General Physics ExperimentⅡ, Department of Physics, Sogang University
http://hompi.sogang.ac.kr/physlab/
Experiment 10. Diffraction and interference of light
1. Purpose
Perform single slit and Young’s double slit experiment by using Laser and computer interface in order to understand
diffraction and interference of light’s wave characteristics.
2. Principle
In 1801, Tomas Young had discovered interference effect which exhibits light’s wave characteristics. When two waves
make superposition, there are maximum energy point and minimum energy point. This phenomenon is called as
“Interference”. In order to occur interference, two waves must have same velocity, frequency, wavelength, and consistent
relative phase.
Let’s assume that rays from single light source passes through closed two slits. If the ray is composed of small particle,
screen which is behind slits will show two bright lines. Young observed, However, several bright lines. It is explained by
the interference of light. Because of diffraction, wave spread out from the edge of slit by passing through two small slits.
For this reason, the interference pattern, which is formed space, is understood like interference pattern in well by
throwing two small stone.
If the distance between two slits is precisely smaller than the distance between slit and screen, ray that passes from two
slit to screen could be assumed as parallel. The waves from two identical sources interfere constructively at points where
the path-length difference is an integer number of wavelengths. The waves interfere destructively where the path-length
difference is a half-integer number of wavelengths. From this experiment, we will understand the concept of quantum
mechanics by doing double-slit experiment while light is analyzed by classical mechanics.
1) Single slit experiment
<Fig. 1> interference pattern in single slit experiment
The diffraction is defined as when light spreads out in circular arcs after passing through a hole in a barrier. All waves
spread out all directions explaining by Huygen’s principle. As shown in Fig. 1 (a), light passing through straight is all in
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General Physics ExperimentⅡ, Department of Physics, Sogang University
http://hompi.sogang.ac.kr/physlab/
same phase. It will make light point at the center of screen. In Fig. 1 (b), two lights which are at the top and bottom make
one wavelength difference.
<Fig. 2> strength of diffraction patterns.
2) Young’s double-slit experiment
As shown in Fig. 3, we will think that light from two slit point S1 and S2 interfere. The light passing through two slits
diffracts and progress with overlapping. If the incident light is monochromatic light, this two waves make interference
pattern when it reaches in screen.
<
Fig. 3> Young’s double slit experiment
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General Physics ExperimentⅡ, Department of Physics, Sogang University
http://hompi.sogang.ac.kr/physlab/
Mathematically, it is expressed as
similar to
In double-slit, we can calculate the distance between two slits, b, by checking number of patterns.
We may reference classical mechanics text book in order to see more detail proving process as follow.
3. Equipment
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General Physics ExperimentⅡ, Department of Physics, Sogang University
http://hompi.sogang.ac.kr/physlab/
Equipment
Number
Laser Diode
1
Lens Holder
2
Power Supply (9V DC)
1
Optical Branch
1
Slit Disk
1
Light Sensor
1
Aperture Bracket
1
Rotary Motion Sensor
1
Linear Translator
1
Aperture Bracket Screen
1
4. Experiment
- Don’t look at the laser at the same height
Caution
- Turn on Laser when you do experiment
- Move Light sensor slowly in order to get exact data
1. Equipment and Set up
<Fig. 4> Rotary sensor setup
<Fig. 5> Light Sensor setup
(1) Set up linear conversion bar as shown in Fig. 4.
(2) Put Light Sensor onto Aperture Bracket and fix by using screw and connect 8pin connector. (3) Fix linear
conversion bar at experiment supporter
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General Physics ExperimentⅡ, Department of Physics, Sogang University
<Fig. 6> diffraction experiment
http://hompi.sogang.ac.kr/physlab/
<Fig. 7> Aperture diffraction pattern
(4) Set up diffraction experiment as shown in Fig. 6.
(5) Put the distance above 70cm between diffraction plate and aperture circular plate.
(6) Turn on laser and choose No. 1 on aperture circular plate. Adjust labjack in order to put light at the center of
aperture hole without diffraction plate.
(7) Put diffraction plate on support fixture and adjust diffraction plate location to put diffraction pattern horizontally
at the center of aperture plate. (We can control the height of light by adjusting at the back of laser diode’s screw.
