Interference Guided Notes
Name:____________________
Interference
Can two different objects occupy the exact same location at the exact same time? Of course not … but two waves can!
When waves meet, they ____________________________ (add together), but then continue traveling unaltered, as if
they had never had contact.
The superimposition of waves can create complex wave patterns.
Constructive interference
Occurs when ________________________________________________________________________
at a given point in space.
Causes an ____________________________and thus energy) at that point.
Causes bright spots / loud sounds / increase in vibration
Destructive interference
Occurs when the ______________________________________________________________________
at a given point in space.
Causes a ________________________________(and thus energy) at that point.
Causes a decrease (or absence!) of light, sound, vibration
Constructive interference  waves are in phase
Destructive interference  waves out of phase
Principle of Superposition
When two waves interfere, the resulting displacement of the medium at any location is the _____________
______________________________________________________________________ at that same location
Position
Blue
Displacement
Red
Total
A
0
0
0
B
3
1
4
Displacement
C
D
E
F
G
Example: To find the resulting wave pattern for the two interfering waves shown above, simply add the
displacements of each wave at each point.
Is this an example of constructive or destructive interference?
You Do: Principle of Superposition
Draw the interference pattern for the two waves shown on the graph below. (Do at least to point J)
Which points have constructive
interference?
Which points have destructive interference?
Constructive or Destructive?
Two boys, each 10 m apart, are splashing, creating identical waves of wavelength 2 m that travel towards each
other. Their little sister is playing in between them, located 4 m away from one boy, 6 from the other.
Will the waves by the sister be big (due to
constructive interference) or small (due to
destructive interference)?
How can we tell?
We have to determine whether the waves are _______________________________, or not. To do this, we
can compare _______________________________ two waves. If the difference in path length equals
______________________(_____________________ 2λ, 3λ, 4λ, etc.) then the waves will be in phase.
X and Y are coherent sources of 2cm waves.
At each of the following points, determine whether they interfere constructively or destructively.
A–
B–
C --
The wavelength of a transverse wave is 4cm. At some point on the wave the displacement is -4cm. At the
same instant, at another point 50cm away in the direction of propagation of the wave, the displacement is
A. 0cm
B. 2cm
C. 4cm
D. -4cm
We have to find out what ________________ of the wave is 50 cm
away.
_______________________________
The other point is _____________________ /
______________________________-
Real life examples of interference
Noise-canceling headphones create sound waves 180o
out of phase with ambient noise to cancel out the sound
Instruments are tuned with a tuning fork by listening for
_____________ – fluctuation in the loudness of the sound
– when both the instrument and the tuning fork are played
together. If the instrument is out of tune, then the
wavelengths of the two notes will not match and will
fluctuate between constructive and destructive interference,
creating the beats. _____________________________
___________________________________________.