Phase difference

  •  The phase constant tells what the wave is doing at t = 0, x = 0
  •  The phase difference: difference between the phases of two waves

These two waves are completely out of phase. Where one has a crest, the other has a trough, so they exhibit completely destructive interference.

For identical sources, Df0 = 0 The path length difference is Dx
The waves interfere completely constructively when they are in phase, such that Dx = ml Path length difference is an integer number of wavelengths
  • The waves interfere completely destructively when they are out of phase
Two speakers emit waves with l = 2.0 m. Speaker 2 is 1.0 m in front of Speaker 1. What can be done to cause complete constructive interference between the two waves?

A. Move speaker 1 forward 1.0 m

 

B. Move speaker 1 forward 0.5 m

 

C. Move speaker 1 backward 0.5 m

 

D. Move speaker 1 backward 1.0 m

 

E. Don't move anything, they already have constructive interference

 

2-D interference is a direct extension of 1-D interference. Identify which points have completely constructive interference and which have completely destructive interference.

Use this interactive PhET simulation to explore superposition, and find locations of completely constructive interference and completely destructive interference for waves emanating from two sources.

Sample problems

Two speakers are in phase, emitting a note of frequency 686 Hz (l = 0.50 m). Speaker A is at the origin, while Speaker B is at (0.00 m, -2.2 m). Where is the first point along the x-axis, to the right of the origin, that you would be able to hear a maximum sound intensity?

Amplitude function

The amplitude function is useful for the special case where the two sources have the same amplitude, and their displacement from each other is known, as well as their initial phase difference.

The black dots in the images above represent two sources of sound waves

  • in phase (left)
  • out of phase (right)

The colors indicate the amplitude of the superposed sound.

The pale green lines show where destructive interference occurs in space.

Simulation source: PhET

Thin-film optical coatings

When light strikes a surface from a lower index of refraction to a higher index of refraction, the light wave undergoes a phase shift of pi radians.

 

When light comes from a higher index of refraction to a lower index of refraction, there is no phase shift.

 

You can think of this in terms of the wave on a string encountering a hard or soft boundary. Reflecting from the hard boundary causes a phase shift, where reflecting from the soft boundary does not.

 

In thin-film coatings, the phase-shifted, light reflected from the outer boundary interferes with the light reflected from the inner boundary.

 

If the interfering waves are in phase, they produce a "strong reflection" because they undergo constructive interference. However, the thickness of the film can be adjusted such that the waves undergo destructive interference, by ensuring that the waves are out of phase when they interfere. Thin-film coatings can be used to create anti-reflective coatings for optical lenses.

For identical sources, Df0 = 0 The path length difference is Dx The waves interfere completely constructively when they are in phase, such that Dx = ml Path length difference is an integer number of wavelengths
Two speakers emit waves with l = 2.0 m. Speaker 2 is 1.0 m in front of Speaker 1. What can be done to cause complete constructive interference between the two waves?
Two speakers are in phase, emitting a note of frequency 686 Hz (l = 0.50 m). Speaker A is at the origin, while Speaker B is at (0.00 m, -2.2 m). Where is the first point along the x-axis, to the right of the origin, that you would be able to hear a maximum sound intensity?
For identical sources, Df0 = 0 The path length difference is Dx The waves interfere completely constructively when they are in phase, such that Dx = ml Path length difference is an integer number of wavelengths
Two speakers emit waves with l = 2.0 m. Speaker 2 is 1.0 m in front of Speaker 1. What can be done to cause complete constructive interference between the two waves?
Two speakers are in phase, emitting a note of frequency 686 Hz (l = 0.50 m). Speaker A is at the origin, while Speaker B is at (0.00 m, -2.2 m). Where is the first point along the x-axis, to the right of the origin, that you would be able to hear a maximum sound intensity?