It effects the result of experiment. Make sure that we choose aperture hole number by checking experiment
results from 1 to 6.)
(8) Measure distance (D) between diffraction plate and aperture circular plate.
(9) Make your experiment room as darkroom for your better experiment conditions.
<Fig. 8> Total setup of Experiment
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General Physics ExperimentⅡ, Department of Physics, Sogang University
http://hompi.sogang.ac.kr/physlab/
2. Data-Studio program Setting
(1) Connect interface box to computer and turn on computer
<Fig. 9> connection of interface box and light sensor
<Fig. 10> Light Sensor
(2) Connect Rotary Motion Sensor and Light Sensor as shown in Fig. 9.
(3) Start PASCO Capstone program. Setup with Hardware Setup menu on the left tab.
(4) Set sample rate as 100Hz at rotary motion sensor setup. Put Resolution as 1440. For Linear combination, set as
Rack&Pinion.
(5) Adjust light sensor gain switch(이득 스위치) as 10x or 100x as shown in Fig. 10.
(6) Launch the time(x) vs Position(y) and vs Intensity(y) graphs.
(7) You can change the measurement by clicking the value on x-axis.
(8) Push Record button and start saving data. Scan diffraction pattern from left to right with carefully moving linear
conversion bar and light sensor.
(9) When scan is done, push stop button and save date.
(10) As shown in below Figure, perform experiments at single-slit and double-slit, respectively. Complete date sheet.
Record the distance from the brightest point to first appeared dark point in the case of single-slit experiment.
(11) Measure the distance to minimize point referring to distance versus intensity table. Record each case of minimize
point. It can easily be measured by using smart curser buttom.
<Fig. 11> Experiment data in single-slit
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General Physics ExperimentⅡ, Department of Physics, Sogang University
http://hompi.sogang.ac.kr/physlab/
<Fig. 12> Experiment data in double-slit
5. Quesions
(1) What is Laser? Laser is summarized by having 4 characteristics. By considering this point, you may think what
characteristic is related to this experiment.
(2) How the diffraction pattern occurs when slit width is small or distance between slit and screen is narrow at singleslit experiment?
(3) In double-slit experiment, how the diffraction pattern occurs when slit width(b) and distance between two slits(d)
are integer of half-integer?
(4) What’s the difference between interference pattern and diffraction pattern?
(5) Let’s think the ration b of d. Does the number of fine pattern increase or decrease when d/b is bigger?
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General Physics ExperimentⅡ, Department of Physics, Sogang University
http://hompi.sogang.ac.kr/physlab/
Experiment DATA SHEET
(1) Single-slit experiment
Single slit space b
A
B
C
1
1
1
2
2
2
3
3
3
4
4
4
5
5
5
6
6
6
Aver
Aver
Aver
1st maximum distance x
(1st dark spot)
Distance between screen D
Laser wavelength
632.8 nm
Theoretical value (sinθ)
Experimental Value (sinθ)
Error (%)
Diffraction pattern on single slit : Condition for creating dark pattern
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General Physics ExperimentⅡ, Department of Physics, Sogang University
http://hompi.sogang.ac.kr/physlab/
Experiment DATA SHEET
(2) Double-slit experiment
Single slit space b
A
B
C
D
1
1
1
1
2
2
2
2
3
3
3
3
4
4
4
4
5
5
5
5
6
6
6
6
Ave
Ave
Ave
Ave
Distance between two slits d
1st maximum distance x
(1st dark spot)
Distance to screen D
mm
Laser wavelength
632.8 nm
Number of fine pattern (2)
Distance between two slits d
(1)
Theoretical value b
Experimental value b
Error (%)
(1) Calculate distance between two slits by measuring interference patterns
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General Physics ExperimentⅡ, Department of Physics, Sogang University
http://hompi.sogang.ac.kr/physlab/
(2) Calculate ration d of b by measuring number of fine patterns within diffraction pattern
(3) By using result (2), calculate b. Make sure that b and d is integer of half-integer when you calculate number of
fine patterns within total pattern.
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