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ContentLearning outcomes
Introduction
Notes
Video / Image
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At the end of this chapter, studentsshould be able to:
Definecoherence.
State the conditions for
interference of light.
State the conditions ofconstructive and destructive
interference.
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Interference is defined as the effectof interaction between two
or morewaves which overlaps or
superposed at a point and at aparticular time from the
sources
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There are 2 type of
interference :
a) Constructive interference
b) Destructive interference
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a) Constructive interferenceis
defined as a reinforcement ofamplitudes of light waves thatwill
produce a bright fringe
(maximum).b) Destructive interferenceis
defined as a total cancellation of
amplitudes of light waves thatwill produce a dark
fringe(minimum).
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Conditions for permanentinterference
Permanent interference betweentwo sources of light only take
placeif they are coherentsources.
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It means
a) the sources must have the
same wavelength or
frequency.
b) the sources must have a
constant phase differencebetween them.
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The light waves that are interfering
must have the same or
approximately of amplitudeto
obtain total cancellationat
minimum or to obtain a good
contrastat maximum
and
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the distance between thecoherent sources should be as
small as possibleof the light
wavelength ().
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is defined as the difference in distance fromeach source to a
particular point.
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a) Constructive Interference.
x2
x1
Path difference, L= |x2x1|
S1
S2
screen
P
L
mL ,.....2,1,0 mwhere :
wavelength
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b) Destructive Interference.
S1
S2
x1
x2
Q
(minimum)
2
1mL
,.....2,1,0 m
where :
wavelength
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Interference pattern for two coherent sources
in phase
Fringe m L
S1
S2
screen
32
4
0
2
0
0
11
2
2
1
0Central bright
1st dark
02
11st dark
1st bright
2 1 1st bright
2
3
2nd dark
3 12
32nd dark
2nd bright
4 2 22nd bright
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a) Constructive Interference.
,.....2,1,0 mwhere :
wavelength
21mL
S1
S2
x1
x2
P (maximum)
L
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b) Destructive Interference.
,.....2,1,0 m
where :
wavelength
mL
S1
S2
x1
x2
Q
(minimum)
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Interference pattern for two coherent sources
in anti-phase
Fringe m L
S1
S2
screen
32
4
0
2
0
0
11
2
2
1
0Central dark
1st bright
02
11st bright
1st dark
2 1 1st dark
2
3
2nd bright
3 12
32nd bright
2nd dark
4 2 22nd dark
