Module 8—Mechanical Waves
Lesson Summary
As you worked through this lesson, you should have developed partial answers to these questions:
- What is path length and path difference?
- What is the relationship between path difference and constructive/destructive interference?
- What are nodal and antinodal lines?
Interference depends on the phase relationship between waves, described in terms of path difference. To reach a common point, the wave fronts must each travel a certain path length, L. When two wave paths are compared and are found to travel different lengths, a path difference, ΔL, exists. The difference in path length is measured in terms of the wavelength of the waves produced by each source.
For constructive interference to occur, the waves must arrive in phase—the path difference must be a whole number of waves. For destructive interference, the waves must be out of phase—the path difference is offset by half a wave. These two conditions can be expressed mathematically. ΔL is path difference, λ is the wavelength, and n is the number of waves.
Constructive Interference |
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Destructive Interference |
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Interference patterns consist of regions of constructive and destructive interference. As such, interference patterns form lines.
Antinodal lines are regions of full constructive interference. Nodal lines are regions of full destructive interference.
Lesson Glossary
antinodal line: areas of full constructive interference
nodal line: areas of full destructive interference
path difference: the difference between two path lengths
path length: the distance between a source and an observer