Department of Energy Argonne National Laboratory Office of Science NEWTON's Homepage NEWTON's Homepage
NEWTON, Ask A Scientist!
NEWTON Home Page NEWTON Teachers Visit Our Archives Ask A Question How To Ask A Question Question of the Week Our Expert Scientists Volunteer at NEWTON! Frequently Asked Questions Referencing NEWTON About NEWTON About Ask A Scientist Education At Argonne Light Waves and Sound Waves
Name:  Shannon B.
Status:  educator
Age:  20s
Location: N/A
Country: N/A
Date: 2000-2001


Question:
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?


Replies:
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 understood.

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.

Tim Mooney


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.

Kenneth Mellendorf


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.

Good luck.

Chris Murphy


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 medium.

Vince Calder


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 collide.

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.

Greg Bradburn



Click here to return to the Physics Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (help@newton.dep.anl.gov), or at Argonne's Educational Programs

NEWTON AND ASK A SCIENTIST
Educational Programs
Building 360
9700 S. Cass Ave.
Argonne, Illinois
60439-4845, USA
Update: June 2012
Weclome To Newton

Argonne National Laboratory