PROFESSOR: Visualization of standing waves
that are electromagnetic requires
that we see fields that are invisible and are
varying at a frequency higher than we can actually
perceive with our eyes.
The visualization of waves that are mechanical is easier.
Here, we've got a spring under tension.
The transfers deflections satisfy the wave equation.
The object in this demonstration is
to see standing waves in a sinusoidal steady state.
Professor Zion will provide a fixed termination at that end.
And I'll try and provide the sinusoidal exhortation.
The first mode's pretty easy.
Here's the second mode.
A little harder to do.
Try that again.
At any location, the response is sinusoidal.
We have a null in the middle, because a wave incident
on that termination comes back and interferes
with the incident wave.
Here are the actual plates and the short.
They're driven at the left by a radial frequency generator.
To see the lamp, we've turned the lights down.
Next, the generator is turned on,
and the lamp is put in place.
I'm rubbing my feet on the carpet
to generate static electricity to help ignite the lamp.
Remember, where there's light, there's
an oscillating electric field.
The dark areas are, therefore, where the electric field
tends to null.
At the right end, we also appear to have a null.
Remember, the right end is shorter.
We can see the nulls even better if the lights are turned out
completely.
What we see is the magnitude or envelope of the standing
wave of electric field.
We can think of the electric field between the plates
as being analogous to the deflection of our spring.
The deflection, like the electric field,
is pinned to 0 at the right end.
A deflection like both the electric and magnetic fields
as an oscillating function of time.
It remains 0 at the null points.
We can measure the magnetic field
with a "one turn" induction coil.
The coil voltage displayed on the scope
indicates the magnetic field perpendicular to its axis.
First, we put the coil at the null in electric field.
The magnetic field is, indeed, tangential to the plates,
and perpendicular to the direction of propagation.
The magnetic field should be a peak
here at the null and electric field.
Now, moving toward the short, where
the electric field is maximum, the magnetic field should null.
Then, at the short, it should peak again.
Note, that the phase shifts by 180 degrees
as we pass through the null.
Watch the peaks and intensity move by a quarter
wavelength as the short is removed.
Again, with the short on and with the short removed.
It is the electric field intensity that is now
the maximum at the right end.
We see this better with the lights out.
By measuring the distance between nulls
and the electric field intensity,
we determine the half wavelength.
It appears to be about 70 centimeters.
Electromagnetic waves propagate on the line
at the velocity of light, c.
So we can deduce the frequency of the source driving
the plates at the left end.
The frequency is equal to the velocity of light 3 times
10 to the eighth meters per second
divided by the wavelength, about 1.4 meters.
So the frequency must be about 210 megahertz.
This is the frequency that we should
deduce from the oscilloscope trace for the magnetic probe.
The period is about 5.2 nanoseconds.
The frequency's about 195 megahertz.
The agreement with the 210 megahertz
we found from the wavelength is consistent with the accuracy
of our measurements.
In visualizing electromagnetic standing waves,
we've used the light illumination
to indicate the electric field intensity and the probe
to measure the magnetic field intensity.
We've observed the distributions of E&H
with open and short circuit terminations.