Re: Path of light...

[William Beaty <billb@eskimo.com>]

On Fri, 28 Mar 1997, Dwight K. Souder wrote:

> 	Greetings everyone.  I overheard someone trying to explain how
> light propagates.  They stated that the electrical and magnetic waves
> move perpendicular to one another.  The resultant is a spiral wave.  He
> then went on to demonstrate with a cord of the magnetic wave moving side
> to side and the electrical wave moving up and down, then with the light
> wave, the end of the cord was spun around in a circle producing a
> spiralling wave down the cord.
> 	How accurate is this?  I've never heard of this "spiralling"
> wave.  Is it a resultant wave from the two waves?
> 	Any input would be helpful.

With *two* superimposed e/m waves, there are two e-field vectors whose sum
is a spirally wave.  Called circular polarization. 

This broke out as controversy on my "weird science" list.  The original
question started as an acoustic analogy: single frequency sound propagates
as a uniform (not pulsed) energy flow because the wave contains a KE wave
and a PE wave in 90deg phase.  When the energy contained in the pressure
and the velocity waves is calculated versus time or versus distance, it is
constant.  The energy travels as a smooth flow, not as ripples.  Is this
analogous to EM? In the usual graph of an EM wave, are the e and b
components at 0deg phase or 90deg temporal phase?  And if they are at 0deg
phase, does this not imply that the energy propagates as Poynting-vector
bursts at twice the wave frequency? 

The discussion petered out, but not before two things were settled: that
Maxwells equations give the the e and b waves at 0deg phase, and that
there is a "galloping misconception" which infects textbooks, a diagram
which depicts a 3D graph of e and b sinewaves with two spatial axes and
one time axis, where e and b are plotted on each of the spatial axes and
where THEY ARE 90DEG PHASE IN TIME.  If we refer to many textbooks for the
right answer, we will find two right answers.  This erroneous diagram is
very probably obeying the dynamics of disease epidemics, and it gives me a
chance to hold forth on one of my favorite concepts: "textbook memetics," 
or the "germ theory of education."     >8-)  !

   Like disease infections that travel between hosts, information travels
   between textbooks when students are taught from one book and then grow
   up to write another, or when a textbook author uses another book as
   reference when writing a new textbook.

   As in epidemics, a particular piece of information can spread 
   exponentially, since the more textbooks it occupies, the more likely
   other textbooks are to aquire it.  The spread might be more than
   exponential, since the more textbooks it occupies, higher its perceived
   credibility.

   Textbooks and humans act as independant populations which can 
   infect each other.  Textbooks are disease vectors.  (Or if you
   want to get Dawkinesque, humans are disease vectors, and a textbook
   author is simply an efficient method that textooks have evolved for
   producing more textbooks!  ;)

   There is a "Typhoid Mary" phenomena.  If a piece of bad information
   gets into an authoritative reference book, it spreads like wildfire.

   Some authors are more prone to the infection than others.  A non-expert
   author who relies more on reference books than on direct knowledge of a
   subject will promote the spread.  Authors who fall for the concept
   "that many textbooks can't be in error" will cause the infection to
   spread faster than exponentially. 

   There are competing infections.  Incorrect information competes with
   correct information for the same environmental niche, like cowpox
   competes with smallpox.  If you *know* that e and b waves are always at
   0deg phase, the "90deg phase" misconception cannot infect you.

   Evolution forces are important.  As it travels from book to book, an
   error can start out as an inadvertant mistake, and be honed and
   polished into a ravening plague which easily conquers the correct
   version of that particular information.

   There is an uninfected population, an infected population, and a
   population which has been innoculated: authors, students, and educators
   who are specifically made aware of the error can no longer become
   infected.

Line up and roll up your sleeves!

......................uuuu / oo \ uuuu........,.............................
William Beaty  voice:206-781-3320   bbs:206-789-0775    cserv:71241,3623
EE/Programmer/Science exhibit designer        http://www.eskimo.com/~billb/
Seattle, WA 98117  billb@eskimo.com           SCIENCE HOBBYIST web page