interactions of waves11
TRANSCRIPT
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
Wave Interference
To determine the amount of interference:
Simply align the waves in time and add the
amplitudes
Amplitudes can be either positive (above
equilibrium) or negative (below equilibrium)
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
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
Wave Interference Examples:
The colors seen in a soap bubble or an oil film
on water are a common example of interference.
Light reflecting off the front and back surfaces of
the thin soap film interferes, resulting in different
colors being enhanced.
Interference of two circular
waves. Absolute value
snapshots of the (real-
valued, scalar) wave field.
Wavelength increasing from
top to bottom, distance
between wave centers
increasing from left to right.
The dark regions indicate
destructive interference.
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
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
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.
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.
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
Wave Reflection & Refraction
The combination of reflection and
refraction enables imaging
Ultrasonic medical imaging
Naval SONAR for detecting submarines
Bats catch mosquitoes
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.
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
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.
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