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

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.

Examples :
 Surface ripples on water
 Seismic S (secondary) waves
 Electromagnetic waves such as radio and light waves
 Light waves. While we can’t see the actual oscillations of light waves, they move in a
transverse manner.
 Guitar strings. When plucked, the string moves up and down, creating a transverse wave,
which then produces a sound.

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.
 Example:
 Sound waves
 Ultrasound waves
 Seismic P-waves
 Tsunami waves
 Waves through a slinky coil

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.

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.
 Example:
 Water waves: Ripples formed on the water surface when a heavy object is thrown into the
water or when a person dives into a water body.
 Sound waves: Sound waves are another example of surface waves.
 Seismic waves: Waves that travel through the Earth’s crust.
 Electromagnetic waves: A type of surface wave that travel through space.

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.

Example:
 Radio and Television waves
 Microwaves
 Medical Examination
 Sterilization
 Communication Devices
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.

Example:
 Sound waves, which are longitudinal waves that travel through air, water, or solids.
 Water waves, which are surface waves that result from the gravitational pull of the moon,
the earth, and the sun on the water.
 Spring waves, which are transverse waves that occur when a spring is stretched and
released.
 Stadium waves, which are waves that move through a crowd of people when they stand
up and sit down in a coordinated manner.
 Battle rope, which are waves that form when a person shakes a long rope up and down or
side to side.