P31220 Lab
Optical Phenomena
Purpose: Students will learn about interference, diffraction, polarization, and real vs. virtual
images. Students will have opportunities to see various other optical phenomena that are difficult
to observe in a lecture situation.
Introduction:
There’s nothing like seeing it with your own eyes. In this lab, you will have a chance to
experiment with a variety of different optical phenomena. Each table in the lab is set up with a
different experiment. Your lab group will spend a few minutes observing and trying to
understand each phenomenon. By the end of lab, you’ll have learned many interesting things!
Polarization:
If you shake your end of a jump rope, you can wiggle the rope in the horizontal plane, the
vertical plane, or in any direction in between. Polarization refers to the plane of vibration of the
light wave. Most light sources, such as an incandescent light bulb, a candle flame, or the Sun
produce non-polarized light. Light can become polarized if it bounces off of a surface at a
glancing angle or passes through an object that restricts the direction of vibration. Some
chemicals, plastics, and crystals can do this, and some even cause the direction of polarization to
rotate as the light passes through. The polarizations of different colors rotate by different
amounts. You will observe this in the lab.
Fig. 1: Passing light through a polarizer is sort of like shaking a rope through a picket fence. The
polarizer restricts the direction of vibration. If the polarizers are crossed, the light can’t get
through.
Real and Virtual Images
In his book “Alice Through the Looking Glass”, Lewis Carroll described how Alice crawled
through the mirror over the fireplace into the room on the other side of the mirror. Of course,
there’s nothing but the chimney on the other side. The room “behind the mirror” is an example of
a virtual image. You can see it, even though the light never actually goes there. The counterpart
of a virtual image is a real image. Light actually does go to the location of the real image, and an
image must be real before it can be projected onto a screen. You probably have more experience
with viewing virtual images than with real images. You will observe both in this lab.
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P31220 Lab
Wave Interference and Diffraction
When the peak of one wave overlaps the peak of another wave, the peaks add together to make a
bigger peak. This is called “constructive interference.” If the peak of a wave coincides with the
trough of a second wave, the waves cancel. This is called “destructive interference.” Destructive
interference is used in noise cancellation headphones, which are worn by people who must work
around loud machinery. Noise cancellation is also used to keep airplane cabins quiet even though
some passengers are sitting ten feet from a screaming jet engine. The Dutch physicist Christiaan
Huygens theorized that each point of a wave acts as a source for generating additional waves.
The waves that you see are a result of constructive and destructive interference among an infinite
number of these wavelets. You may wish to look up “Huygens wavelets” and “noise
cancellation” later.
Diffraction occurs when a single wave spreads out after striking a small barrier or passing
through a small slit. This is why you can’t see tiny bacteria and viruses with an optical
microscope. The size of these critters is comparable to a wavelength of light, so considerable
diffraction occurs. The wavelets passing through a slit (or around a critter) enhance and cancel
each other in specific directions depending on the wavelength and slit width. The basic
geometry for single-slit diffraction is given in Fig. 2.
θ
Slit Width d è
è
Distance between slit and screen L
Path Difference
Screen
Fig. 2: Geometry for diffraction and interference. Because the two black rays connecting the
edges of the slit and the screen are very nearly parallel, the two angles θ are considered to be
equal. This is a very good approximation. The slight path difference is what causes constructive
and destructive interference at particular values of θ.
n=2
n=1
n=0
n=1
n=2
Fig. 3: Diffraction pattern for single-slit diffraction. The quantity n is the “order” of diffraction.
If you shine a laser through a single slit (and you will, in this lab), you’ll see a pattern similar to
that in Fig. 3. The light is brightest where constructive interference occurs, and dim where
destructive interference occurs. The mathematical condition for diffraction is
Equation 1
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P31220 Lab
where d is the slit width and
, where x is the distance between the center of the pattern and
the dark spot in the pattern, and L is the distance between the slit and the screen. These distances
are easily measured and can be used to determine either the wavelength of the laser or the width
of the tiny slit.
In this lab we will use “diffraction” when referring to single-slit patterns and “interference” when
referring to multiple-slit patterns. Equation 1 and Figure 2 work for both, except that a few
quantities need to be redefined as follows:
n
d
Single Slit
Diffraction
Measured from center (n=0) to
pattern minima (dark areas)
Double Slit
Interference
Measured from center (n=0) to pattern
maxima (bright areas)
Slit width
Spacing between slits
n = 10
n=7
n=3
n=2
n=1
n=0
n=1
n=2
n=3
n=7
n = 10
The double-slit pattern consists of a series of equally spaced fringes. In actual practice, the slit
openings in a double-slit pair each have some width. If you shine a light through an actual double
slit, you will see the evenly spaced double-slit pattern superimposed on top of the single-slit
diffraction pattern, as shown in Fig. 4. You will observe and measure both patterns in this lab.
Fig. 4: Double-slit patterns for three different wavelengths of light. The regularly spaced dark
stripes are the double-slit pattern, which is superimposed on the single-slit pattern. Compare this
to Fig. 2. The n=7 fringes in the double-slit pattern are missing from the red pattern because they
are superimposed on the n=1 minima from the single-slit pattern.
About the Cornell Slitfilm, which is shown on the next page: The Slitfilm was developed in the
1950’s. It contains several combinations of slits for systematic studies. The slits are sandwiched
between two plates of glass. You’ll shine a laser through the slitfilm at Tables 1 and 2. You’ll
put it up to your eye and look through it at Tables 3 and 4. The key to the “slitfilm” is given in
Fig. 5 on the next page.
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P31220 Lab
Fig. 5: Cornell slitfilm slide, showing slit configurations and widths.
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P31220 Lab
What to do in this lab:





