What is a Wave?
So waves are everywhere. But what makes a wave a wave? What characteristics, properties, or behaviors are
shared by the phenomena that we typically characterize as being a wave? How can
waves be described in a manner that allows us to understand their basic nature
and qualities?
A wave can be described as a disturbance that
travels through a medium from one location to another location. Consider a slinky
wave as an example of a wave. When the slinky is stretched from end to end
and is held at rest, it assumes a natural position known as the equilibrium or
rest position. The coils of the slinky naturally assume this
position, spaced equally far apart. To introduce a wave into the slinky, the
first particle is displaced or moved from its equilibrium or rest position. The
particle might be moved upwards or downwards, forwards or backwards; but once
moved, it is returned to its original equilibrium or rest position. The act of
moving the first coil of the slinky in a given direction and then returning it
to its equilibrium position creates a disturbance in the
slinky. We can then observe this disturbance moving through the slinky from one
end to the other. If the first coil of the slinky is given a single
back-and-forth vibration, then we call the observed motion of the disturbance
through the slinky a slinky pulse. A pulse is a
single disturbance moving through a medium from one location to another
location. However, if the first coil of the slinky is continuously and
periodically vibrated in a back-and-forth manner, we would observe a repeating
disturbance moving within the slinky that endures over some prolonged period of
time. The repeating and periodic disturbance that moves through a medium from
one location to another is referred to as a wave.
But what is meant by the word medium? A medium is a substance or material that carries the
wave. You have perhaps heard of the phrase news media. The news media refers to the various institutions (newspaper offices,
television stations, radio stations, etc.) within our society that carry the
news from one location to another. The news moves
through the media. The media doesn't make the news and the media isn't the same
as the news. The news media is merely the thing that
carries the news from its source to various locations. In a similar manner, a
wave medium is the substance that carries a wave (or disturbance) from one
location to another. The wave medium is not the wave and it doesn't make the
wave; it merely carries or transports the wave from its source to other
locations. In the case of our slinky wave, the medium through that the wave
travels is the slinky coils. In the case of a water wave in the ocean, the
medium through which the wave travels is the ocean water. In the case of a sound
wave moving from the church choir to the pews, the medium through which the
sound wave travels is the air in the room. And in the case of the stadium wave, the medium through which the stadium wave travels is the fans that are
in the stadium.
Particle-to-Particle Interaction
To fully understand the nature of a wave, it is important to
consider the medium as a collection of interacting particles. In other words, the medium is composed of parts that are capable of
interacting with each other. The interactions of one particle of the medium
with the next adjacent particle allow the disturbance to travel through the
medium. In the case of the slinky wave, the particles or
interacting parts of the medium are the individual coils of the slinky. In the
case of a sound wave in air, the particles or
interacting parts of the medium are the individual molecules of air. And in the
case of a stadium wave, the particles or
interacting parts of the medium are the fans in the stadium.
Consider the presence of a wave in a slinky. The first coil
becomes disturbed and begins to push or pull on the second coil; this push or
pull on the second coil will displace the second coil from its
equilibrium position. As the second coil becomes displaced, it begins to push
or pull on the third coil; the push or pull on the third coil displaces it from
its equilibrium position. As the third coil becomes displaced, it begins to
push or pull on the fourth coil. This process continues in consecutive fashion,
with each individual particleacting to
displace the adjacent particle. Subsequently, the disturbance travels through
the medium. The medium can be pictured as a series of particles connected by
springs. As one particle moves, the spring connecting it to the next particle
begins to stretch and apply a force to its adjacent neighbor.
As this neighbor begins to move, the spring
attaching this neighbor to its neighbor begins to stretch and apply a force on its
adjacent neighbor.
A Wave Transports Energy and Not Matter
When a wave is present in a medium (that is, when there is a
disturbance moving through a medium), the individual particles of the medium
are only temporarily displaced from their rest position. There is always a
force acting upon the particles that restores them to their original position.
In a slinky wave, each coil of the slinky ultimately returns to its original
position. In a water wave, each molecule of the water ultimately returns to its
original position. And in a stadium wave, each fan in the
bleacher ultimately returns to its original position. It is for this reason,
that a wave is said to involve the movement of a disturbance without the
movement of matter. The particles of the medium (water molecules, slinky coils,
stadium fans) simply vibrate about a fixed position as the pattern of the
disturbance moves from one location to another location.
