Optical_properties_of_microwaves.doc

Optical properties of microwaves
05物理学 李明阳
Microwaves have similar properties as light waves, the reflectance, interference, and diffraction phenomenon observed for visible light and X-ray can be reconstructed by using microwaves.
By observing and measuring the reflection, interference, and diffraction of microwaves, can be well understood as light and microwaves are all electromagnetic waves.

1. Principle
⑴ Reflection of microwaves
Microwave obeys the reflection law of light. If the microwave from the emission antenna is incident on a metal plate with an incident angle of α, the received microwave by the receiving antenna will be the strongest provided that when the reflection angle β is equal to the incident angle α.
⑵ Single-slit diffraction of microwaves
The principle of microwave diffraction is the same as that of visible light. ,diffraction will occur, when a microwave passes through a narrow slit, whose width is comparable with the wavelength of the microwave. The distribution of the intensity of the microwave hinter the slit is not uniform. Let λ be the wavelength, a be the width of the slit, then the diffracted wave will be the minimum, when the diffraction angle satisfies the following equation:
,
and it will be maximum, when the diffraction angle satisfy the equation
.
⑶ Double-slit interference of microwaves
Microwaves obey the law of interference of light. When a planar microwave is incident on a metal plate with two narrow slits, each of the slit is a source for secondary waves, and the waves from the two slits interfere hinter the metal plate. Naturally, there occurs also diffraction, when microwave passes through each slit. Eventually the result is the overlap of the diffraction and the interference. In order to investigate only the interference of the two center diffraction waves from the two slit, the slit width should taken to be close to the wavelength of the microwave. For example, for λ=32mm, a=40mm, the diffraction angle of the first order minimum is nearly 53°. So when a large value of b is taken, then the diffraction effect will be small, and vise versa.

From the law of interference of light, it is to known that, the intensity of microwaves hinter the plate will be increased due to interference, when the interference angle φ satisfy the equation
,
and it will be decreased due to interference, when the interference angle φ satisfy the equation
.

2. Apparatus and Instrumentation
The main apparatus of this experiment is a microwave spectrometer . The spectrometer can be divided into three functional units, namely emission unit, the receiving unit, and the rotation platform unit. The emission unit composed of a fixed arm, a solid-state microwave source, an attenuator and an emission antenna. The solid state source generates microwaves with a wavelength in the neighborhood of 32 mm. The generated microwave is attenuated to a proper intensity and then emitted through the bell antenna. The receiving unit composed of a movable arm, a receiving antenna, a crystal detector and an indicator. The microwave from the receiving antenna is detected by the crystal detector, and the relative power level of the received microwave is then indicated. The rotation unit gives information of angle between the axis of emission antenna and receiving antenna.

3、Contents of Experiment
(1) Experimental verification of the reflection law of electromagnetic waves
For different incident angle of 30º, 35º, 40º, 45º, 50º, 55º and 60º, measure the corresponding reflection angles.
(2) Single-slit diffraction
Measure the intensity distribution of the diffracted waves hinter the single slit, find the diffraction angles of the first order minimum and first order maximum, and compare them with the theoretical ones.
(3) Double-slit interference
Measure the intensity distribution of the microwaves hinter the double slit, find the interference angles of the zero-order, first-order and second-order minimum as well as the first-order and second-order maximum, and compare them with the theoretical ones.

4、 Data record and processing
⑴ Reflection of microwaves

左/度
右/度
平均值/度
30
35
22
28.5
35
41
27
34
40
47
32
39.5
45
49
39
44
50
52
46
49
55
56
53
54.5
60
60
62
61

⑵ Single-slit diffraction of microwaves
角度/度
电流/μA
角度/度
电流/μA
50
1
-2
66
48
16
-4
62
46
31
-6
64
44
21
-8
68
42
11
-10
68
40
16
-12
66
38
24
-14
52
36
15
-16
42
34
3
-18
32
32
1
-20
26
30
10
-22
18
28
29
-24
11
26
31
-26
10
24
16
-28
8
22
2
-30
12
20
4
-32
10
18
34
-34
4
16
54
-36
1
14
47
-38
0
12
40
-40
2
10
50
-42
4
8
64
-44
3
6
64
-46
1
4
69
-48
0
2
80
-50
0
0
78

⑶ Double-slit interference of microwaves
角度/度
电流/μA
角度/度
电流/μA
50
8
-2
56
48
4
-4
32
46
0
-6
14
44
1
-8
3
42
18
-10
0
40
50
-12
1
38
62
-14
4
36
50
-16
14
34
22
-18
24
32
2
-20
30
30
0
-22
26
28
3
-24
16
26
12
-26
4
24
40
-28
0
22
70
-30
0
20
82
-32
0
18
66
-34
0
16
44
-36
2
14
26
-38
3
12
12
-40
8
10
6
-42
8
8
4
-44
3
6
9
-46
0
4
26
-48
0
2
50
-50
0
0
64

⑷双缝干涉的有关计算
微波波长λ=c/υ=0.03375m，a=b=40mm
从图像中得到0级峰的x=19.9112°，1级峰的x=38.1143°。
而由（a+b）sinφ= （k+1/2）λ
得0级峰的φ=19.1773°，一级峰的φ=38.1143°所以误差为：
E1 =（19.1773°－19.9112°）/ 19.1773°=3.83%
E2 =（38.1143°－39.2579°）/ 38.1143°=3.00%

5、Problems
(1) How should the slit-width be chose n by the single-slit diffraction measurement?
Answer：The width that the slit-widthshould near the wave-length of microwave.
(2) By the single-slit diffraction, how will the position of the first-order maximum changes when the wavelength increases?
Answer：When the wavelength increases，the first-order maximum will close to the zero-order maximum.

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