The instructions are on the Data Sheets. Perform the experiments and answer the Analysis
Questions.
You may visit the experiment tables in any order. There are six different types of lab
stations.
Please budget your time! You shouldn’t need more than 15 minutes at any lab station,
with the possible exception of Law of Malus.
When you are finished with a set of apparatus, please vacate the table immediately if another
group needs to use it. Don’t be shy about asking to use a table if a group appears to be done
with it!
Don’t forget to visit the Displays! You can do this at any time during the lab.
Safety Warning:
Do not let the beam of laser light or its mirror
reflection enter your eye!
White board at front of room
Instructor
Table 1
Table 2
Table 3
Sl
itf
il
m
la
m
ps
Table 6
Table 7
Table 5
Table 11
Table 4
Table 8
Table 10
Table 9
Table 12
Displays
Fig. 6: Room layout
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P31220 Lab
Name: ________________________________
Lab Section: ________________ T.A.’s ____________________________________________
Today’s Lab Partners: ___________________________________________________________
Data Sheet and Analysis Questions
Table 1: Single-Slit Diffraction with a Laser
A laser is shining through a single slit in the slitfilm. The slit width can be found on the key on
Fig. 5 of this handout. The diffraction pattern is on the white board at the front of the room.
Your job: Use Equation 1 to measure the wavelength of the laser. Sketch the diffraction pattern
and show your work below.
Hint: You’ll find it helpful to mark the positions of the minima using a whiteboard
marker.
CAUTION: Don’t stare into the laser beam or allow it to strike your eye. Beware
of reflected beams as well as direct beams.
Wavelength of HeNe laser:
___________________ nm
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P31220 Lab
Name: ________________________________
Lab Section: ________________ T.A.’s ____________________________________________
Today’s Lab Partners: ___________________________________________________________
Table 2: Double-Slit Diffraction with a Laser
A laser is shining through a double slit in the slitfilm. The slit width and spacing can be found on
the key in Fig. 5 of this handout. The diffraction pattern is on the white board at the front of the
room.
Your job: Use Equation 1 to measure the wavelength of the laser. Sketch the interference
pattern and show your work below.
Hint: You’ll find it helpful to mark the positions of the interference fringes using a
whiteboard marker.
CAUTION: Don’t stare into the laser beam or allow it to strike your eye.
Wavelength of HeNe laser:
___________________ nm
1. Compare your result from Table 1 with your result from Table 2. Do the two wavelengths
agree? Explain.
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P31220 Lab
Name: ________________________________
Lab Section: ________________ T.A.’s ____________________________________________
Today’s Lab Partners: ___________________________________________________________
Tables 3 and 4: Observations with the Cornell Slitfilm
To use the slitfilm, orient the glass plate so that it looks like Fig. 6 when you look at it. Hold the
slitfilm close to your eye and look at one of the special lamps through the slits. The lamps are
located on a high shelf at the side of the lab. Both lamps are straight-filament aquarium bulbs.
One lamp produces white light. The other is covered with red and blue filters so that you can see
the effects due to changing the wavelength. Red light has a longer wavelength than blue light.
Single slits: Use the left-most column of slits, as seen in Fig. 5. Move up and down the column
to change the slit width.
2. How is the diffraction pattern affected by changing the width of a single slit?
3. Which slit gives you the most spread-out diffraction pattern? ________________________
4. Based on what you see, how much does the size of the diffraction pattern change when the
slit width is doubled?
Double slits: Use the right-most column of slits, as seen in Fig. 5. These are double slits with
constant slit width. Move up and down to change the spacing between slits.
5. How does changing the spacing affect the interference pattern?
6. Compare the patterns from the red and blue lights. How does changing the wavelength of
light affect the interference pattern?
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P31220 Lab
Name: ________________________________
Lab Section: ________________ T.A.’s ____________________________________________
Today’s Lab Partners: ___________________________________________________________
7. Is your answer to question 6 consistent with what you see when you observe white light
through the slits?
Multiple slits: Use the fourth column from the left, as seen in Fig. 5. The slit spacing and width
are constant, but the number of slits varies.
8. Describe what happens to the pattern as the number of slits increases.
