Physics 4 Laboratory
Young’s Double-Slit Interference Experiment
Prelab Exercise
Please read the Procedure section and try to understand the physics involved and how the experimental procedure works. These questions should help you to be prepared for a productive
experience in lab. Complete this exercise and bring your responses to lab with you.
1. Young’s double-slit experiment was historically important because it provided convincing
evidence that light is a wave phenomenon. What physical principle of waves does Young’s
experiment exploit?
2. You will use both red and green lasers in performing the experiment. The wavelengths of the
light the lasers produce is unknown. Nevertheless, both lasers emit visible light, so you know
a range of wavelengths which must include them. What is that wavelength range (in nm)?
3. Knowing the colors of the two lasers, can you narrow down your estimate of the two laser
wavelengths? Calling the wavelengths λRed and λGreen , make a rough estimate for of each
wavelength (in nm).
4. You will make a slit pair using two razor blades in close contact. Clearly, the resulting slit
separation will be much less than, say, a centimeter. But it will also be much larger than,
say, a micron (micro-meter). Make a rough estimate about how wide your slit spacing will
be.
5. When you shine the red laser through your slit pair at a screen two meters away, what do
you expect to see? The beam diameter of the laser is roughly 2 mm. Draw a sketch of the
image you expect to see on the screen. Indicate the distance we are calling “fringe spacing.”
6. Using your estimated values for wavelength and slit spacing, what fringe spacing do you
expect to see? In answering this question, develop an equation for the fringe spacing.
7. Do you think you can measure this expected fringe spacing with just a ruler?
8. Since we can’t compute the slit spacing without the wavelength of the laser, and vice versa,
and we don’t know either of them, the procedure asks you to compute the laser wavelength
from your observations of the image pattern produced by putting a wire in the laser beam.
Explain how putting a single obstacle (like a wire) in the beam is similar to putting a slit
pair in the beam.
9. When you conduct the experiment, you will need to compute wavelength from fringe spacing
and wire diameter. What equation will you use?
10. When you conduct the experiment, you will need to compute slit spacing from wavelength
and fringe spacing. What equation will you use?
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Equipment
• Optical bench.
• Projection screen.
• Red laser pointer.
• Green laser pointer.
• Ruler.
• Laser mount.
• Slide mount.
• Glass microscope slides.
• Candle and matches.
• Razor blades and binder clip.
• Enameled wire.
• Dowel.
• Wire cutters.
• Cellophane tape.
Objective
The objective of this experiment is to investigate double-slit interference of light. You will exploit
the relationship among slit spacing, wavelength, and fringe spacing to measure the wavelength of
light starting with only a ruler as a standard of length. The investigation will also illustrate how a
well-characterized slit pair can be used to determine the wavelength of an unknown light source.
The procedure illustrates how averaging of multiple measurements can increase the precision of a
measurement.
Procedures
1. Prepare a slit pair.
• Collect a thin film of carbon black on part of a microscope slide by holding the slide in the
orange part of a candle flame. Above the flame, the combustion gases contain mostly H2 O
and CO2 , but inside the orange part of the flame, combustion is incomplete and there are
many carbon atoms available to precipitate onto the relatively cool glass of the slide. While
blackening the slide, keep it moving so as not to overheat it, and deposit just enough carbon
to make the slide opaque. You want as thin a film as you can get while making the slide
nearly opaque.
• Scribe a double slit in the carbon film with a pair of razor blades pinched tightly together.
Use the corners of the two blades and be sure that both corners are in contact with the glass
before making a quick stroke across the glass.
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• You may have to try this procedure several times before you get a clean slit pair. You can
make several slit pairs on the slide and then choose your best one. If you need to start over,
just wipe the soot off the slide with a paper towel.
2. Measure the double-slit interference pattern with a red laser.
• Place the red laser pointer and your slide into holders to steady them. Arrange the apparatus
so that the laser shines through the slits and onto a screen a few meters away. You should
be able to use the screen provided on your lab bench, but you might find it interesting to
project onto a distant wall.
• Measuring the spacing between a single pair of fringes will be inaccurate, but if you measure
the distance covered by many fringes and then divide by the number of fringes you can make a
more accurate measurement of the basic fringe spacing. By measuring the spacing of different
fringes many times and averaging, you can reduce your error even further. If you are really
ambitious, you could repeat the measurement for more than one screen range.
• Make a data table with columns for measured screen range, number of fringes counted, the
measured width of the counted fringe array and the computed fringe spacing. Be careful
about what you observe here. There are really two patterns of fringes on the screen: there’s
a single-slit pattern due to the width of you slits, and there’s a double-slit pattern due to the
separation of the slits. You want the double-slit pattern; it’s the smaller of the two. Check
your measured slit spacing against the estimate you prepared for the pre-lab to convince
yourself you are measuring the right thing.
