Reflection, Transmission & Refraction Semester 6
of Microwaves
Objectives:
➢ Operational test
➢ Transmission and absorption of microwaves by polystyrene body
➢ Transmission and absorption of microwaves by water
➢ Transmission and absorption of microwaves by the human body
➢ Transmission and absorption of microwaves by a metal body
➢ Microwaves reflection
➢ Microwaves refraction
➢ Total reflection of the microwaves
➢ Microwaves polarization
➢ Microwaves polarization plane
➢ Diffraction of microwaves due to a slit
➢ Diffraction of microwaves due to a double slit (Young’s experiment)
Materials/ Components Required:
★ Microwave transmitter with holder ★ Tray support
★ Microwave receiver with holder ★ 150 X 150 steel sheet
★ 12 V DC power supply ★ Protractor with pin
★ Three-terminal cable for power ★ Paraffin prism
supply with extensions ★ 11 slits grating metal sheet
★ Loudspeaker ★ Single slit grating metal sheet
★ BNC - double banana cable ★ Two slits grating metal sheet
★ Expanded polystyrene panel ★ Linear ruler
★ Tray
Schematic of the setup:
IISER-TIRUPATI 1
Reflection, Transmission & Refraction Semester 6
of Microwaves
Theory:
The electromagnetic spectrum is the set of all possible frequencies of
electromagnetic radiation. This spectrum is continuous, but a conventional and indicative
subdivision in various intervals or frequency bands is possible. The possible frequency
or equivalently wavelength range goes from zero to infinity, maintaining the relationship
of inverse proportionality between the two quantities.
Microwaves are EM radiation with a wavelength between radio waves and infrared
radiation. Although we consider them separate from radio waves, microwaves are
included in the UHF and EHF parts of the radio spectrum. Still, they have specific
characteristics due to their high frequency. The boundary between the microwaves and
the neighbouring radiation ranges is unclear and can vary according to the different
fields of study. The microwaves are between 0.1 m (frequency of about 2-3 GHz) and 1
mm (frequency of about 300 GHz). Above 300 GHz, the absorption of EM radiation is so
intense that the atmosphere can be considered opaque at these frequencies.
Dangers arising from exposure to microwaves:
Exposure to microwaves, like sunlight, can increase the incidence
of cataracts in later life. There are no protections against
microwaves like sunglasses against the sun. A microwave oven
with a defective door can be a source of risk. For the same
reason, it is good to avoid being in the place of the antenna
emission of powerful aircraft radars and look directly and closely
at the anti-theft sensors that use radar technology.
IISER-TIRUPATI 2
Reflection, Transmission & Refraction Semester 6
of Microwaves
The microwave optics kit comprises a transmitter, a receiver, a loudspeaker, an
articulated rail and other components.
This kit can be used for performing various experiments on microwaves: it is possible,
first of all, to demonstrate that microwaves have the same characteristics as light waves
and cause the same phenomena of reflection, refraction, interference, diffraction, etc.
Procedure: (Rail mounting)
➢ Hook the short arm (with the
connection flange) to the long
one (with the pivot)
➢ Put the washer in the pivot
➢ Insert the protractor on the pivot,
at 0°
➢ Screw the black PVC yoke
onto the pin
At the end of the assembly, the right arm must rotate independently of the left one.
IISER-TIRUPATI 3
Reflection, Transmission & Refraction Semester 6
of Microwaves
Task 1: Operational test
● Align the receiver horn with the transmitter horn.
● Set the internal modulation of the transmitter.
● Turn the speaker volume knob to the left.
● Arrange the system as shown in the figure below.
● Connect the power supply.
● Turn the volume control slowly: you should hear a fixed frequency sound of 550
Hz with increasing amplitude. (Use DSO instead of Speaker to see the output)
Task 2: Transmission and absorption of microwaves by polystyrene
panel
The absorption and transmission of microwaves provide important qualitative
information about the physical properties of substances interacting with electromagnetic
waves. Let’s take into account, for example, the absorption. When crossing a layer of
thickness x, it is observed that the intensity of the transmitted radiation follows the
Lambert’s law:
(1)
where k is the absorption coefficient of the material at a known frequency.
If, for a given frequency, a material has k = 0, then it has zero absorption. Of
course, the same material can be transparent at certain frequencies and absorb other
ones. It is known that electrical insulators are generally transparent to microwaves and
visible light, instead, they strongly absorb ultraviolet radiation. It is common to see that
solid insulators such as diamond, quartz, and kitchen salt are transparent crystals. If the
insulating material is an ionic crystal, we can observe a strong absorption in the infrared
region. The first type of absorption is due to the electrons of the solid, the second one is
due to the oscillations of the ions. Semiconductors absorb visible light. This is the reason
why silicon solar cell panels appear black. A high absorption coefficient is associated
with a high reflectivity. Metals absorb and reflect on the whole spectrum, even in the far
infrared and microwave regions.
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Reflection, Transmission & Refraction Semester 6
of Microwaves
● To perform this experience, arrange the system as shown in the figure below.