9. Try to discern a relationship between the number of slits and the number of dim maxima
between the bright maxima. What did you come up with?
Diffraction Gratings: Diffraction gratings are made with many parallel slits. Use the third
(middle) column of the slitfilm. Pay particular attention to the grating in the middle of the
slitfilm, which has the most slits.
10. Observe the red and blue lamp with the middle grating. Describe what you see.
11. How is the interference pattern affected by the wavelength of light?
12. Diffraction gratings are commonly used to spread out the different colors of light. Observe
the white light with the middle grating. Compare the order of the colors on the left and right
sides of the grating. What do you see?
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P31220 Lab
Name: ________________________________
Lab Section: ________________ T.A.’s ____________________________________________
Today’s Lab Partners: ___________________________________________________________
Tables 5 and 6: Observations of Polarization Phenomena
You have three pieces of polarizing film, a desk lamp, and some transparent objects. Observe the
phenomena and describe what you see.
CAUTION: Handle the polarizers by their edges. Try not to get fingerprints on them.
13. Put two of the polarizers together. View the desk lamp through the polarizers. Rotate one of
the polarizers back and forth. What happens?
14. Arrange two of the polarizers so that they block all of the light from the lamp. Insert a third
polarizer between the first two polarizers. Rotate the third polarizer. What happens?
15. Try to explain what you saw happen when you inserted the third polarizer. Discuss it in your
group and do your best to explain it on your own. Don’t worry about having a correct
explanation at this time. The point here is for YOU to make a guess. Check yourself by
looking up the correct explanation later.
16. Many polymers can rotate the direction of polarization by an amount that depends on the
wavelength of light, the thickness of the polymer, and the amount of internal stress in the
polymer. Arrange the two polarizers so that they block either the maximum or minimum
amount of light. Observe and describe what happens when you insert the sample of mica and
the sample of cellophane between the polarizers. What do you see?
17. Do your computer, calculator, or cell phone screens use polarized light? How can you tell?
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P31220 Lab
Name: ________________________________
Lab Section: ________________ T.A.’s ____________________________________________
Today’s Lab Partners: ___________________________________________________________
18. Stress test: You have a plastic object, as shown in Fig. 7. You also have a clear plastic fork.
Place these between the crossed polarizers. You should see the colors and patterns change
when you squeeze the object or gently press on a tine of the fork. Engineers sometimes make
plastic models to figure out forces and stresses inside of the things they are designing. Did
you see the effect?
Squeeze Here
Squeeze Here
Fig. 7: Object for viewing internal stress with polarizers.
19. Calcite Crystal: Calcite is a very common mineral with some rather uncommon properties.
Put the calcite crystal down on top of the dot in Fig. 8 below. Look at the dot through the
crystal. Rotate the crystal on the paper. What do you see?
CALCITE
Fig. 8: Put the calcite crystal on the paper.
20. Calcite Crystal: Put the calcite crystal on top of the dot in Fig. 8 above. Use one of your
polarizers to figure out whether the light coming through the calcite crystal is polarized.
What did you determine?
21. Use a polarizer to check whether your computer, calculator, or cell phone screens use
polarized light. If you have sunglasses with you, check whether the two lenses are polarized
in the same direction. (They’re not always made properly.) What did you find out?
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P31220 Lab
Name: ________________________________
Lab Section: ________________ T.A.’s ____________________________________________
Today’s Lab Partners: ___________________________________________________________
Law Of Malus Data
Angle
(degrees)
Voltage
Reading
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P31220 Lab
Name: ________________________________
Lab Section: ________________ T.A.’s ____________________________________________
Today’s Lab Partners: ___________________________________________________________
Tables 7, 8, and 9: Law of Malus
Polarizers
Solar Cell
Lamp
Rubber Band
Fig. 9: Apparatus for Law of Malus. Please don’t cook the polarizers with the lamp!
If you put two polarizers together and rotate one of them, the intensity of the light will be
described by the equation
Equation 2
where I/Imax is the ratio between the measured intensity of light and its maximum intensity, and θ
is the angle between the two polarizers.

