single frequency) sources of light with good phase stability…
• …so we will use lasers as our light sources in this lab.

Equipment Data Collection and Analysis