Transcript Optics and Sound - Moline High School

OS3 Wave Properties/Models
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 Model-Something
used to represent
something else

Can be bigger, smaller or same size
 Models
have limitation-never have the exact
same properties as the real object
Models of Light
 Wave
model
 Particle model
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 Wave
model-thought of as a traditional
wave that can transfer energy
 Pros: Waves reflect, refract, have
wavelengths and frequencies
 Cons: Wave needs a medium and can NOT
travel through a vacuum-light can
 Particle
Model-Light thought of as particles
 Pros: particles reflect, affected by gravity,
change speed in a new medium, have color,
do not need a medium
 Cons: particles do not refract (bend)
according to Snell’s Law
 Scientists
invented their own model for light
called a photon
 Photon-Particle-like bundle of energy that
moves like a wave

Contains both particle and wave properties
 Wave
A disturbance in a medium
A true wave must have a medium to pass through
 Examples
gas, etc.
of mediums: Air, water, metal,
 Waves
transfer energy from one place to
another with little transfer of the medium
itself
1.
Transverse Waves
2. Longitudinal Waves
Transverse
Waves-The direction of
propagation (motion) of the wave is
perpendicular to the disturbance

Examples: Water waves, light waves, radio waves,
stretched strings of musical instruments
Longitudinal
Waves-The propagation is
parallel to the disturbance
 Example:
Sound waves
Disturbance
motion
Rarefactionmolecules
spread out
Compression- air
molecules
squished
together
 Crest-High
point on a wave (greatest
disturbance)
 Trough-Low points on a wave
 Rest position- The location of the medium
when there is no disturbance
Crest
Wavelength ( )
Rest position
Amplitude
Trough
 Amplitude-The
distance from the rest
position of a wave to the crest or trough


The maximum displacement from rest position
NOT the distance from the top of a crest to the
bottom of a trough
 Wavelength-The
distance between identical
points on adjacent waves
 From crest to crest or trough to trough
 The length of one complete wave

Represented by the symbol λ (lambda)
Spot Check
What
letter represents the wavelength?
What letter represents the amplitude?
Wavelength = A
Amplitude = D
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Spot Check
What
interval represents 1 full
wavelength?
Wavelenght
= B-F OR A-E OR C-G
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 Frequency-
The number of waves passing a
point each second or how often waves are
passing

NOT how fast the waves are moving
 Symbol:
f
 Measured in waves/sec, cycles/sec, 1/sec or
hertz (Hz)
 Period-
The amount of time it takes for one
wave to pass a point
 Period is symbolized by T
 Measured in seconds
 Frequency
and period are inversely related
(reciprocals)
f=1/T
or T=1/f
Example:
Example:
A pendulum makes 2 back
and forth swings in 1 sec.
 Frequency = 2 Hz
 Period=1/2 second (time needed to
complete 1 vibration)
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Example:
Tim
Ahlstrom of Oconomowoc, WI holds
the record for hand clapping: 793 times
in 60 seconds
Calculate
the frequency
Calculate the period
Frequency = 793/60 = 13hz
Period = 1/13 = .077s
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 Bumblebees
flap their wings at ~130
flaps/sec.

Produce a sound of 130 Hz
 Honeybees

Produces a higher pitched sound of 225 Hz
 Mosquito

flap their wings at 225 flaps/sec.
flaps its wings at 600 flaps/sec.
Produces a high-pitched sounds of 600 Hz
 The
speed of a wave depends on the medium
(material) the wave moves through
 For example: In air, all sound waves whether
they have high frequency or low frequency
all travel at the same speed
 Wavelength
and frequency vary inversely to
produce the same wave speed for all sounds
 Long
wavelengths have low frequencies
 Short wavelengths have high frequencies
 In
general, the more rigid the material
(molecules closer together), the faster the
wave moves

Example: Sound waves travel fastest in solids
and slowest in gases
Light or Sound?
Which
travels faster
light or sound?
Light:3 X 108 m/s
Sound:343 m/s

Phet Simulations
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Reflection and Transmission
 http://www.kettering.edu/physics/drussell/
Demos/reflect/reflect.html
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To Recap….
 Does
changing frequency affect the speed of
a wave?
 NO!
 Does changing wavelength affect the speed
of a wave?
 NO!
 Does amplitude affect the speed of a wave?
 NO!
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 The

formula for the speed of a wave is:
Wave speed = wavelength X frequency
V=λf
 V=wave
speed (m/s or cm/s)
 λ=wavelength (m or cm)
 f=frequency (waves/s, cycles/s, 1/s, or Hz)
 If
a sound wave has a frequency of 396 Hz
and a wavelength of 0.86 meters, what is the
wave speed?
 1. List the variables



f=396 Hz
λ=0.86 m
V=?
 2.



Set up the equation
V=λf
V=(0.86 m)(396 Hz)
V=340 m/s
Example:
Calculate
 V=λf
 2.5 = 5.0 f
 f = .5 hz
the frequency of the waves.
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Example:
 If
the frequency of a wave triples, what
happens to the wavelength?
λ = 1/3
 If
the frequency of the wave triples, what
happens to the velocity?
v =same (only dependent on meduim)
V=λf
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 The
energy of a wave depends primarily on
its frequency
 The only energy problems we do will be
related to the energy of light
 Formula:
 E=energy
E=hf
(Joules, J)
 f=frequency (waves/s or Hz)
 h=Planck’s constant (6.6 x 10-34 Js)
 Standing
wave-A wave in which parts of the
wave remain stationary and the wave
appears to be not traveling
 Results from the interference between an
incident (original) wave and a reflected one
 Node-Any
part of a standing wave that
remains stationary
 Antinode-The positions on a standing wave
where the largest amplitudes occur

Example: Different standing waves can be
produced by shaking the rope at different
frequencies

Phet Simulations
 Superposition-The
adding of waves
 Interference-When
2 or more waves overlap
 Constructive
interference
(reinforcement)Wave crests overlap to
produce an increase in
wave amplitude
Destructive
interference
(cancellation)-When a
crest and trough
overlap, resulting in a
wave of decreased
amplitude
 Interference
produces beats
 Beat-Result of alternate cancellation and
reinforcement of 2 sound waves with slightly
different frequencies
 The
vibration of an object that is made to
vibrate by another vibrating object that is
nearby

One object vibrates to make another object
vibrate
 Example:
The sounding board in a musical
instrument
 The
frequency that requires the least
amount of energy to continue the vibration
 Resonance-Resound
or sound again
 When the frequency of forced vibrations on
an object match the object’s natural
frequency
 Results in a dramatic increase in amplitude
 Example:
Swinging
When pumping you pump with the natural
frequency of the swing
Even small pumps or pushes from someone
else will produce large amplitudes if
delivered in rhythm with the natural
frequency of the swinging motion
 Compression-A

pulse of compressed air
Air molecules push into their neighbors
 Rarefaction-A
disturbance in air in which the
pressure is lowered
 Loudness
is a physiological sensation sensed
in the brain
 Subjective but related to sound intensity
 Roughly, loudness follows the intensity
decibel scale
 Longitudinal
Waves transfer energy as the disturbance
is in the same direction as the wave (aLONG the wave)
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