Re: Waves

[Maurice Barnhill <mvb@UDEL.EDU>]

Carl E. Mungan wrote, among other things:
> Ludwik asks what is a wave? Maybe that's a good question to start with -
> anyone care to take a crack? In my mind, I was counting pulses as waves -
> after all, you could Fourier compose many sinusoids to get a pulse. I am
> however ruling out standing waves - there does have to be a net energy
> transport, hence a traveling wave.

A wave is a pattern of movement in some medium in which the pattern
moves in a different way than the medium does.  In light, the medium is
electromagnetic fields, which do not in themselves move at all; instead
there is a time-varying value for the field at each point. The wave is
in the pattern of variation of the fields at each point.

In some cases the medium undergoes a net displacement as a result of the
way, but the displacement is different than the movement of the pattern.
A classic example of this kind of wave is a crowd startled by a sudden
noise.  The people nearest to the noise step away from it, forcing the
next line of people to move back a step, which forces the next line of
people to step back, and so forth.

A shock wave is a pattern of changes in the form of motion of the
particles in a medium, where the location of the change may move at a
different speed or direction than the particles themselves [this
definition may not be general enough].  Clearly a shock wave can carry
momentum.

In an oscillatory wave, the particles of the medium undergo cyclical
motion with no net displacement.  Most but not all contributions to this
thread had this kind of wave in mind.

This is all off the top of my head, so I expect some corrections, but I
think that I got the gist of the definitions correct.
--
Maurice Barnhill, mvb@udel.edu
http://www.physics.udel.edu/~barnhill/
Physics Dept., University of Delaware, Newark, DE 19716