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Modern Physics: Lab 6-C Name _________________________ Double-Slit Interference Hour _______ Purpose: Observe characteristics of double slit interference patterns. Determine the equation for bright fringes in double-slit interference on a screen. Equipment: Computer with internet access Transparency wave patterns Overhead markers Ruler Laser Diffraction Grating of known slit separation Preparation: In 1801 Thomas Young showed that when light was separated into two separate beams, it would create an interference pattern. This experiment was crucial in proving that light was a wave. The diagram at right shows two light rays passing through two slits. Each light ray spreads out in a circular pattern, with the dark circles representing crests in the light ray and the white circles representing troughs in the light ray. Any point where two crests (or two troughs) overlap creates constructive interference and produces brighter light. Any point where a crest overlaps with a trough creates destructive interference and produces darkness. Part 1: Two-Dimensional Patterns formed by Interfering Waves In this part of the lab, you will create two-dimensional interference patterns and use them to predict what light interference will look like when viewed on a screen. 1. Obtain two sets of semicircles printed on clear transparency paper. The dark solid circles represent crests in the light ray and the lighter, dotted circles represent troughs in the light ray. Carefully position the two sets of circles together so that their centers are approximately 3 cm apart. Be sure that the edges of the semicircle patterns form a straight line, as shown in the diagram at left. Then use a small amount of tape to fasten the two transparencies together. The centers of the two patterns now represent two light sources in a double-slit interference pattern. 2. Use a transparency marker to mark all of points where two crests in the wave pattern are aligned. (NOTE: Your circle pattern will have many more points than those marked in the diagram above.) a) What type of interference (constructive or destructive) would occur at these points? b) What type of light (bright or dim) would be seen at these points? Modern & Contemporary Physics Chapter 6 ~ B Newitt 3. When you have marked all of the points where crests in the wave pattern align, look for rows of constructive interference points. Connect each row of constructive interference points with a line, as shown in the diagram at left. These lines show the locations where crests in the two wave patterns will overlap, producing bright spots of light. 4. To see what an interference pattern will look like when viewed on a wall or “screen,” place a piece of scratch paper across the semicircle transparencies as shown on the diagram at left. First, position the paper reasonably close (approximately 1-2 cm) to the centers of the two semicircle patterns. Adjust the paper until its edge seems to be parallel to the edge of the two semicircle pattern. Note that it is not necessary for the edge of the paper to pass through any of the constructive interference dots. 5. Measure the distance between each pair of lines. Record the values along “Line A” on the diagram below. 6. Move the paper to a medium distance away from the centers of the two semicircle patterns and repeat Steps #4 and #5. Record the values for the distances between each pair of lines along “Line B” on the diagram below. 7. Move the paper to a greater distance away from the centers of the two semicircle patterns and repeat Steps #4 and #5. Record the values for the distances between each pair of lines along “Line C” on the diagram below. Line C Line B Line A 8. Along what line are the constructive interference spaced closest together? 9. Along what line are the constructive interference spread farthest apart? Modern & Contemporary Physics Chapter 6 ~ B Newitt Part 2: Creating an Equation for the Distance Between Fringes In this part of the lab, you will determine the equation used to calculate the distance between bright fringes for a double-slit interference pattern. 10. Open the web site, located at http://tutor-homework.com/Physics_Help/double_slit_experiment.html. 11. The distance between the bright fringes of light is abbreviated with the variable x. On the diagram above, label the arrow showing the distance between the fringes with the variable x. 12. One of the factors that determines the distance between bright fringes is the wavelength (λ) of the light. What is the wavelength of the light used and the color of that light when the program first begins? (Note: Be sure to record the label correctly!) 13. To determine the effect of the wavelength on the distance between the bright fringes, increase and decrease the wavelength of the light and observe the effect on interference pattern. Record your observations in the space below. 14. How does the color of the light change . . . when the wavelength is increased? . . . when the wavelength is decreased? Modern & Contemporary Physics Chapter 6 ~ B Newitt 15. The distance between the two slits is called the slit separation and is abbreviated with the variable d. On the diagram on the previous page, label the arrow showing the slit separation with the variable d. 16. To determine the effect of the slit separation on the distance between the bright fringes, increase and decrease the slit separation and observe the effect on interference pattern. Record your observations in the space below. 17. The distance between the two slits and the screen is called the length and is abbreviated with the variable L. On the diagram on the previous page, label the arrow showing this length with the variable L. 18. To determine the effect of the length on the distance between the bright fringes, increase and decrease the length (labeled as “Distance to the Screen” in the applet) and observe the effect on interference pattern. Record your observations in the space below. 19. What factor(s) are directly related to the distance between fringes? 20. What factor(s) are inversely related to the distance between fringes? 21. The basic form of the equation for the distance between bright fringes in a double slit interference pattern is shown below. Complete each space in the equation with the following quantities: Wavelength (λ) Slit Separation (d) Length to Screen (L) Modern & Contemporary Physics x x = Chapter 6 ~ B Newitt Part 3: Testing the Equation with a Laser In this part of the lab, you will test your equation by using it to calculate the wavelength of a laser light shining through small slits. The directions for this section of the lab will need to be followed exactly in order to prevent injury or damage to the equipment. Be sure to ask your teacher if you have any questions! 22. Once you have successfully derived the interference equation, you will use it to determine the wavelength of a laser. In this situation, the laser will actually shine through a diffraction grating, which provides many slits instead of just two. This will produce a pattern that has a much greater proportion of dark destructive interference than bright constructive interference. However, the equation for the distance between the bright fringes of light will still apply. 23. Be sure that the diffraction grating has been attached to the slit holder so that the laser can shine through the 100 lines/mm square. The slit separation for the 100 lines/mm segment is 1.0 x 10-5 m. 24. Without turning the laser on, position it so that it is able to shine through the diffraction grating on to the screen. If necessary, increase the height of the laser and diffraction grating by setting them on a pile of books. 25. Be sure that the sliding cover on the front of the laser is open and that everyone is clear of the path of the laser beam. Then turn on the laser and observe the interference pattern on the screen. Measure any quantities that are needed to calculate the wavelength of the laser light. (NOTE: Be sure not to move the laser while you are measuring the interference pattern.) Then calculate the wavelength of the laser light. Record your data and show your calculations in the space below 26. Do you think your answer for wavelength is an appropriate value, considering the color of the light? 27. How do you think the interference pattern on the screen would change if a green laser was used instead or red? Justify your answer. (NOTE: If time allows, ask your teacher to show you the interference pattern formed by a green or violet laser.) Modern & Contemporary Physics Chapter 6 ~ B Newitt