MICROWAVE DIFFRACTION AND INTERFERENCE
The object of this experiment is to observe interference and diffraction
of microwave radiation, and to use the observations to measure the wavelength
of the microwaves. The absorption of microwaves by materials will also be
observed.
Introduction:
Electromagnetic waves that have wavelengths in the microwave band can
be generated with a vacuum tube device h o w n as a klystron or a solid state
device called a Gunn oscillator. Microwave radiation can also be genetated by a
magnetron, used in domestic ovens and high power transmitters.
The intensity of'the microwave beam emerging from the microwave horn
is within the limits of what is considered safe. However, to be on the safe side,
avoid any unnecessary exposure of any part of your body to the radiation. It is
especially advisable not to expose your eyes to the source of the microwaves at
close range.
Double Slit Interference
The objective of this part of the experiment is to observe the interference
of microwaves and to use these observations to f i d their wavelength.
Microwaves spreading out from a single source can be divided by
openings in a metal screen. The beams coming from the two openings are
coherent, will spread out (diffract) -and interfere with each other. For some
locations the interference will be constructive and for others it will be
destructive with gradual changes from constructive to destructive in between.
The type of interference is seen to depend on the angle$, at which the
waves are observed. The angle is measured with respect to the centre line
drawn through the midpoint between the two openings in the screen and
perpendicular to the screen. The values of the angles for which constructive
interference should be observed are given by the following equation:
d sin 0
where d
h
rn
=
rnh
(2)
is the centre to centre space between the screen openings,
is the wavelength of the waves and
is a number that takes on integer values only
(i.e. m = 0, 1, 2, 3, etc.).
Procedure
I.
2.
3.
4.
,
5.
Obtain the apparatus with the microwave generator and receiver mounted
on the board with a protractor and swivel at one end. Mount the
generator at the end with the protractor and set the receiver directly
opposite so the units face each other.
Set the screen with two openings in front of the transmitter horn and
adjust the horn to screen distance so that the received power at 90' is a
maximum. Adjust the gain on the amplitude to 80 p A (or a value as
close to this as possible). Note that the received power is very sensitive
to changes in distance between the transmitter and the horn. If possible,
tape each of these in place on their stands to ensure that they do not
move duriig your data taking.
Record receiver meter readings as a function of angle up to about 60' on
both sides of the centre. Take readings at 2' intervals.
Plot the data for relative power vs. angle. Draw a smooth curve through
the points. Determine the angle separating each side maximum from the
centre maximum and average. Estimate the uncertainty in this angle.
Measure the centre to centre separation of the two openings in the screen.
Use equation (2) and the average location of the maxima to get an
estimate of the microwaves' wavelength and the uncertainty of this value.
Single Slit Diffraction
The image of a point source passing through a small opening can be
characterized as a fuzzy cental image surrounded by concentric fuzzy bands of
light. The angular separation of the first dark spaces is inversely related to the
size of the opening, a, and proportional to the wavelength;
where k is a constant of proportionality which depends slightly on the
shape of the opening. For a rectangular opening, k = 1, but for a circular
opening, k = 1.22. The objective is to observe the single slit diffraction pattern
and obtain a rough estimate of the wavelength of the microwave source.
I.
Take the screen with the 3 cm opening and mount it in front of the
transmitter. Adjust the spacing of the screen and horn by sliding the
base of the transmitter forward or back to obtain a maximum received
power at 90'. Adjust the gain on the receiver to an amplitude of
approximately 80 pa. ( These readings are a relative measure of received
power so the gain must be kept constant for each set of measurements,
but may be adjusted before each set.) Make measurement of received
power every 2' from 90' to 150' and again from 90' down to 30'.
Draw a smooth curve throuzh the points.
2.
3.
4.
5.
Plot the data for relative power against angle.
Repeat the measurements of I. above, using the screen with the
single 1.5 cm opening placed in front of the horn and adjusted as before.
Adjust the received power at 90' so that it is the same as that obtained
at 90' using the double slit. Plot relative power versus angle on same
graph as the double slit. Comment on any similarities between the two
patterns.
Estimate the angular separation, 0, of the minimum power points on each
side of the central maximum for each pattern.
Note that the larger opening results in a narrower pattern of received
power. If possible, estimate the wavelength of the microwaves usirig the
formula (1) for single slit diffraction.
Absorption of Microwaves by Wet and Dry Materials
The object of this part of the experiment is to observe the ability of
water to absorb microwaves.
Procedure
1.
2.
3.
Adjust the horns to 90 and the received power to maximum. With no
screens crumple a dry paper towel and place it in the receiver horn.
Record the meter reading.
Remove the paper towel from the horn and wet it. The towel should be
damp but not dripping. Place the damp towel in the receiver horn and
record the new meter reading. Compare this reading to the one with the
dry towel in the horn. What can you conclude from these observations?
Try other types of materials as available, plastics, cloth, etc.