WAVES:
A way to transmit energy
Waves are defined as a periodic
disturbance that carries energy from one
place to another.
A periodic disturbance is one that
happens over and over again at regular
intervals.
.
Like a bouncing ball, the motion
repeats over and over again.
There are two kinds of waves you
should recognize.
Longitudinal or compression
waves
and transverse waves.
Longitudinal waves, like sound waves, are
mechanical waves because they must have
a material to travel through.
The material they travel through is called
the “medium”.
The medium can be any material.
Air
Rock
Water
Wood
Anything!
Sometimes longitudinal waves are referred
to as compression waves because they are
caused by a vibrating object compressing
and rarefying the atoms or molecules of
the medium.
Vibrating
object
Alternating areas of compression and rarefaction in the
medium.
•By vibrating back and forth, the vibrating object
alternates between compressing and rarefying the
molecules of the medium.
Areas of
compression are
high pressure areas
where the molecules
are squeezed closer
together by a
vibrating object.
Areas of rarefaction are
low pressure areas where
the vibrating object pulls
the molecules further
apart.
In longitudinal waves the molecules of the medium
vibrate along with the vibrating object. Molecules of
the medium vibrate back and forth on each side of a
fixed resting point, transferring their kinetic energy
to adjacent molecules. In this way the energy is
transferred, but the molecules remain in place.
Longitudinal waves get their name because the
vibration that puts energy into the wave vibrates in
the same plane in which the energy moves.
Transverse Waves
In a transverse wave the particle displacement is
perpendicular to the direction of the energy
transfer.
The particles do not move along with the wave, they simply
vibrate up and down about their individual rest positions as the
wave passes by.
Pick a single particle and watch its motion.
The wavelength of a wave is the distance
between successive vibrations. Wavelength
is measured in meters. Here is how
wavelength is measured on a transverse
wave.
This diagram shows how wavelength can
be measured on a longitudinal wave. It
is measured from compression to
compression.
The Amplitude of a wave determines how much
energy a wave is carrying. This can easily be
seen in a transverse wave where the wave height
is the amplitude. Obviously, the higher the wave
crest the higher the amplitude, and the greater
the wave energy.
In longitudinal waves amplitude is a measure of
the degree of compression. The more compressed
the compressions are the greater the amplitude.
Low amplitude ( Less
compression)
High amplitude ( greater
compression)
Wave velocity is a measure of how fast the
energy is being transferred. Wave velocity is
measured in meters per second (m/s).
Wave frequency, for a transverse wave, is a
measure of how many wave crests pass a point in one
second. Wave frequency is measured in Hertz.
1Hertz is equal to 1wave per second. If 5 waves
pass the smiley in one second, then the wave
frequency is 5Hz or 5 waves per second.
Longitudinal wave frequency is how many compressions
pass a point in one second.
Wave period, for transverse and longitudinal waves,
is the time it takes for one complete wave ( one
wavelength) to pass a point. If it takes .3seconds
for a single wave to pass the smiley then the wave
period is .3s. Wave period is measured in seconds.
The relationship between wave frequency, wave length, and wave
velocity is expressed in a very simple equation, V=f λ. In this
equation V is velocity measured in m/s, f is frequency measured
in Hz, and λ is wavelength, measured in meters. λ is the Greek
letter Lambda. Shown below is an example problem using this
equation.
EXAMPLE 1: What is the velocity of a longitudinal
wave with a frequency of 2000Hz and a
wavelength of .17m?
Solution
Three ways to use the wave
equation.
Two More Equations
Wave period is measured in seconds per wave.
Wave frequency is measured in waves per second…
Hmmm… they look like they are the
inverse of each other!
What is the frequency of a wave with a period of .17s
Just find the inverse of .17s
The answer is 5.9Hz
What is the Period of a wave with a
frequency of .046Hz?
Just find the inverse of .046Hz
The answer is 21.7s
THE DOPPLER EFFECT:
an apparent change in frequency
caused by the motion of the wave
source
The source of a wave can be
stationary, such as a bell, a
light bulb, or even a ripple
spreading outward in a pond of
water, or it can be moving.
When the source of a wave is
moving the waves are
compressed closer together in
the direction of motion and
spread further apart behind
the source. This changes the
frequency of the wave.
Stationary
source
Moving
source
THE DOPPLER EFFECT AS IT
APPLIES TO SOUND WAVES
Joe hears a
low frequency
Moe hears a
higher
frequency
Here is a situation in which you will recognize
the Doppler effect. The the driver of a car
keeps a hand on the horn constantly as he
approaches and then passes an observer. Listen
to the high pitch as the car approaches and
the low pitch after the car passes the
observer.
DOPPLER
EFFECT
THE DOPPLER EFFECT AS IT
APPLIES TO LIGHT WAVES
Low frequency long
wavelength end of
the spectrum
An observer on the Earth
sees light shifted towards the
red ( lower frequency) end of
the spectrum. This is
referred to as the RED
SHIFT.
To be
continued…