Harrison County High School
Waves
A wave is a disturbance that carries energy
through matter or space (356)
We generally discuss two types of waves:
1.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)
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 hurricane’s
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
Wave energy spreads out over time
Most waves are created
by vibrations
V
As the energy of a wave
spreads out, it creates
wave fronts
As the wave fronts get larger, the energy is
spread out over a larger area
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
There are three wave forms
1. Transverse waves have perpendicular motion
2. Longitudinal waves have parallel motion
3. Surface waves produce circular motion
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
The characteristics of the transverse wave can
be diagramed using the sine wave (Figure
11-9, p 365)
Crest
Wavelength (l)
Amplitude (A)
Trough
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
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
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)
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
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.
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=lf
Practice, p 370, Questions 1-4
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)
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 Einstein’s 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)
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)
When the source of the sound is stationary,
the sound wave fronts reach both observes
with an equal frequency and therefore equal
pitch
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
The Doppler Effect occurs in both mechanical
(e.g. sound) and non-mechanical (e.g. EMR)
waves.
Trivia: What happens with the situation below?
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
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.
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
Interference
When different waves occur in the same
place, they combine together to produce a
single wave…this 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”
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