Waves are said to be an energy
transport phenomenon. As a disturbance moves through a medium from one
particle to its adjacent particle, energy is being transported from one end of
the medium to the other. In a slinky wave, a person imparts energy to the first
coil by doing work upon it. The first coil receives a large amount of energy
that it subsequently transfers to the second coil. When the first coil returns
to its original position, it possesses the same amount of energy as it had
before it was displaced. The first coil transferred its energy to the second
coil. The second coil then has a large amount of energy that it subsequently
transfers to the third coil. When the second coil returns to its original
position, it possesses the same amount of energy as it had before it was
displaced. The third coil has received the energy of the second coil. This
process of energy transfer continues as each coil interacts with its neighbor. In this manner, energy is transported from one
end of the slinky to the other, from its source to another location.
This characteristic of a wave as an energy transport phenomenon
distinguishes waves from other types of phenomenon. Consider a common
phenomenon observed at a softball game - the collision of a bat with a ball. A
batter is able to transport energy from her to the softball by means of a bat.
The batter applies a force to the bat, thus imparting energy to the bat in the
form of kinetic energy. The bat then carries this energy to the softball and
transports the energy to the softball upon collision. In this example, a bat is
used to transport energy from the player to the softball. However, unlike wave
phenomena, this phenomenon involves the transport of matter. The bat must move
from its starting location to the contact location in order to transport
energy. In a wave phenomenon, energy can move from one location to another, yet
the particles of matter in the medium return to their fixed position. A wave
transports its energy without transporting matter.
Waves are seen to move through an ocean or lake; yet the
water always returns to its rest position. Energy is transported through the
medium, yet the water molecules are not transported. Proof of this is the fact
that there is still water in the middle of the ocean. The water has not moved
from the middle of the ocean to the shore. If we were to observe a gull or duck
at rest on the water, it would merely bob up-and-down in a somewhat circular
fashion as the disturbance moves through the water. The gull or duck always
returns to its original position. The gull or duck is not transported to the
shore because the water on which it rests is not transported to the shore. In a
water wave, energy is transported without the transport of water.
The same thing can be said about a stadium wave. In a stadium wave, the fans do not get out of their seats and walk
around the stadium. We all recognize that it would be silly (and embarrassing)
for any fan to even contemplate such a thought. In a stadium wave, each fan
rises up and returns to the original seat. The disturbance moves through the
stadium, yet the fans are not transported. Waves involve the transport of
energy without the transport of matter.
In conclusion, a wave can be described as a disturbance that
travels through a medium, transporting energy from one location (its source) to
another location without transporting matter. Each individual particle of the
medium is temporarily displaced and then returns to its original equilibrium
positioned.
Check Your Understanding
1. TRUE or FALSE:
In
order for John to hear Jill, air molecules must move from the lips of Jill to
the ears of John.
False.
A sound wave
involves the movement of energy from one location to another, not the movement
of material. The air molecules are the particles of the medium, and they are
only temporarily displaced, always returning to their original position.
2. Curly and Moe are conducting a wave experiment using a
slinky. Curly introduces a disturbance into the slinky by giving it a quick
back and forth jerk. Moe places his cheek (facial) at the opposite end of the
slinky. Using the terminology of this unit, describe what Moe experiences as
the pulse reaches the other end of the slinky.
When the slinky
reaches the end of the slinky and hits Moe in the cheek, Moe expereinces a pulse of energy. The energy originated
on Curly's end and is transported through
the medium to Moe's end. The last particle on Moe's end transports that energy
to Moe's cheek.
3. Mac and Tosh are experimenting with pulses on a rope. They
vibrate an end up and down to create the pulse and observe it moving from end
to end. How does the position of a point on the rope, before the pulse comes,
compare to the position after the pulse has passed?
The point returns to its original position. Waves (and
pulses) do not permanently displace particles from their rest position.
4. Minute after minute, hour after hour, day after day, ocean
waves continue to splash onto the shore. Explain why the beach is not
completely submerged and why the middle of the ocean has not yet been depleted
of its water supply.
Ocean waves do
not transport water. An ocean wave could not bring a single drop of water from
the middle of the ocean to shore. Ocean waves can only bring energy to the
shore; the particles of the medium (water) simply oscillate about their fixed
position. As such, water does not pile up on the beach.
5. A medium is able to transport a wave from one location to
another because the particles of the medium are ____.
a. frictionless
b. isolated from one
another
c. able to interact
d. very light
Answer: C
For a wave to be
transmitted through a medium, the individual particles of the medium must be
able to interact so that they can exert a push and/or pull on each other; this
is the mechanism by which disturbances are transmitted through a medium.