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 - 1 -
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 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 - 2 -
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 -
3. Equipment 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 Caution - Don t look at the laser at the same height - 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 - 4 -
<Fig. 6> diffraction experiment <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 - 5 -
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 Science Workshop program. Setup rotary motion sensor at digital sensor window and light sensor at analogue sensor window. Be sure whether it works. (4) Set sample rate as 100Hz at rotary motion sensor setup. Put Division/Rotation as 1440. Choose Position at measure option. Put Rack at linear combination window. (5) Adjust light sensor gain switch as 10x or 100x as shown in Fig. 10. (6) Launch the time(x) vs Position(y) or Intensity(y) graph. (7) Use axis-value changeable function. When we put mouse on time(x)-axis and click left button, we can easily change value of x-axis. Choose Position. Then, we can get Intensity(y-axis) and Position(x-axis) in one graph. (8) Push start 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. - 6 -
<Fig. 11> Experiment data in single-slit <Fig. 12> Experiment data in double-slit - 7 -
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 single-slit 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? - 8 -
Experiment DATA SHEET (1) Single-slit experiment Single slit space b A B C 1 1 1 1 st maximum distance x (1 st dark spot) 2 2 2 3 3 3 4 4 4 5 5 5 6 6 6 Aver Aver Aver Distance between screen D Laser wavelength 632.8 nm Theoretical value Experimental Value Error (%) Diffraction pattern on single slit : Condition for creating dark pattern - 9 -
Experiment DATA SHEET (2) Double-slit experiment Single slit space b Distance between two slits d A B C D 1 1 1 1 2 2 2 2 1 st maximum distance x (1 st dark spot) 3 3 3 3 4 4 4 4 5 5 5 5 6 6 6 6 Ave Ave Ave Ave Distance to screen D Laser wavelength mm 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. d = nλ D x (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. - 10 -