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1
5.2
Interference
Coherence
TwoSlit Interference
Thin film Interference
Interference Effects
Interference is a general property of waves.
A condition for interference is that the wave
source is
coherent.
Interference between two waves gives
characteristic interference patterns due to
constructive
and
destructive interference
.
Coherence
For two waves to show interference they
must have coherence.
Two waves are coherent if one wave has a constant
phase relation to the other
coherent
incoherent
phase
shift
x
2
∆
φ= π
λ
∆
x
Coherence
Light from
two separate
light bulbs is
Incoherent
Light from a single
light bulb passing
through a small slit is
coherent
Laser light is coherent
Interference
δ
=0
Sum
Constructive Interference
λ
E
Distance >
Interference
δ
=
2
λ
Destructive Interference
distance
E
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Interference
δ
=
λ
Constructive Interference
path difference
path difference =
δ
=r
2
–r
1
In phase
Condition for constructive interference
m
δ
λ
=
Condition for destructive interference
1
()
2
m
=+
m = 0 +
1, +
2,….
r
1
r
2
Superposition of waves
at A shows interference
due to path differences
A
Order number
m
Coherent
waves
barrier
Interference pattern of water waves
Interference Pattern
Constructive
Destructive
Interference pattern due to
two spherical waves
Coherent
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 Spring '09
 Smith

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