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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