The Properties of Microwaves©98
Experiment 8
Objective: To observe and measure reflection, absorption, polarization, and interference
of microwaves.
DISCUSSION:
Electromagnetic radiation occurs whenever charged bodies are caused to oscillate.
If the oscillation is simple harmonic, the radiation is in the form of electromagnetic
waves that have definite frequencies, wavelengths and amplitudes. The character of the
radiation, as we perceive it, is determined by the frequency. For example, a frequency of
60 Hz is associated with the radiation from the wires carrying commercially generated
alternating currents. Frequencies from a few tenths of a megahertz to several hundred
megahertz are used in commercial radio and television. Frequencies of about 1014 Hz are
associated with visible light.
1. Reflection (Fig. 1)
When a plane wave is reflected from a surface, the angle i at
which the incident wave Ii strikes the reflecting surface equals
the angle r at which the reflected wave Ir leaves the surface.
As shown in Fig. 1, the angles of incidence and reflection are
measured relative to the normal to the surface.
2. Absorption (Fig. 2)
Radiation incident upon a medium may be partially
reflected (Ir), partially transmitted through the medium (It),
and partially absorbed by the medium (Ia). The intensity of
the absorbed radiation may be measured indirectly by
measuring the incident radiation intensity Ii and subtracting
from it the sum of the reflected and transmitted radiation
intensities. This is expressed mathematically in terms of
the incident radiation Ir which has been measured directly.
I I I I
i
r
t
Ii
Reflecting Surface
The frequency to be used in this experiment is about 1010 Hz and produces waves
whose wavelength is about 3 cm. This wavelength is characteristic of that class of
electromagnetic radiation called microwaves. Like all frequencies of electromagnetic
radiation, these waves may be reflected, absorbed, polarized, and experience interference.
Each of these phenomena is to be examined in this laboratory:
i
r
Ir
Figure 1
Ii
Absorbing
Medium
Ir
It
Figure 2
(1)
a
3. Polarization (Fig. 3)
In an electromagnetic wave, the fluctuating electric and magnetic field vectors lie in a
plane perpendicular to the direction of propagation. Ordinarily, during the course of
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several cycles of oscillation, the electric and magnetic fields point in any and all
directions in this plane, with the electric vector and magnetic vector at any instant at
right angles to one another. If all directions of the electric field except one are
suppressed, then the wave is linearly polarized. Whether or not a wave is linearly
polarized may be determined by allowing the wave to pass through a medium that
absorbs or reflects all electric fields except those pointing in a given direction,
creating a plane of polarization. In Fig. 3, the plane of polarization is the x-z plane,
meaning that all the electric vectors of the wave lie in that plane. If a wave is linearly
polarized and if the medium is oriented so that the wave passes unhindered through
the medium, a rotation of the medium about an axis along the direction of
A: The electric part of an E&M wave.
X-axis
B: The magnetic part of an E&M wave.
X-axis
Z-axis
Y-axis
Electric Vector
Z-axis
Y-axis
Magnetic Vector
C: A linearly polarized (in the x-z plane)E&M wave.
X-axis
Z-axis
Y-axis
Figure 3: Linear polarization of an E&M wave.
propagation, through an angle of 90°, prevents the passage of the wave.
4. Interference (Fig. 4)
Whenever a sinusoidal wave traveling to the right is superimposed upon a
sinusoidal wave traveling to the left, each with the same amplitude, frequency and
speed, interference of the two waves occurs in such a way as to set up what are
known as standing waves. A schematic representation of a standing wave is
shown in Fig. 4. At a node, the vibrating medium (in this case, the
electromagnetic field) remains fixed. Between the nodes are loops, at the center
of which the medium oscillates sinusoidally with a maximum amplitude. The
maximum amplitude points are called antinodes. The wavelength of the
standing wave is twice the distance d from one node to the next.
(2)
  2d
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To set up standing waves in this experiment, we place a reflector opposite the
generator. The waves reflected from the reflector have the same frequency and
wavelength, and nearly the same amplitude, as do the waves incident on the reflector.
But because they move in opposite direction, they interfere with the incident waves so as
to produce standing waves.
The detector is too large to place in the region of the standing wave. We therefore
place a very small reflector, called a probe, in the region of the standing wave to reflect
some of the intensity in the standing wave to the detector, which is aligned at a right
angle to the line connecting the generator and probe. This probe is small enough so that
it does not seriously upset the conditions which produce the standing wave.
If the probe is placed at a node of the standing wave, little or no intensity is
reflected to the detector. If the probe is placed at an antinode, a maximum amount of
intensity is reflected to the detector. Moving the probe therefore allows the measurement
of the wavelength if the wave since this motion detects the positions of the nodes and
antinodes of the standing wave.
For best results, the probe and the detector are held in a fixed position and the
standing wave is adjusted by moving the reflector in or out in a precise manner. Because
there is always a node at the reflector, the standing wave is shifted a distance equal to the
distance through which the reflector is moved.
EXERCISES:
1. Reflection
a. Arrange the apparatus as shown in Fig. 5, so that
the angle subtended at the reflector by the generator
and detector is 90°. The reflector consists of a
metallic sheet.
b. Rotate the reflector to find that orientation at
which the power received by the detector is a
maximum.
c. Determine whether the angle of incidence of the
wave is equal to the angle of reflection.
d. Repeat this experiment for angles other than 90°.
Reflector
Detector
Generator
Figure 5: Reflection of microwaves.
2. Absorption
a. Arrange the apparatus as shown in
Fig. 6. In this arrangement, with no
absorbers intervening, the detector
receives as much energy as possible
from the generator. The reading of the
intensity Ii received by the detector
from the generator may be changed by
Generator
Detector
Place various
absorbers here
Figure 6: Absorption of microwaves.
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adjusting the ‘gain control’ of the detector. Set this reading to 100 units. This value
now corresponds to the incident intensity Ii as seen in Fig. 2.
b. Insert an absorber with a plane surface (such as a book) between the generator and
the absorber. Orient the book so that the waves reflected from its surface are directed
at right angles to the incident waves, as in Fig. 5. Measure the reflected intensity Ir
received by the detector.
c. With the apparatus aligned as in Fig. 6, measure the transmitted intensity It
received by the detector.
d. Subtract the sum of the intensity reflected and transmitted from the incident
intensity to find Ia, the intensity absorbed.
e. Repeat this exercise for other items suggested by the instructor.
3. Polarization
a. Arrange the apparatus as shown in
Fig. 7, with the polarizing grid between
the detector and the generator. Orient the
Generator
Detector
polarizer so that the grid is horizontal.
Polarization grid
Measure the intensity received by the
Figure 7: Polarization of microwaves.
detector.
b. Repeat part a, with the grid of the polarizer at 45° to the horizontal.
c. Repeat part a, with the grid of the polarizer orientated vertically.
d. The polarizer transmits only the component of the electric vector which is
perpendicular to the grid bars. Why perpendicular and not parallel?
e. What do you conclude about the polarization of the waves emitted by the
generator?
4. Interference
a. Align the apparatus as seen in Fig. 8., with the
detector at a right angle to the line created by
the generator, probe and reflector.
b. Move the reflector in or out by rotating the
screw until a maximum reading on the
detector is observed. Record this position;
there is now an antinode at the detector.
c. Move the reflector in or out until a minimum
reading on the detector is observed. Record
this position; there is now a node at the
detector.
d. Using these positions and Eq. (2), find the
wavelength of the electromagnetic wave.
Screw Adjustment
Reflector
Probe
Detector
Generator
Figure 8: Interference of microwaves.
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