If you've ever been to the ocean, you've seen a wave. However, in
physics, a wave is more than a swell in the sea. A wave is one of
two ways in which energy is transfered. In the past, we have
talked only of particles. If you throw a ball at a brick wall,
the target might gain some kinetic energy, and the ball would
lose some. The ball itself had to move over a distance. The ball
is a moving particle. If you tie a rope to a brick wall, and then
shake the rope, the target also gains energy. The rope, however,
transfered the energy as a wave and didn't actually have to move
toward the wall. In this lesson you will earn what a wave is as
well as some basics about wave behavior. Then you will have the
opportunity to learn more by studying light, which is really a
type of wave.
Mechanical waves are waves that require what we call a "medium."
Ocean waves, sound waves and even a wiggling rope are mechanical
wave examples. By "medium" we mean that the wave travels through
a substance. The sound travels through the air. The air would be
the sound's medium. The properties of the medium often affect the
behavior of the wave, as you will soon learn.
Electromagnetic waves do not require a medium. Light, X-Rays and
radio waves are examples of electromagnetic waves. Light, for
example, can travel through air or the vacuum of space without
trouble. Electromagnetic waves are generally very fast- light
moves at 299 792 458 m/s. That's fast.
The best way to learn waves in by learning about mechanical
waves. There are three types of mechanical waves. The first type
is the transverse wave. This is the vibration of the medium in a
direction perpendicular to the direction of the wave. The vest
way to explain it is for us to draw it.
The second type of wave is a longitudinal wave. The medium moves
parallel to the direction of the wave. Sound waves are
The third type is the surface wave. These are waves like the
waves of the ocean. Only the surface ripples, the rest of the
medium does not. If you dive deep under the ocean, you wont be
able to feel the waves high above you.
Waves tend to repeat themselves. This is the nature of the wave.
There will be one wave pulse, then another, then another, in a
rhythmic pattern. The time it takes for the wave to repeat itself
is called the wave's period, and is given the symbol T. The
inverse of the period, or 1/T, is called the frequency (f). This
is the number of wave repeats in one second. You will need to
know both of these to continue your study of waves.
Waves have several components. Study the diagram below very
carefully. The wavelength of a wave is the distance between the
crests of the wave. You will notice that this distance is the
same as the distance between the troughs of the wave, so the
wavelength can really be either. Wavelength is given the symbol
Lambda (no, not Lambada, that's the forbidden dance), or . The distance between the equilibrium and the
crest is called the amplitude, or wave height. The larger the
amplitude, the more energy that the wave has.
Physicists like you and me love to make calculations. You do,
don't you? For instance, you probably are dying to find out how
to calculate the velocity of a wave. This shouldn't be hard for
you. Velocity is measured in meters per second, or m/s. What part
of a wave is measured in meters? The wavelength is, of course.
And what part is measured in seconds? The period. So the velocity
is measured as the wavelength over the period.
v = / T
Since the period is the same as 1 over the frequency, we can
simplify this equation to:
v = f
The speed of a mechanical wave does not depend upon the amplitude
of a wave. Really, the properties of the medium have the greatest
effect on speed. A wave with a large amplitude moves at the same
speed as a wave with a small amplitude, provided the mediums and
wavelengths are the same.
When a wave changes mediums, it is a big deal. Say we tie one end
of a rope to a brick wall, then send a wave down the rope. The
wave tries to change from the medium of the rope to the slower,
denser medium of the wall. It isn't likely that the wall will
start to vibrate. Instead, the wave will bounce off of the wall,
and return down the other side of the rope. This is called wave
reflection. When a wave enters a slower medium, there will be a
reflected wave which is 180 degrees out of phase. (180 degrees
out of phase means that it is on the other side of the
If we tie a rope to a thread, and send a wave down the rope, it
will definitely move the thread. It is changing to a lighter,
faster medium. There will still be a reflection though. It will
be small, but it will be present. It won't be out of phase
though. When a wave changes to a faster medium, the reflected
wave is in phase with the original wave.
We use the words "slow medium" to refer to a heavier, more dense
medium. This is because a wave gets slower when entering a more
rigid medium. Likewise, when a wave enters a less rigid medium,
it enters a "fast medium."
Waves sometimes end up hitting each other. If we send a wave down
each side of a rope, they will interact when they come together.
There is a scientific rule that we follow for the interaction of
two waves. It is called the principle of superposition, and it
states that "the displacement of a medium caused by two waves is
the sum of the displacements of the individual waves." What this
means is that the two waves will combine into one bigger or
smaller wave when they meet. This is called wave interference. If
two waves are in phase, then they will make a single, big wave.
The crest of this big wave is called the anti-node. If the two
waves are out of phase, the resulting wave will be equal to the
larger wave minus the smaller wave. If two waves that are out of
phase meet, and they have the same displacement, then they will
cancel out in the place where they join. This point is called a
node. There will be waves up and down the medium, except at the
A standing wave is really a simle concept, but it sounds
difficult. When the period of the wave is the same as the time it
takes to cover distance to a node point and back, you have a
standing wave. If you've ever heard of someone shattering a glass
with the sound of their voice, they used a standing wave. In that
particular case, the standing wave of sound causes something
called resonance. This makes the glass vibrate exactly the same
way the persons voice is, and then the vibration destroys the
glass. A resonance occurs when one object causes another to
vibrate with the same frequency.
To learn more about light, read the lessons on light. Light is an
electromagnetic wave, but now that you know some wave basics, you
can use it as an example to learn the many ways that waves can
behave in the universe. Many properties of light waves also work
with other types of waves.