● The transmitter and the receiver are 25-30 cm far from the centre of rotation of
the rail.
● Connect the output of the receiver to the DSO
● Now, place the supplied polystyrene panel in the middle and observe that there is
no attenuation of the signal. Repeat the test by sending the received signal to a
measuring instrument if necessary.
Task 3: Transmission and absorption of microwaves by water
● Repeat the previous experience using the plexiglass tray without water.
● Also, in this case, no signal attenuation is recorded
● You can try again with other electrically insulating materials (for example, wood,
cork, etc.), but the result will be the same.
● The insulating materials let the microwaves go through.
● Depending on the type of material, a partial, more or less evident, reflection of the
microwaves may occur.
● Fill the tray with water (in the picture, it is faintly coloured to show it). Place
the tray on its longer side, as shown in the figure below.
The signal is absent; the water absorbs the microwaves. The law that describes
this phenomenon of absorption is: I = I0 · e - k x
This means the water layer does not allow microwaves to reach the receiver. This is
precisely why submarines and boats do not use radar since the latter is based on the
transmission and reception of microwaves, but sonar works at much lower frequencies.
Check if the same thing happens even when the microwaves cross the short side of the
tray.
IISER-TIRUPATI 5
Reflection, Transmission & Refraction Semester 6
of Microwaves
Task 4: Transmission and absorption of microwaves by the human body
● Further confirmation that the microwaves are absorbed by water can be seen by
placing the hand between the transmitter and the receiver, as in the figure below.
● Also, in this case, the signal is almost attenuated. The explanation of the
phenomenon lies in the fact that, on average, 75% of the human body is made of
water.
Task 5: Transmission and absorption of microwaves by a metal body
● Repeat the previous experience by placing a metal plate between the transmitter
and the receiver, as shown in the below figure.
● There is no microwave passage in these conditions, and the signal is absent.
Task 6: Microwave reflection
● Arrange the system as shown in the below figure.
● The reflecting plate must be aligned with the zero of the protractor, while the
branch on which the transmitter is arranged must form an angle of 45° with the
zero of the protractor.
● Switch on the devices and select the internal modulation.
● Adjust the amplitude of the received signal over the DSO screen.
IISER-TIRUPATI 6
Reflection, Transmission & Refraction Semester 6
of Microwaves
● Then, while holding the left branch of the guide, slowly rotate the right one.
● You will notice that the received signal increases in amplitude and becomes
maximum when the angle formed by the left branch with the zero of the protractor
is 45°.
● This proves that the signal reflected by the plate is maximum when the angle of
reflection is equal to the angle of incidence (below figure)
Task 7: Microwave refraction
● An optical prism can produce the refraction of light, so even a paraffin prism
produces a refractive effect on the microwaves that follows the same optical laws
as the refraction of light rays.
● For this experience, arrange the system as shown in the below figure.
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Reflection, Transmission & Refraction Semester 6
of Microwaves
● It is advisable to place the transmitter 25 cm away from
the prism to minimise the microwaves reaching the
receiver without passing through the prism, which must
be arranged as shown in the side figure.
● In this position, the angle at the vertex is A = 45°, while the value of the angle of
incidence i can be read on the upper goniometer. Choose, for example, i = 30°.
● If the receiver is aligned with the transmitter, it receives no signal because the
wave beam sent by the transmitter is deviated from the prism.
● Holding the left branch, slowly rotate the right one and take note of the position at
which the signal reaches the maximum value (below figure).
● You can thus evaluate the total
deviation angle δ, taking into account
the position on the lower protractor of
the index of the branch on which the
receiver is placed.
● From geometric considerations,
we have that δ = i + e - A
● Using this formula, knowing the values
of i, A, δ, you can calculate the value
of the angle e
(side figure)
IISER-TIRUPATI 8
Reflection, Transmission & Refraction Semester 6
of Microwaves
Task 8: Total reflection of the microwaves
The refraction index is a quantity that depends, at a given temperature, both on
the material and the wavelength of the incident radiation. Paraffin is a variable mixture of
alkanes (Cn, H2n+2), characterised by refractive indices ranging from 1 to 1.4. This
experience aims to determine the refractive index of the supplied paraffin at a frequency
of 10.5 GHz, which will approximately have a value between 1 and 1.4.
In this experience, we use total reflection to pass a wave from a propagation medium
with a refractive index of n to the air. The index of refraction n is linked to the limit angle
l from the following relation:
n = 1 / sen (l)
For this reason, we should know the value of l to calculate the value of n.
● Arrange the system and place the prism, as shown in the figure below.
● The wave reaches the prism perpendicularly, so it is not deflected. It then
continues in the same direction and reaches the inclined face of the prism
according to an angle of 45°
● If this angle is greater than the limit angle, the ray is totally reflected. Otherwise, it
is refracted.
● To check it, hold the left arm of the rail and slowly rotate the right one.
● You can thus verify that the signal is maximum when the arm is rotated about 90°
in the direction of the totally reflected ray (see the below figure).
