Transcript Waves Interference 6.6:Interference of Waves 6.7

J. Pulickeel
SPH3U1
January 2010
6.6: Wave Interaction
So far we have been considering one wave experiencing
changes.
Consider the case of two waves coming into contact with each
other. What do you think could happen?
When two waves meet while traveling along
the same medium it is called INTERFERENCE.
Let’s consider two waves moving towards each other, both
having a positive upward amplitude.
What will happen when they meet?
2
Constructive Interference
 They will ADD together to produce a greater amplitude.
 This is known as CONSTRUCTIVE INTERFERENCE.
What do you notice about the amplitude and the width?
3
Constructive Interference
Point P is called a
SUPERCREST at
this location
Have you seen a SUPERBOUNCE?
Destructive Interference
 Now let’s consider the opposite, two waves moving
towards each other, one having a positive (upward)
and one a negative (downward) amplitude.
 What will happen when they meet?
5
Destructive Interference
 This time when they add together they will produce a
smaller amplitude.
 This is know as DESTRUCTIVE INTERFERENCE.
The Principle of Superposition
At any point, the resulting amplitude of two interfering waves is the
algebraic sum of the dipslacements of the individual waves.
6
Use the principle of superposition to determine the resulting pulse
when the pulses shown are superimposed on each other.
(The point of overlap should be at the horizontal midpoints of the
pulses.)
6.7: Resonance
 Every object has a natural frequency which is
determined by the source.
 Consider a child on a swing. If you push her at the
right time (at the right frequency or in phase) the
child will swing higher and higher.
 The transfer of energy from one objet to another
having the same frequency is called resonance
Resonance
 This is why
 your windows shake when a truck drives by...
 Coffee shoots out of a cup in your car.
 Glass breaks when opera singer sings...
Where are the
antinodes?
Where are the
nodes?
Show
 Helicopter video
 Tacoma Narrows Bridge video
 Rice Resonance
Show Tuning Fork
Experiment...
6.8: Standing Waves
Standing waves are created when waves
travelling in opposite directions have the
same amplitude and wavelength.
Standing waves are easier to analyze because
they seem stationary.
When waves of equal amplitude and shape
interfere they can create nodes (0 points)
and anti-nodes (supercrests and super
troughs)
(a) (c) (e) show destructive interference
(b) (d) show constructive interference.
(f) shows the resulting “standing wave
interference pattern” created as the
waves continually pass through one
another.
http://www.walter-fendt.de/ph14e/stwaverefl.htm
6.9: 2D Interference
 Consider two sources S1 and S2 with identical waves in
phase with the same amplitude.
http://phet.colorado.edu/simulations/sims.php?sim=Wave_Interference
2. Twin water bugs Jimminy and Johnny are both creating a series of circular
waves by jiggling their legs in the water. The waves undergo interference and
create the pattern represented in the diagram at the right. The thick lines in the
diagram represent wave crests and the thin lines represent wave troughs. Several
of positions in the water are labeled with a letter. Categorize each labeled
position as being a position where either constructive or destructive interference
occurs.
Constructive Interference: A and B
Destructive Interference: C, D, E, and F
A standing wave is formed when ____.
a. a wave refracts due to changes in the properties of the medium.
b. a wave reflects off a canyon wall and is heard shortly after it is formed.
c. red, orange, and yellow wavelengths bend around suspended atmospheric particles.
d. two identical waves moving different directions along the same medium interfere.
My Answer
Official Answer D
The number of nodes in the standing wave shown in the diagram at the right is ____.
a. 6
b. 7
c. 8
d. 14
My Answer
Official Answer C
The number of antinodes in the standing wave shown in the diagram above right is ____.
a. 6
b. 7
c. 8
d. 14
My Answer
Official Answer
B
The number of nodes in the entire pattern is ___.
My Answer
a. 7
b. 8
c. 9
d. 16
Official Answer C
Of all the labeled points, destructive interference occurs at point(s) ____.
a. B, C, and D
b. A, E, and F
d. C only
e. all points
My Answer
c. A only
Official Answer
A