2. Wave Interference
Two different material objects can never occupy the same
space at the same time.
Because mechanical waves are not matter but rather are
displacements of matter, two waves can occupy the same
space at the same time.
The combination of two overlapping waves is called
superposition.
Superposition Principle : When two waves interfere, the
resulting displacement of the medium at any location is the
algebraic sum of the displacements of the individual waves
at that same location.
Works for both longitudinal waves and for transverse waves
3. Wave Interference
Simplyalign the waves in time and add the
amplitudes
Amplitudes can be either positive (above
equilibrium) or negative (below equilibrium)
4. Wave Interference
Constructive Interference:
If the amplitudes are of the same sign, the
wave is reinforced and grows bigger
Destructive Interference:
If the amplitudes are of opposite sign, the
wave is diminished and grows smaller
6. Wave Interference
Examples:
Sound – creates beats
• Constructive = Louder sound
• Destructive = Softer or no sound
Light
• Constructive = Brighter Light; change in color
• Destructive = Dark or no light
Water
• Constructive = Larger crests
• Destructive = No visible wave
7. Wave Interference
Examples:
Interference of two circular
waves. Absolute value
The colors seen in a soap bubble or an oil film snapshots of the (real-
on water are a common example of interference. valued, scalar) wave field.
Light reflecting off the front and back surfaces of Wavelength increasing from
the thin soap film interferes, resulting in different top to bottom, distance
colors being enhanced. between wave centers
increasing from left to right.
The dark regions indicate
destructive interference.
8. Wave Reflection
Reflection-A wave will bounce off an object
Wave reflection from surfaces depends on
the characteristics of the surface
Smooth hard surfaces reflect best
Rough soft surfaces reflect poorly
Energy not reflected is absorbed or
transmitted through the material
9. Wave Reflection
Law of Reflection- A wave bounces off at
the same angle it hits. (angle measured
with respect to the normalline
Think of arrows pointing in the direction of
the wave motion
Angles Equal
10. Wave Reflection
What happens to the
motion of a wave when
it reaches a boundary?
At a free boundary,
waves are reflected.
At a fixed boundary,
waves are reflected
and inverted.
11. Wave Reflection - Sound
Echoes are produced
when sound is reflected.
An echo can only be
heard by the human ear
when the time interval
b/w the echo and the
original sound is greater
than 0.1 s and the
distance b/w the person
and the reflecting surface
is greater than 17 m.
If smaller than 17 m, then
called Reverberation.
15. Wave Refraction
If there is a change in the characteristics
of a medium, waves are bent
This occurs because different parts of the
wave front travel at different speeds
Think of a marching around a curved track
The inside people have to move more
slowly than the outside people to keep the
lines straight
19. Wave Reflection & Refraction
The combination of reflection and
refraction enables imaging
Ultrasonic medical imaging
Naval SONAR for detecting submarines
Bats catch mosquitoes
20. Standing Waves
A standing wave is produced
when a wave that is
traveling is reflected back
upon itself.
Appear to stand still
There are two main parts to
a standing wave:
Antinodes – Areas of
MAXIMUM AMPLITUDE
Nodes – Areas of ZERO
AMPLITUDE.
23. Natural Frequency
Objects have ―natural‖ frequencies
The frequency that they vibrate at when
disturbed
Based on their size and structure
Guitar strings are an example
24. Resonance
Reinforcing of an object’s natural frequency so that
the amplitude increases quickly
If you have ever been talking in a bathroom and
notice that certain notes are very loud—that’s
resonance: that loud note is the natural frequency
of that room.
Think about a swing on a playground - You go high
when you pump the swing at its natural vibration
frequency
Resonance is how a soprano can break a glass
with her voice.
25. Resonance
Tacoma Bridge
7, 1940, one of the most famous incidents
involving the collapse of a bridge occurred.
The Tacoma Narrows Bridge (formerly the
Tacoma Suspension Bridge) was a mile-long
bridge on Route 16 in Washington State. The
original bridge was built with faulty
construction which yielded a potentially
tragic situation when four months after its
completion, the bridge collapsed in the face of
what many recall as a light breeze.
Fortunately, there were no cars on the bridge
at the time of the collapse, hence no human
lives lost. A dog walking on the bridge during
the incident, did in fact lose his life as a result
of the collapse – the light breeze caused the
bridge to “resonate” until the amplitude
became to great for the infrastructure