Question 24
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
Reference: Past Exam Paper – November 2018 Paper 12 Q29
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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