Q#24 (Past Exam Paper – November 2018 Paper 12 Q29)
Two sources of microwaves P and Q produce coherent waves with a phase difference of 180°.
The waves have the same wavelength λ.
At the point S there is a minimum in the interference pattern produced by waves from the two sources. The distance (QS – PS) is called the path difference.
In the expressions shown, n is an integer.
Which expression represents the path difference?
A nλ
B ½ nλ
C (n + ½) λ
D (2n + ½) λ
B ½ nλ
C (n + ½) λ
D (2n + ½) λ
Solution:
Answer: A.
When 2 sources of waves that are in phase interfere:
Constructive interference (maxima): path difference = nλ where n is an integer
Destructive interference (minima): path difference = (n+1) λ / 2
However, if the sources has a phase difference of 180°, the above conditions no longer hold for constructive and destructive interference. The conditions are reversed.
Constructive interference (maxima): path difference = (n+1) λ / 2
Destructive interference (minima): path difference = nλ
A minima is produced at S, indicating that destructive interference occurs. So, path difference = nλ.
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