Post by akh2103
I'm a recent humanities grad trying to get a grasp of EM. I can
understand/visualize sound waves propagating through a medium. But I
can't understand or visualize EM waves in the same way. I get that EM
waves are 'fundamental' and can't be broken down/explained in terms of
smaller parts (the way a sound wave can be analyzed in terms of
molecules). but i still can't 'see' EM waves. Can anybody help?
After looking at some of the replies, I suspect you may still be having
Sound waves are transmitted mechanically through a medium of some sort. A
simple and reasonably accurate explanation of the process is that a molecule
is mechanically disturbed by the sound source. The molecule then bumps into
another molecule and another. If there is no medium, (as in a vacuum) there
is no sound.
EM is different.
Chances are you have run a plastic comb through your hair, waved it over
little pieces of paper and seen them move. The paper was influenced by the
Electric (E) Field associated with the electric charges on the comb. You
would see the same effect in the vacuum of space.
Chances are you have waved a permanent mgnet over pieces of iron filings and
seen them move. The filings were influenced by the Magnetic (M) Field
associated with the magnet. You would see the same effect in the vacuum of
In both of these experiments, as you move the comb or magnet away, the
effect diminishes and ultimately becomes unmeasurable.
But something that seems "magical" can occur when Electric and Magnetic
fields combine. The combined effect can be measured at interstellar
The "why" is way too complex for an email. But we can talk a bit about how
it originated and what's going on.
In the mid-to-late 1800s, scientists, experimenters and mathematicians were
attempting to develop a set of mathematical formulas that would accurately
describe their measurements of Electrical and Magnetic phenomena.
James Clerk Maxwell derived a set of equations that now bear his name:
Maxwell's Equations. One form of these equations is called "Differential
Differential Equations had been around for quite a while, and mathematicians
had developed general solutions for many of them -- including the forms
discovered by Maxwell.
One solution of Maxwells Equations leads to what is called a "Wave
Equation," A wave equation describes the characteristics of a sinusoidal
wave. 19th century scientists quickly realized that, according to Maxwell,
it *should* be possible to generate en EM Wave that could "fly" through
Within a few years, Heinrich Hertz was able to generate those waves. And
that led to radio, TV, cell phones, GPS, Radar etc. etc.
Since numerous mechanical devices also generate sinusoidal waves, it is
tempting to look at how these devices operate and then attempt to apply
those principles to EM.
Here is a simple example: a pendulum. When we lift the free end, we impart
Potential Energy (PE) to it. When we release it, the pendulum will swing
downward until it reaches the bottom. At that time, all the PE has been
changed to Kinetic Energy (KE) and the arm will swing upward. At the top of
the swing (disregarding friction and air resistance) the pendulum will stop,
reverse its path and repeat the process "forever."
Thus, the energy goes from PE to KE to PE... etc.
This type of motion is called harmonic motion and is described by a
Since an EM wave is composed of an E component and an M component, it is
tempting to jump to the conclusion that there is an energy exchange from E
to M to E...etc.
But there is a *huge* difference between the two processes In the case of
the pendulum (and other similar mechanical harmonic devices) the PE and KE
are out of phase. That means that when the PE is at a maximum amplitude, the
KE is zero, and vice-versa.
In the case of the EM wave, when the E is maximum, so is the M. When the E
is zero, so is the M. Put another way, E cannot *cause* M and vice versa.
The actual cause of E and M is electrical charges and their motion. An EM
wave is called a "transverse" wave. Sound waves, etc are "longitudinal"
waves. They are very different.
Confusing? Yep. Don't feel badly if you have difficulty. A lot of really
smart and well educated folks are in the same boat!
I hope this helps! Good luck in learning more about this very fascinating