Lab Assignment 9: Interference
InstructoràOverview
Electromagnetic radiation such as visible light exhibits behavior that is particlelike in some physical situations and wave-like in others. This phenomenon is
known as wave-particle duality. For example, in the photoelectric effect, which
we will be examining this term, light can be modeled as quantized !rticles«nown as photons. If a photon exceeds a certain energy level known as the work
function, it can eject an electron from a metal surface. This lab examines the
wave nature of light which is manifested in interference and diffraction.
This lab is based on Lab 25 of your eScience Lab kit.
Our lab consists of two components. Here is a high-level view:
n Experiment 1, you will replicate the famous Youngàdouble slit
experiment using some foil and a laser pointer.
n Experiment 2, you will continue to explore interference by executing a
single slit experiment. Based on the resulting diffraction pattern, you will
calculate the width of the aperture.
Take detailed notes as you perform the experiment and fill out the sections
below. This document serves as your lab report. Please include detailed
descriptions of your experimental methods and observations.
What you, need for this lab activity
From the eScience kit
luminum foil
lothes pin
aser pointer

irror support
ush pin
uler
ape measure
You provide
ook(s) for
mechanical support
tility knife
(optional)
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Physics II
Experiment Tips/Comments:
‰nstead of using the push pin to create your slits, I suggest using an X-Acto)ke blade. I used a utility blade as pictured below and found that the blade
approach made smoother slits. The push pin tended to tear the thin
aluminum foil. The straight edge of the protractor was helpful to insure
consistently straight slits.
n assistant is very helpful for this lab activity. I set up the foil system as
follows:
”he clothes pin, held in the mirror support, holds the foil. I placed a mass on
the mirror support to insure that it stays upright and stable.
”he distance from the foil to the wall was ~2.3 meters in my case.
—hile I directed the laser beam through the slit(s), my assistant measured the
various dimensions of the resulting diffraction pattern that was projected on
the target wall.
…xercise case when using the laser. Do not stare directly into the beam and
be aware of reflections from the foil.
”ake your time when creating the slits. This step is crucial to the success of
the experiment. Like all experiments, careful preparation is critical. The
resulting diffraction patterns are worth the time spent on the slit preparation!
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Physics II
Date:
Student:
Abstract
Introduction
Material and Methods
Experiment 1: YoungàDouble Slit Experiment
Results/Observations
Double Slit Experiment Data
Wavelength (nm)
Distance from foil to wall (m)
Number of bright spots
Distance between first and last bright spot (m)
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Physics II
Experiment 2: Single Slit Diffraction
Results/Observations
Single Slit Experiment Data
Width of center peak (cm)
Width of slit for a single peak
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Physics II
Experiment 1: YoungàDouble Slit Experiment nalysis and Discussion
Based on your experimental results, please answer the following
questions:
1. If light hits two slits, what pattern would you expect to see on the wall if you
model light as particles? What pattern would you expect to see if light
behaves as a wave? Explain your answers.
2. As long as the distance to the wall is much greater than the separation of the
slits, the spacing between the bright spots is uniform. Using data from the
Double Slit Experiment Data table, compute the distance between two bright
spots.
3. How close together were you able to make your slits? In other words,
calculate the quantity d in the relation ?y = ?L/d . Show all of your work.
4. If you were to use a green light instead of a red light, how would that change
the pattern?
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Physics II
5. The same effect is not observed when white light from a light bulb is used.
Explain why white light doesn produce a fringe pattern.
6. A diffraction grating is like a double slit except it contains a large number of
equally parallel slits. What would you predict the differences would be in the
pattern seen on the wall?
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Experiment 2: Single Slit Diffraction nalysis and Discussion
Based on your experimental results, please answer the following
questions:
1. What happened to the diffraction pattern as you increased the width of the
slit?
2. If you shined the light through a door, would you observe a diffraction
pattern? Explain your reasoning with physical detail.
3. Use the results of this experiment to explain why you can hear, but not see
someone who is standing out-side the door if you are standing right around
the corner of the doorway.
4. The width of the center peak (w) can be calculated using the following
equation:
2?L
, where R is the width of the slit, ? is the illumination wavelength,
a
and L is the distance from the foil to the viewing surface.
w=
Use this equation to determine the width of the slit that caused the diffraction
pattern. Show your work.
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Physics II
Conclusions
References
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