● This means that l is slightly less than 45°, corresponding to a refractive
index of about 1.4.
IISER-TIRUPATI 9
Reflection, Transmission & Refraction Semester 6
of Microwaves
Task 9: Microwave polarization
As pointed out in the theoretical introduction,
an electromagnetic wave with a well-defined
frequency is constituted by a set of two fields: an
electric field (E) and a magnetic field (B),
perpendicular to each other. If the electric vector
and the magnetic vector oscillate in the same
direction, the electromagnetic wave is polarized.
To determine if the electromagnetic waves produced by the transmitter are
polarized, you can perform the following experiment. At the back of the transmitter, there
is a protractor, shown below in Figure a. Slightly unscrewing the handwheel, it is possible
to rotate the transmitter up to 90° in both directions of rotation, as shown in Figure b.
● Now arrange the system as shown in the figure below.
● set the internal modulation and adjust the volume of the received signal to a
medium value.
● If you rotate the transmitter slowly, you will see that when it is placed almost
perpendicular to the receiver, the signal is extinguished, as the receiver includes a
polarization filter.
● By connecting the receiver output to a tester set for alternating voltage
measurements, you can verify that, within experimental errors, the intensity of the
received signal is proportional to the cosine of the angle of rotation.
IISER-TIRUPATI 10
Reflection, Transmission & Refraction Semester 6
of Microwaves
Task 10: Microwave polarization plane
Previous experience has shown that the electromagnetic wave produced by the
transmitter is linearly polarized, i.e., both the electric and the magnetic vectors oscillate
in the same direction.
● With the following experience, you can determine the direction of oscillation.
● Set up the system as shown in the figure below, and when the internal modulation
is activated, adjust the intensity of the acoustic signal to an average value.
● Then, place the 11 slits grid between the transmitter and the receiver as shown in
the below figure. You will notice that the acoustic signal maintains its intensity.
● The acoustic signal is extinguished by turning the grid 90° degrees, as shown in
the figure below. Remembering what is explained in the introduction, it is possible
to conclude that the electric vector of the wave produced by the transmitter
oscillates in a horizontal plane.
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Reflection, Transmission & Refraction Semester 6
of Microwaves
Task 11: Diffraction of microwaves due to a slit
When a wave reaches an obstacle with a slit slightly wider than the wavelength,
the phenomenon of diffraction occurs. Beyond the slit, the energy does not spread
uniformly but presents maximum points alternate to minimum points. This strange
distribution is explained by the Huygens-Fresnel principle, according to which all the
points of the slit belonging to the same wavefront behave like coherent wave sources.
This means that in a generic point of the space beyond the slit, the intensity will be the
result of the overlapping of these elementary waves. On the screen,n will be alternating
maximum and minimum intensity points. You can check it using the system shown in the
figure below. Ensure the transmitter is no more than 30 cm away from the center of the
rail.
In the central position, the intensity is maximum, but when
the receiver moves slowly, a sequence of minimums and
maximums is detected. It can be shown that the angular
distance between the central maximum and the first minimum
satisfies the following relationship
Sin =
where α is the displacement angle of the receiver,
λ is the wavelength, and d is the width of the slit.
Being λ = 2,85 cm and d = 5 cm,
the first minimum must be at the angle
of rotation of the receiver of about 34° (below figure).
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Reflection, Transmission & Refraction Semester 6
of Microwaves
Task 11: Diffraction of microwaves due to a double slit
(Young’s experiment)
If we now use the two slits grating metal sheet as shown in the below Figure, the
radiation diffracted by each slit will interfere with each other.
This is Young's experiment. The effect obtained is a superposition of two
phenomena: the interference between the elementary waves produced by the two slits
and the diffraction obtained from each of the two slits (See the slit pattern).
First, set the transmitter and receiver in the positions indicated in the previous
experience, replacing the single-slit grating metal sheet with the one with two slits. The
received signal is maximum in this situation, as the two elementary waves travel the
same distance. By rotating the receiver, it is possible to study the result of this overlap
for different angles (Below figure).
If a is the distance between the two slits, the first-order maximum of the interference
figure is given by the following relation:
Sin =
IISER-TIRUPATI 13
Reflection, Transmission & Refraction Semester 6
of Microwaves
Precautions:
● Dangers arising from exposure to microwaves:
● Exposure to microwaves, like sunlight, can increase the incidence of
cataracts in later life. There are no protections against microwaves like
sunglasses against the sun.
Results & Inference:
References:
➔ Optics, Eugene Hecht and A.R. Ganesan (2002) 4th Edition, Addison Wesley Longman.
➔ Introduction to Modern Optics, Fowles (1989) Dover Publications.
➔ Fundamentals of nonlinear optics by Powers, Peter E. (2015) CRC Press.
➔ Introduction to solid state physics by Kittel, Charles (2016) Wiley India.
➔ The Art of Experimental Physics, D.W. Preston and E.R. Dietz (1991) John Wiley.
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