Page 217 - text book form physics kssm 2020
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Chapter 5
Waves
Interference with Coherent Source of Waves
Interference of waves is the superposition of two or more waves from a coherent source of
waves. Two sources of waves are coherent when the frequency of both waves is the same and
the phase diff erence is constant. Superposition of waves produces constructive interference and
destructive interference.
Constructive interference occurs when two crests are in superposition.
Before superposition During superposition After superposition
P Q Q P
A 2A A
Combined displacement = A + A = 2A
Figure 5.55 Constructive interference between two crests
Constructive interference also occurs when two troughs are in superposition.
Before superposition During superposition After superposition
–A –A
–2A
P Q Q P
Combined displacement = (–A) + (–A) = –2A
Figure 5.56 Constructive interference between two troughs
Destructive interference occurs when a crest and a trough are in superposition to produce zero
combined displacement.
Before superposition During superposition After superposition
P P
A A
–A –A
Q Q
Combined displacement = A + (–A) = 0
Figure 5.57 Destructive interference between a crest and a trough
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