Categories of Waves
Waves come in many shapes and forms. While all waves share
some basic characteristic properties and behaviors, some waves can be distinguished from others
based on some observable (and some non-observable) characteristics. It is
common to categorize waves based on these distinguishing characteristics.
Longitudinal versus Transverse Waves versus Surface
Waves
One way to categorize waves is on the basis of the direction
of movement of the individual particles of the medium relative to the direction
that the waves travel. Categorizing waves on this basis leads to three notable
categories: transverse waves, longitudinal waves, and surface waves.
A transverse wave is a wave in which particles of the medium move
in a direction perpendicular to the direction that the wave moves.
Suppose that a slinky is stretched out in a horizontal direction across the
classroom and that a pulse is introduced into the slinky on the left end by
vibrating the first coil up and down. Energy will begin to be transported
through the slinky from left to right. As the energy is transported from left
to right, the individual coils of the medium will be displaced upwards and
downwards. In this case, the particles of the medium move perpendicular to the
direction that the pulse moves. This type of wave is a transverse wave.
Transverse waves are always characterized by particle motion being perpendicular to wave motion.
A longitudinal wave is a wave in which particles of the medium move
in a direction parallel to the direction that the wave moves.
Suppose that a slinky is stretched out in a horizontal direction across the
classroom and that a pulse is introduced into the slinky on the left end by
vibrating the first coil left and right. Energy will begin to be transported
through the slinky from left to right. As the energy is transported from left
to right, the individual coils of the medium will be displaced leftwards and rightwards.
In this case, the particles of the medium move parallel to the direction that
the pulse moves. This type of wave is a longitudinal wave. Longitudinal waves
are always characterized by particle motion being parallel to wave motion.
A sound wave traveling through air is a classic example of a
longitudinal wave. As a sound wave moves from the lips of a speaker to the ear
of a listener, particles of air vibrate back and forth in the same direction
and the opposite direction of energy transport. Each individual particle pushes
on its neighboring particle so as to push it forward. The collision of particle #1 with its neighbor serves
to restore particle #1 to its original position and displace particle #2 in a
forward direction. This back and forth motion of particles in the direction of
energy transport creates regions within the medium where the particles are
pressed together and other regions where the particles are spread apart.
Longitudinal waves can always be quickly identified by the presence of such
regions. This process continues along the chain of particles until the sound wave
reaches the ear of the listener. A detailed discussion of sound is
presented in another unit of The Physics Classroom Tutorial.
Waves traveling through a solid medium can be either
transverse waves or longitudinal waves. Yet waves traveling through the bulk of
a fluid (such as a liquid or a gas) are always longitudinal waves. Transverse
waves require a relatively rigid medium in order to transmit their energy. As
one particle begins to move it must be able to exert a pull on its
nearest neighbor. If the medium is not rigid as is the case with fluids, the particles
will slide past each other. This sliding action that is characteristic of
liquids and gases prevents one particle from displacing its neighbor in a
direction perpendicular to the energy transport. It is for this reason that
only longitudinal waves are observed moving through the bulk of liquids such as
our oceans. Earthquakes are capable of producing both transverse and
longitudinal waves that travel through the solid structures of the Earth. When
seismologists began to study earthquake waves they noticed that only
longitudinal waves were capable of traveling through the core of the Earth. For
this reason, geologists believe that the Earth's core consists of a liquid -
most likely molten iron.
While waves that travel within the depths of the ocean are
longitudinal waves, the waves that travel along the surface of the oceans are
referred to as surface waves. A surface wave is a wave in which particles of the medium
undergo a circular motion. Surface waves are neither longitudinal nor
transverse. In longitudinal and transverse waves, all the particles in the
entire bulk of the medium move in a parallel and a perpendicular direction
(respectively) relative to the direction of energy transport. In a surface
wave, it is only the particles at the surface of the medium that undergo the
circular motion. The motion of particles tends to decrease as one proceeds
further from the surface.
Any wave moving through a medium has a source. Somewhere
along the medium, there was an initial displacement of one of the particles.
For a slinky wave, it is usually the first coil that becomes displaced by the
hand of a person. For a sound wave, it is usually the vibration of the vocal
chords or a guitar string that sets the first particle of air in vibrational
motion. At the location where the wave is introduced into the medium, the
particles that are displaced from their equilibrium position always moves in
the same direction as the source of the vibration. So if you wish to create a
transverse wave in a slinky, then the first coil of the slinky must be
displaced in a direction perpendicular to the entire slinky. Similarly, if you
wish to create a longitudinal wave in a slinky, then the first coil of the
slinky must be displaced in a direction parallel to the entire slinky.
