Download Interference of Waves

Interference of Waves
Key Question
• What happens when 2 waves meet?
• Animations:
▫ http://phys23p.sl.psu.edu/phys_anim/waves/ind
exer_waves.html
▫ http://www.earthguide.ucsd.edu/earthguide/diag
rams/wave_interference/wave_interference.html
▫ http://www.acs.psu.edu/drussell/demos/superpo
sition/superposition.html
Wave Interference
• When pulses travel towards each other, the
pulses interfere with each other for an instant
and then continue travelling unaffected.
• All waves behave this way.
• Wave Interference – occurs when 2 or more
waves act simultaneously on the same particles
of a medium
Let’s Play with Waves
• What will happen when a crest and a trough meet?
• What will happen when two crests meet?
combined
waveform
wave 1
wave 2
Two waves in phase
Two waves out of phase
Destructive
Interference
• The waves will cancel
out totally or
partially
• Exactly out of phase
– totally cancels –
node.
Constructive
Interference
• Occurs when waves
build each other up,
resulting in the
particle having a
larger amplitude
• Exactly in-phase:
super-crest and
super-trough
Resulting Wave
Principle of Superposition:
The displacement from equilibrium of each
particle in a wave is a result of the energy
transferred to that particle. The resultant
displacement of two or more waves is the
algebraic sum of individual displacements at one
time, since the energy of the particle would be
increased/decreased.
Example 1
Add the two waves below to determine the resulting wave.
Example 2 (Stationary)
Add the two waves below to determine the resulting wave.
Constructive or Destructive?
Homework
1. Research noise cancelation headphones and give a one paragraph
description of how they apply the principle of superposition. Include a
diagram.
2. Complete Interference of Waves Worksheet
3. The principle of superposition can be used to explain why pulses are
inverted when they reflect from the fixed end of a spring.
a. What must the amplitude of a wave always be at the fixed end?
b. If you consider the amplitude at the fixed end to be the result of the
superposition of the incident and reflected waves, why does this
necessarily imply that the reflected wave is inverted?
c. Sketch a graph that shows the incident and reflected waves would
appear at the fixed point.
4. A washing machine sometimes bounces vigorously for a few
moments at one or more of its spin cycle speeds. Explain the physics at
work here.