Light Waves and Sound Waves
Name: Shannon B.
I teach sixtth grade and we are currently studying sound
and basic wave characteristics. One of my students asked : Do light
waves or sound waves have width to them if you were to look at them from
an aerial view rather than from the side?
Yes, they do, but it is not very instructive to go into this aspect of
sound or light before the basic notions of how waves behave are
Scientists are always doing this kind of thing: reducing something to
its simplest form, and understanding how that behaves, THEN looking at
complications. If you look at complications before thoroughly
understanding the simple stuff, what you find is that you lack the
tools to understand or even describe the complicated stuff.
Eventually you'll find that to describe and understand the three
dimensional structure of a real sound wave, you have to know the
details of its source, and how the source size compares with the sound
wavelength; you have to understand diffraction; and you need lots and
lots of math.
But you can learn a great deal about sound and waves by thinking only
in terms of waves you can draw, and what you learn from this applies to
waves of all kinds.
From an aerial view, waves are similar to from a side view. The actual
width from either direction depends a great deal on the structure of the
device emitting the waves. Consider a flashlight or a laser. From above or
the side, a flashlight beam is wide. From above or the side, a laser beam
is very narrow.
If you are speaking of a single wave within the beam, waves traveling
through space are three-dimensional. A light wave oscillates in all
directions perpendicular to its direction of motion. The wavefront of a
sound wave is a two-dimensional "surface" traveling through space.
Wave behavior from "above" can be easily seen in a pool of water. When you
drop a rock into the water, the waves move out in concentric circles from
the center of the drop. Unless there is something in the way of the wave,
it will continue to grow out in circles until the wave is dissipated. If a
wave is constrained be some device (say a wall), it will continue to bounce
of the containment and thus have some sort of width and height. Take a
megaphone for example. It is conical in shape and thus the waves will move
off in an ever expanding cone away from the speaker. If the speaker points
the megaphone straight up, then the waves should continue upward in an ever
expanding cone. If he points it parallel to the ground, the cone will
expand until it hits the ground and then the shape is changed due to the
sound "bouncing" off the ground.
One of the best ways to show wave behavior is using a pool of water and
objects in the pool to show how waves go around walls, through slits, etc.
A kiddie pool, a few rocks, and some pieces of boards of different shapes
and slits in them will help explain well the behavior of waves.
Light is the oscillation of an electric vector,E, and a magnetic vector, B,
which are perpendicular to one another and also to the direction of
propagation. So if a light wave has "width" perpendicular to the direction
of propagation, I'd say it is the magnitude of those vectors. In the wave
model for light the intensity, I, is ~E^2, so that could be called the
"width" of the wave perpendicular to the direction of propagation.
Sound is the transmission of a pressure wave through some material -- let's
say air. The molecules of the material, in addition to being moved by the
oscillating pressure wave, also are subject to random thermal motions, so I
suppose that the "width" of a single sound wave would be the mean free path
length of the random motions of the atoms/molecules of the particular
Sound waves are compression waves. That is, they are variations in the
density of the air. If you were to take very fast measurements of the air
ressure near a source of sound the air pressure would rise and fall like a
wave. Sound waves travel as a result of momentum transfer when air molecules
There is no such thing as a single sound wave. They originate from a
source and travel out in all directions (in three dimensions) so, yes, they
have a "width". They travel out as an expanding bubble so the width is
increasing is they travel.
Light waves are a different phenomenon. They are electromagnetic in
nature. The magnetic and electric fields rise and fall very rapidly as they
travel. But they do not depend on collisions to travel. There are single
waves of light (called photons). While there are interesting interference
effects that could be used to argue that light waves do have a width that
width is very small. For purposes of sixth grade students I would say that a
light wave has no width.
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Update: June 2012