WAVE
- a disturbance in a medium in which a) there is no gross migration
of the medium; and b) energy is transported from point A to point
B.
A novel example of a wave that fits this operational definition
is a stadium wave wherein people stand and then sit on cue, causing
the wave to move rapidly through the stands. The pulse moves through
the crowd , but the people (the medium in this example) do not
move laterally at all. A stadium wave is an example of a transverse wave
in which whatever movement of the medium that exists occurs at
right angles to the direction of propagation. Water waves are
transverse, so, too, are stadium waves and light waves.
A second kind of wave is a longitudinal
wave, in which the displacement of the
medium is parallel to the pulse moving through the medium. A homely
example of a longitudinal wave is a row of domino standing on
end. Pushing over the first domino causes it to fall onto the
second; the second falls on the third,and so-on. It should be
noted that pushing the domino in some direction perpendicular
to the direction of the line form does nothing to establish a
longitudinal wave. You may have noticed a second longitudinal
wave if you have ever been near a railroad yard when a train has
pulled out; there you have noticed that the engine starts to move
first, followed one-by-one by the other cars. There is a very
audible pulse that runs through the length of the train. As we
shall see, sound is another example of a longitudinal wave.
This APPLET shows the difference between longitudinal and transverse
waves
http://www.cbu.edu/~jvarrian/applets/waves1/lontra_g.htm
Torsional waves round out the different kinds of waves; in this
scenario, the displacement of the medium is at right angles to
the direction of propagation as well as perpendicular to the displacement
of the other two kinds of waves. We will not do much with them;
they are mentioned here for the sake of completeness. The twisting
action seen on the Tacoma Narrows bridge before it collapsed was
due to torsional waves.
We need to establish the definition of some new
terms.
Wavelength = [symbol: lambda] the straight line distance from
any point on a wave to the corresponding point on the next succeeding
wave [measured in distance units, usually meters or fractions
thereof].
Frequency = [symbol: f ] the number of waves that pass some
observation point in 1 second [ measured in units called Hertz
1 Hz = 1/sec].
Period = [symbol T ] the time it takes one wavelength to
pass the observation point [usually measured in seconds or fractions
thereof] [note that
f = 1/T ]
Velocity = [symbol v] the speed of a wave in a given direction
[measured in conventional units for velocity]
Displacement = the distance that the medium moves from equilibrium
as the wave passes through.
Amplitude = maximum displacement
Phase
= two wave sources are in phase if they produce identical waves
at the same time. Differences in phase are usually expressed as
a phase angle.
How waves combine
Superposition = the addition of two or more waves to produce a new wave. Superposition can take one two forms: constructive interference (waves combine to form a bigger wave) and destructive interference (waves combine to form a smaller wave)
This page was last reviewed by mgosselin 10/09/2005