PhysicsLAB Resource Lesson
Interference of Waves

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When two or more waves simultaneously and independently travel through the same medium at the same time, their effects are superpositioned. The result of that superposition is called interference. There are two types of interference: constructive and destructive.
  • Constructive interference occurs when the wave amplitudes reinforce each other, building a wave of even greater amplitude.
  • Destructive interference occurs when the wave amplitudes oppose each other, resulting in waves of reduced amplitude.

In the diagrams that follow, two arbitrary waves have been superpositioned, with their resultant interference patterns shaded in. Regions of constructive interference are labeled with "C" and regions of destructive interference with a "D." Note that the regions alternate, but are not necessarily equal in size. A mini-lab to practice locating and identifying regions of constructive and destructive interference is provided.
 
 
 In regions labeled with a "C," are the amplitudes of the original "green" and "blue" waveforms inside or outside of the shading?

 In regions labeled with a "D," are the amplitudes of the original "green" and "blue" waveforms inside or outside of the shading?

 
Standing waves
 
When two identical waves travel through the same medium at the same time but in opposite directions, a special interference pattern called a standing wave is formed. Within a standing wave, regions of constructive interference are called antinodes and regions of destructive interference are called nodes
 
This name is derived from the impression that the wave appears to be "standing still" since the nodes and antinodes are not being translated from one end of the medium to the other even though the wave's energy is continuously traveling "back and forth."
 
The lowest frequency to produce a standing wave pattern in a medium is called the fundamental, or the 1st harmonic. As additional "loops" are inserted, overtones are produced. A loop equals a distance of ½λ. In each case, since the medium has not changed, the wave speed remains constant and we see evidence of the relationship that the wavelength is inversely proportional to the frequency.
 
L = 1 loop
L = 0.5 λo
 
f = fo
L = 2 loops
L = 1.0 λ1
 
f1 = 2fo
L = 3 loops
L = 1.5 λ2
 
f2 = 3fo



 
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