### Waves Interactions

```http://www.physicsclassroom.com/mmedia/waves/lt
m.cfm
Boundary
 Where 2 media meet
 Where wave conditions change
 Reflection, transmission, or both can occur
Waves at Boundaries
A wave travels from one medium to another (i.e. air into
water)
- Waves enter the new medium upright
- How much energy is transmitted into the new medium
depends on how different the media are.
- The greater the difference in the media the less
transmission
-
More of the energy is reflected (returns to the original
medium)
Waves at Boundaries
When the wave goes from low density to high density the reflected wave is
flipped (In this case it would return on the bottom)
Waves at Boundaries
Free/Open/Loose Boundary
Boundary that can move
- Boundary moves in response to the wave
- When wave travels from:
- More dense to less dense
-
Wave remains upright upon reflection
- Less dense to more dense
-
Wave flips upon reflection
Fixed Boundary
Boundary that does not move
- Most energy is reflected
- Wave flips upon reflection (crest becomes a trough)
Boundary Conditions
Free Boundary
Boundary that does move
- Most energy is reflected
- Wave maintains orientation upon reflection (crest
stays a crest)
Boundary Conditions
Absorption
In the real world
- some of the energy is absorbed by the medium in the
form of heat (usually as a result of friction)
- Some of the wave amplitude is lost as a result
Interference
Principle of Superposition
-when the particles of a medium are affected by multiple
waves at the same time
-the oscillation of each particle is the combination of the
oscillation that would occur as a result of each
individual wave
Types of Interference
Constructive Interference:
When multiple waves add up to create a larger
amplitude
Types of Interference
Destructive Interference:
When multiple waves add up to create a smaller
amplitude
Interference
Constructive Interference
Destructive Interference
Standing Waves
 occur when a wave is confined between two
boundaries
 Wave is reflected back and forth
• As the frequency
changes the standing
wave pattern changes
• low frequency, long
wavelength
• high frequency, short
wavelength
Standing Waves
Standing Waves applet
Standing Waves
Interference occurs between the wave and its reflection
Where destructive interference occurs, nodes form
Node – particle remains at equilibrium
Where maximum constructive interference occurs,
antinodes form
 Antinode – particle is farthest from equilibrium
Diffraction
Bending of waves around a boundary
- no change in medium
Huygen’s Principle
- Each molecule that is part of a wave
front emits its own wave front
- Individual wave fronts are circular
- When many circular wave fronts overlap
only the combined affect is seen
- Very far from an individual source, the
combined wave front appears straight
Diffraction
 When a straight wave front hits a boundary the edges
of the individual wave fronts show
Diffraction
The amount of bending that is noticeable depends on
the wavelength and the size of the opening
Wavelength ≥ slit size → more bending
Wavelength < slit size → less bending
Diffraction and Interference
When there are multiple openings the diffracted waves
will overlap and interfere
Constructive interference occurs
when wave fronts are in phase
i.e. where crests happen at the
same time and place (where lines
overlap on the diagram)
Destructive interference occurs
when wave fronts are out of phase
i.e. where a crest and a trough
happen at the same time and place
(where lines overlap spaces on the
diagram)
Diffraction and Interference
 Where constructive
interference occurs an increase
in amplitude occurs (brighter
area on diagram)
 Wave form is magnified
 Where destructive
interference occurs a decrease
in amplitude occurs (grey
areas on diagram)
 Wave form is cancelled out
What Depends on What
 Frequency: depends on source
 Amplitude: depends on energy
 Speed: depends on medium
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