Connects with SciGen Unit E4
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Quick Content Refresher for Busy Professionals
What types of waves are there, and what do they have in common?
Waves transfer energy from one place to another without transferring matter, and they can be classified into two
major categories: mechanical and electromagnetic. Mechanical waves require a material medium. Electromagnetic
waves do not.
Waves can also be classified as either longitudinal or transverse. Mechanical waves can be either longitudinal or
transverse, while electromagnetic waves are always transverse.
In longitudinal mechanical waves (like sound), particles in the medium move back and forth parallel to the direction
of energy travel. (Think motion along the path of longitudinal waves.)
In transverse mechanical waves (like waves on the surface of water), particle in the medium move back and forth
perpendicular to (transverse to) the direction of energy travel. While mechanical waves come in both longitudinal
and transverse varieties, electromagnetic waves are always transverse. (See E4t3, “How can energy travel when
matter doesn’t?” for more discussion of how electromagnetic waves travel without a material medium.)
What do waves have in common?
For all their variety, the phenomena we group under the heading of waves have some things in common, and the
choice of the word “wave” as a collective name suggests that water waves may be a good prototypical illustration
of their commonalities. (This illustration requires some qualification when extended to compression waves and
waves that can travel through a vacuum, of course.)
So, consider the ripples that spread outward from a pebble that falls into a pond. The falling pebble creates a
disturbance and then disappears below, adding no more energy to the system. We are interested in the response
to the disturbance: waves.
The main thing to notice is that the energy of the disturbance travels a long way even though the water does not.
The water through which the waves travel moves a little, but basically stays in the same location. This transfer of
energy without transfer of matter is an essential characteristic of waves.
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Waves have alternating peaks (crests) and valleys (troughs). Even compression waves, like sound, can be described
in terms of crests and troughs, although in such a case these terms describe a graph of compression rather than
the more literal peaks and valleys of waves in water.
Transverse waves:
Longitudinal waves and corresponding wave graph:
A restoring force (a force that tends to return a physical system to equilibrium, in this case gravity) brings peaks
back toward their original level, but the water overshoots because of its momentum, creating a new valley where
the peak was, and vice versa.
The speed at which the ripples spread does not depend on the size of the splash that started them. Ripples from
bigger pebbles have peaks that are farther apart, not faster-moving peaks. Instead, the speed depends on the
medium (water, in the case of our ripples).
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When a set of ripples encounters a barrier, they are reflected. The angle of reflection is equal to the angle of
incidence.
If there are other ripples (from another stone, or from reflections), the two sets of ripples combine in interference
patterns as they pass through each other. Where peaks combine with peaks, or troughs with troughs, there is
constructive interference (enhancing the highs and lows of the peaks and troughs, respectively). Where peaks
combine with troughs, there is destructive interference, bringing the surface of the water nearer to its resting level.
The wave propagates from the origin only as long as energy is put into the system. Waves are a response to
disturbance, and new waves will cease to form when a disturbance ceases.
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How do waves differ?
As stated earlier, waves can be either mechanical or electromagnetic. Below are some ways the two major
categories of waves differ.
Mechanical Waves
Electromagnetic Waves
Need a medium?
Yes
No
Speed
Depends on the properties of the
medium (such as density and
compressibility)
Constant (c = the speed of light) in a
vacuum, but can be slowed down by
traveling through different media
Transverse?
Sometimes (for example, waves on the
surface of water)
Always: EM waves are a type of transverse
vibration of space itself
Longitudinal?
Sometimes (for example, sound waves)
No
Mechanical waves
An event causes a displacement in some kind of medium (which can
be any state of matter: solid, liquid, or gas). The displacement sends a
disturbance quite far through the medium, even though each part of
the medium makes only small motions (back and forth, or up and
down, or both). The speed of such a wave depends on the properties
of the medium (such as density and compressibility) rather than on
what kind of event created the wave.
Electromagnetic waves
Electromagnetic waves such as visible
light are always transverse. They are
not a vibration of matter, and can travel
through a vacuum. In a sense, they are
a vibration of space itself. In fact, light
travels fastest through empty space—
matter slows it down (or even absorbs
or reflects it, depending on the EM
wavelength and type of matter
involved).
The wavelike nature of light is
shown by its interference patterns, as
well as by the way it reflects and how it
bends when going through prisms and
lenses. But in some situations, light also
acts as if it is made of particles (called
photons) that travel at the speed of the
light wave but concentrate the wave
energy in individual packets, with the
amount of energy in each packet
depending on the color of the light.
Longitudinal mechanical waves
In the simplest mechanical-wave
situation, a push on the medium
creates a compression wave that
moves in the same direction as
the push. The material right next
to the initial push moves slightly,
leading it to push on the adjacent
material. This effect causes a
sequence of compressions that
move farther and farther away
from the initial push, even though
each part of the medium only
moves slightly, then
decompresses and settles back at
its original position (until and
unless it gets another push).
Sound is a compression wave of
this kind.
Transverse mechanical waves
In some situations, mechanical
waves travel perpendicular from
the push that creates them. These
transverse waves can occur
along a surface where displaced
liquid is restored to its average
level by gravity (e.g., water-
surface waves), or as shear
waves within solid materials (e.g.,
S-type earthquake waves), or
along something linear like a rope
or cable. Transverse waves
cannot travel through the interiors
of gases or liquids, since the
weak attraction between the
molecules of those kinds of
materials will not pull on each
other hard enough to cause a
transverse wave.
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