Download Historical burdens on physics 119 Electromagnetic transverse waves

Historical burdens on physics
119 Electromagnetic transverse waves
Subject:
Right at the beginning of the chapter about waves students learn the definition of the concepts longitudinal and transverse wave:
“For a transverse wave the displacement of the individual sections of the
wave carrier is perpendicular to the direction of propagation. For a longitudinal wave they oscillate back and forth in the direction of propagation.”
Later, when the subject is electrodynamics, they learn:
“Light can be polarized. Thus, it is a transverse wave whose E and B fields
oscillate perpendicularly to the direction of propagation.“
Usually, the distribution of the electric and the magnetic field strength in
space is illustrated by a figure like that of our Fig. 1.
x
y
E
z
H
Fig. 1. “Snapshot” of the electric and the magnetic field strength of a sine wave
Deficiencies:
According to the definition which our students learn, in a transverse wave
the wave carrier moves perpendicularly to the direction of propagation. If
we take this definition literally, then an electromagnetic wave is not a transverse wave, since nothing is moving in such a wave. Of course, one might
argue that the statement is not meant literally, but just in the way we speak
normally when we say that the temperature or the stock-market price “is
moving”.
However, the “movement” seems to be taken too seriously by the students.
We suspect that part of the fault is the picture of Fig. 1 which is never missing in the text books: A snapshot of the movement of the vector tip of the
electric and the magnetic field strength.
You can easily find out that something is not understood when performing a
physics examination at the University. Ask for the field line picture within the
room where the examination takes place for the radio waves coming from a
nearby radio station, the students usually reply by sketching the picture of
Fig. 1. When you point out that this is not a field line picture, the students
are usually perplexed. Apparently, they interpret the image of Fig. 1 in the
sense of our citation: a movement. What makes the interpretation of the
figure somewhat difficult is the fact that first a spacial coordinate system is
drawn, and then two other physical quantities E and B are represented. We
know the procedure from mechanics, where we often draw force vectors in
a scene that represents an object in normal space. In our case, there is the
additional difficulty that the values of E and B change from point to point,
and that their functional dependency is shown for only one space coordinate. The suggestion of an oscillation in the sense of a movement is rather
strong.
Fig. 2. Field line picture of a periodic electromagnetic wave
Origin:
A somewhat unreflected take-over of the definition of the concepts longitudinal and transverse wave from mechanics to electrodynamics. There may
exist a historical reason why the oscillation metaphor is so widely used in
electrodynamics. In former times students learned: “Light is a transverse
ether wave.” And that was meant in the sense of the mechanical definition
of the concept transverse wave.
Disposal:
Explain the field strength distribution in a (periodic) wave with a drawing like
that in Fig. 2, instead that of figure 1.
Friedrich Herrmann, Karlsruhe Institute of Technology