At this point it should be clear that you do not have enough information to determine either the
wavelength of the laser or the slit spacing. To find the slit spacing, you need a laser of known
wavelength. The next few steps describe a procedure for calibrating the wavelength of the green
laser. You will then use the green laser to measure the slit separation. With a known slit separation
you can find the wavelength of the red laser.
The idea is to use a short piece of wire of known diameter to create an interference pattern similar
to the double-slit pattern. A wire placed across a laser beam in place of the slit pair will also
produce an interference pattern. Light passing around the two sides of the wire will interfere with
each other just as light passing around the two sides of the barrier between the two slits. But
superimposed on top of that double-slit pattern will be the main beam plus an interference pattern
due to light farther from the wire. The wire interference pattern may be somewhat more difficult
to interpret, but the primary fringe spacing will obey the same law as the double slit.
3. Measure the wire thickness.
• Wrap a wire around a piece of dowel until you have (at least) a centimeter or so of wrapping.
Take care to make the turns tight against each other. Count the number of turns.
• Divide the length of the wrapping by the number of turns to get the wire diameter.
• This step will take some care. The more closely spaced your turns and the more of them you
can wrap, the more accurate will be your result. It helps to tape the end of the wire to the
dowel before you start wrapping. Don’t lose count!
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4. Measure the wavelength of the green laser.
• With your green laser and a piece of wire, devise a setup similar to the one used to observe
double-slit interference with the red laser and slit pair.
• Use a similar procedure to measure the fringe spacing. Make several measurements with
different numbers of fringes, and average your results to derive a measurement of the green
laser wavelength.
• Record your data neatly in a suitable table.
5. Measure the slit spacing with the green laser.
• Set up your apparatus to make a double-slit pattern with the green laser and your slit pair.
• Make several appropriate measurements and average your results to derive a value for the slit
spacing.
• Record your data neatly in a suitable table.
6. Determine the wavelength of the red laser.
Use your original data and your measured value of slit width to determine the red laser wavelength.
You should compute it for each fringe spacing measurement and then average your results.
Final Report
For the homework portion of this lab exercise, prepare a brief report of your procedure and results.
This report is not intended to be a complex or long-winded document, but it should give you an
opportunity to organize your thoughts about the experiment and to present a coherent account of
it. Use the principles and typographical standards we’ve developed in the “evaluative experience”
to be direct and organized. Strive for the brevity that comes with being as direct as you can be.
Include what you need to in order to explain yourself completely—no more, no less. Here is an
outline of what you should include.
Title
Author
Although your lab work was collaborative, please write your own report. Indicate the names of
your lab partners.
Abstract
The abstract is a very brief, one paragraph précis of what you did and what your results were. It
should not be more than 100 words—just enough to let a reader know what the paper is about.
The only quantitative information should be the wavelengths of your two lasers, the wire diameter
and the slit width.
Introduction
This experiment was first conducted in 1801 and is considered one of the landmark experiments
in physics. Why is this experiment important? Why do we care enough to repeat it over 200
years later? Briefly explain the physics of interference from a double slit, developing the basic
mathematical relationship you exploit in the experiment. You don’t need to explain interference—
that’s background your reader should understand—but you should address all issues pertinent to
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this particular experiment. What is interfering with what in this case? How does the fringe spacing
depend on the wavelength and the slit spacing? Why?
You should also give an outline of the procedure and what it accomplished.
Apparatus and Procedure
In this section give a more complete account of the steps you followed and how they fit together.
With what you were trying to measure in mind, what did you do to conduct your study? How did
your procedure result in an answer? Include simple diagrams to help you in your explanation of
the theory. How and what did you measure during lab and how do these measurements allow you
to calculate your intermediate and final results? This section will be quite similar to the procedure
described in this handout, but instead of saying what you should do, say what you actually did.
(You don’t need to include descriptions of any blind alleys you explored, unless you think you have
discovered a technique that you want to share with the world.) Include tables of data recorded,
and any computed results. If you combine measured data and computed results in the same table
be sure to indicate clearly which is which. Be sure to point out your major results. For example,
after you describe how you determined the diameter of the wire, you can say what actual diameter
you found.
Analysis
Explain how your individual results relate to each other. Are all of your results consistent? Does
everything make sense and seem physically reasonable? Make a simple estimate of the accuracy of
your determination of your final results (the two laser wavelengths, the wire diameter, and the slit
separation).
When performing your error estimate, do not use the wavelength figures that may be printed on
the laser pointers. Estimate your error based on the value spread of your data. Computing the
standard deviation of your averaged data is a good approach.
If something went wrong, this is the place to analyze why. Sometimes experiments go badly. A
compelling analysis of failure will be worth as much as success as far as your homework score goes
(although poor execution could possibly damage your participation score).
Conclusion
Restate your results and summarize the most important aspects of the experiment. Think about
other things that this lab can do, discuss other possible applications of your procedure. Discuss
why this method is a powerful experimental tool. Feel free to add any other remarks you feel are
appropriate—but remember that brevity and directness are paramount.
References
Cite any sources you may have used in preparing your report.
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