Harrison County High School

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Harrison County High School
Waves
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A wave is a disturbance that carries energy
through matter or space (356)

• We generally discuss two types of waves
• Mechanical
• 2. Electromagnetic

Mechanical waves require a medium (or substance)
in which to travel
Electromagnetic (EM) waves do not require a medium
Therefore, EM waves can travel through a vacuum
(like space)
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Recall that energy is the ability to do
work work is the movement of an object over a
distance
When an ocean wave (think of a hurricanes waves)
reach a boat, the waves move the boat (usually
violently and sometimes destructively).
Since the boat moves, work is done, therefore the
waves must transfer energy
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Wave energy spreads out over time
Most waves are created by vibrations
As the energy of a wave spreads out, it creates
wave fronts
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As the wave fronts get larger, the energy is
spread out over a larger area
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Diagram 11-4 p 359
The spring pulls the mass upward when released,
compresses, then pushes the mass back in the
direction where is started from
If this motion can continue (forever), it is
called simple harmonic motion
If this motion fades out over time, it is called
damped harmonic motion
The motion of particles in a medium acts like the
motion of the springs
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There are three wave forms
1. Transverse waves have perpendicular motion
2. Longitudinal waves have parallel motion
3. Surface waves produce circular motion
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Characteristics of a Transverse Wave
All transverse waves have similar shapes,
regardless as to how big they are or what medium
they travel through
An ideal transverse wave produces a shape that is
represented by a sine curve
Mathematically, the sine curve is produced from
the function f(x) A sin q p
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The characteristics of the transverse wave can be
diagramed using the sine wave (Figure 11-9, p
365)
Crest
Wavelength (l)
Trough
Amplitude (A)
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The characteristics of a longitudinal wave can be
represented by a slinky (Figure 1-10, p
366)
Compression
Rarefaction
The amplitude (A) of a longitudinal wave is
determined by the density or pressure on the
medium, converted to a transverse or sine wave
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Wavelength is the distance between the crests or
troughs of two waves, OR the distance between
compressions or rarefactions of two waves
The symbol for wavelength is the Greek character
lambda (lc), l
Since wavelength is a measurement of distance,
the SI unit is the meter
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Period (T) is the amount of time required for one
full wave to pass a given point
Since period is a measurement of time, the SI
unit is the second (s)
Frequency is the number of vibrations (or waves)
that occur in a 1.0 s time interval
Frequency 1 / period
f 1 / T
The SI units for frequency is equal to 1/s, and
is called a hertz (Hz)
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Light is a form of Electromagnetic radiation (EMR)
EMR and the electromagnetic spectrum results from
the vibration of an atom
The EM spectrum occurs in a wide range of
frequencies and wavelengths
There are seven regions of the EM spectrum
determined by specific frequencies and wavelengths
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EM Spectrum
Increasing frequency
Radio
Micro
Infrared
Visible
UV
X-Ray
Gamma
Increasing wavelength
Each band of the EM spectrum has different uses
or applications. See Table 11-1, p 368.
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Wave Speed is the speed with which the wave is
moving through a medium
Since, speed distance / time, then
S wavelength (m) / period (s)
S l / T
Since frequency 1 / T, then
S l f
Practice, p 370, Questions 1-4
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The speed of a wave depends on the medium
Kinetic theory explains the differences in wave
speed (p 371)
Gases molecules spread far apart, vibrations
must travel a long ways before transferring
vibration to another molecule
Waves do NOT travel fast in gases (like air)
Liquids molecules are closer together which
allow the vibrations to transfer much easier
Waves travel moderately fast in liquids (like
water)
Solids molecules are tightly packed together
allowing vibrations to transfer through the
entire mass of molecules almost immediately The
greater a solids density, the faster the wave
speed
Waves travel extremely fast through solid (like a
steel railroad track)
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The wave speed of EM waves in a vacuum (e.g.
light in space) is finite or constant
Light speed (c) is approximately 3.0 x 108 m/s or
186,000 miles/s
According to Einsteins theory of general
relativity, no speed can exceed the speed of
light (c).
EM waves slow considerably when passing through
mediums (e.g. air or water)
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Doppler Effect
The pitch of a sound, how high or low it is, is
determined by the frequency at which sound waves
strike the eardrum in your ear
The higher the frequency of sound, the higher the
pitch
The movement of an object toward a subject
compresses sound waves (and increases the pitch)
whereas movement away from a subject rarefies the
sound waves (decreasing the pitch)
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When the source of the sound is stationary, the
sound wave fronts reach both observes with an
equal frequency and therefore equal pitch
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When the source of the sound is moving toward a
subject, the sound wave fronts are compressed,
creating an apparent increase in frequency, and
therefore a much higher pitch.
High pitch
Low pitch
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The Doppler Effect occurs in both mechanical
(e.g. sound) and non-mechanical (e.g. EMR) waves.
Trivia What happens with the situation below?
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Wave Interactions
When waves interact with an obstacle, three
things can happen
1. Reflection the bouncing back of a wave as
it meets a surface or boundary
2. Diffraction the bending of a wave as it
passes an edge or an opening
3. Refraction the bending of a wave as it
passes from one medium to another
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Reflection
Waves reflect at free boundaries See Figure
11-16 A, p 374.
Waves reflect and invert at fixed boundaries.
See Figure 11-16 B, p 374.
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Diffraction
Diffraction is the bending of waves around an
obstacle. See Figure 11-17, p 375.
Refraction
Refraction is the bending of waves as they pass
from one medium to another (and the wave speed is
changed)
No change in wave speed no refraction
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Interference
When different waves occur in the same place,
they combine together to produce a single
wavethis is called Interference
Waves are always added together in Interference
If the resulting interference wave is greater
than either original waves, the result is called
constructive interference
If the resulting interference wave is smaller
than largest original wave, the result is called
destructive interference
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Standing Waves
Standing waves are produced when an original wave
and a reflected wave of the same amplitude and
frequency interfere with each other.
Standing waves produce what appears to be a wave
that does not move and contains regions with no
vibrations (i.e. nodes) and maximum vibrations
(i.e. antinodes)
See Figure 11-23, p 380