Electromagnetic versus Mechanical Waves
Another way to categorize waves is on the basis of their
ability or inability to transmit energy through a vacuum (i.e., empty space).
Categorizing waves on this basis leads to two notable categories: electromagnetic
waves and mechanical waves.
An electromagnetic wave is a wave that is capable of transmitting its
energy through a vacuum (i.e., empty space). Electromagnetic waves are produced
by the vibration of charged particles. Electromagnetic waves that are produced
on the sun subsequently travel to Earth through the vacuum of outer space. Were
it not for the ability of electromagnetic waves to travel to through a vacuum,
there would undoubtedly be no life on Earth. All light waves are examples of
electromagnetic waves. Light waves are the topic of another unit at The Physics
Classroom Tutorial. While the basic properties and behaviors of
light will be discussed, the detailed nature of an electromagnetic wave is
quite complicated and beyond the scope of The Physics Classroom Tutorial.
A mechanical wave is a wave that is not capable of transmitting
its energy through a vacuum. Mechanical waves require a medium in order to
transport their energy from one location to another. A sound wave is an example
of a mechanical wave. Sound waves are incapable of traveling through a vacuum.
Slinky waves, water waves, stadium waves, and jump rope waves are other examples of mechanical waves; each
requires some medium in order to exist. A slinky wave requires the coils of the
slinky; a water wave requires water; a stadium wave requires fans in a stadium;
and a jump rope wave requires a jump rope.
The above categories represent just a few of the ways in
which physicists categorize waves in order to compare and contrast their behaviors and
characteristic properties. This listing of categories is not exhaustive; there
are other categories as well. The five categories of waves listed here will be
used periodically throughout this unit on waves as well as the units on sound and light.
Check Your Understanding
1. A transverse wave is transporting energy from east to
west. The particles of the medium will move_____.
a. east to west only
b. both eastward
and westward
c. north to south
only
d. both northward
and southward
Answer: D
The particles
would be moving back and forth in a direction perpendicular to energy
transport. The waves are moving westward, so the particles move northward and
southward.
2.A wave is
transporting energy from left to right. The particles of the medium are moving
back and forth in a leftward and rightward direction. This type of wave is
known as a ____.
|
a.
mechanical |
b.
electromagnetic |
|
c.
transverse |
d.
longitudinal |
Answer: D
The particles
are moving parallel to the direction that the wave is moving. This must be a
longitudinal wave.
3. Describe how the fans in a stadium must move in order to
produce a longitudinal stadium wave.
Answer:
The fans will
need to sway side to side. Thus, as the wave travels around the stadium they
would be moving parallel to its direction of motion. If they rise up and sit
down, then they would be creating a transverse wave.
4. A sound wave is a mechanical wave, not an electromagnetic
wave. This means that
a. particles of the medium move perpendicular to
the direction of energy transport.
b. a sound wave
transports its energy through a vacuum.
c. particles of
the medium regularly and repeatedly oscillate about their rest position.
d. a medium is
required in order for sound waves to transport energy.
Answer: D
Mechanical waves
require a medium in order to transport energy. Sound, like any mechanical wave,
cannot travel through a vacuum.
5. A science fiction film depicts inhabitants of one
spaceship (in outer space) hearing the sound of a nearby spaceship as it zooms
past at high speeds. Critique the physics of this film.
Answer:
This is an
example of faulty physics in film. Sound is a mechanical wave and could never
be transmitted through the vacuum of outer space.
6. If you strike a horizontal rod vertically from above, what
can be said about the waves created in the rod?
a.
The particles vibrate horizontally along the direction of the rod.
b. The particles vibrate vertically, perpendicular to the
direction of the rod.
c. The particles vibrate in circles, perpendicular to the
direction of the rod.
d. The particles travel along the rod from the
point of impact to its end.
Answer: B
The particles
vibrate in the direction of the source which creates the initial disturbance.
Since the hammer was moving vertically, the particles will also vibrate
vertically.
7. Which of the following is not a characteristic of
mechanical waves?
a.
They consist of disturbances or oscillations of a medium.
b. They transport energy.
c. They travel in a direction that is at right angles to the
direction of the particles of the medium.
d. They are created by a vibrating source.
Answer: C
The
characteristic described in statement c is a property of all transverse waves,
but not necessarily of all mechanical waves. A mechanical wave can also be
longitudinal.
8. The sonar device on a fishing boat uses underwater sound
to locate fish. Would you expect sonar to be a longitudinal or a transverse
wave?
Answer: Longitudinal
Only longitudinal
waves are capable of traveling through fluids such as water. When a transverse
wave tries to propagate through water, the particles of the medium slip past
each other and so prevent the movement of the wave.