Re: Wave phase reversal on reflection

["James S. Marsh" <jsmarsh@UWF.EDU>]

Here's a take on this question which may help.    First, the wave must be
continuous
across the boundary.  This is the boundary condition.

 The wave consists of an incoming wave with amplitude Ai, a transmitted
wave with amplitude At, and a reflected wave with amplitude Ar.  The
continuity boils
down to Ai+Ar=At, since Ai+Ar is the net amplitude on the incoming side of
the boundary.
This means Ar=At-Ai.

Now in the situation where the transmitted wave amplitude is smaller than
the incomiing
wave amplitude, At<Ai, then  the reflected amplitude must have the opposite
sign to the incoming amplitude.  In other words the reflected amplitude
must be
negative relative to the incoming amplitude.   The phase reversal is just
the sign shift
of this amplitude.

So the question boils down to, under what conditions is the transmitted
amplitude
smaller than the incoming amplitude.   This question can be discussed in terms
of impedence matching at the interface, as well as from several other
points of view.

JSM

> My colleague asked if there were a good explanatiion for the phase
> reversal when a wave reflects from a boundary where the new speed will be
> less and not when going from slow to fast. I don't have one for her. Can
> anyone help?
>
> Ken Fox
> AP/IB Physics Teacher
> Smoky Hill High School, CO