Set up the apparatus as shown in Fig. 9. Make sure that the light is not in contact with the
polarizers, as they will overheat.

Connect a DMM to the solar cell so that it is measuring voltage.

Measure voltage vs. polarizer angle. Move only one polarizer. The starting angle doesn’t
matter, but you will want to make sure that you cover all 360 degrees of rotation. Be careful
not to bump the lamp while you are taking your data.

Make a calculated column in Graphical Analysis to divide all of your data points by your
maximum voltage. Graph V/Vmax vs. θ. Plot data points ONLY. Remove the connecting
lines. Add error bars. The error in your angle measurement is probably less than 3 degrees,
or better than 1% of 360 degrees. Therefore you may use the 2% error in the DMM reading
as your error.

Now that you have your data points on the graph, you will next plot the theoretical curve.
Create a calculated column containing the formula (cos(("X" + 0)*pi/180))^2+0. This is
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P31220 Lab
Name: ________________________________
Lab Section: ________________ T.A.’s ____________________________________________
Today’s Lab Partners: ___________________________________________________________
equivalent to the equation
where θ is the measured polarizer
angle. The parameters φ and yo are numbers that you may need to adjust to match the theory
curve with your data. They are set to zero to start with. You can change them by editing the
“column definition” in Graphical Analysis. The angle φ adjusts for the fact that the initial
angle between your two polarizers probably wasn’t zero. The parameter yo moves the theory
curve up and down. It should be zero but may need adjusting if there is much stray
background light in the room. The equation includes a units conversion from degrees to
radians because Graphical Analysis assumes that angles are given in radians.

Once you have the best fit you can get between theory and your data, print your graph and
turn it in with your lab. Before printing:
o Make sure that the data POINTS are shown, and the theory CURVE (connect dots) is
shown. Do not show the “data points” for the theoretical curve.
o Make sure that the colors in the graph are dark before printing.
22. How well were you able to observe the expected relationship between intensity (voltage) and
polarizer angle?
Tables 10, 11, and 12: Real and Virtual Images
Virtual image in a plane mirror:
23. Look at yourself in the mirror. Where does your image appear to be located: on the mirror,
in front of the mirror, or behind the mirror?
Now test this idea by doing an experiment to measure the exact location of the image.

Set up a mirror and a nail and piece of paper as in Fig. 10. Be sure to CAREFULLY mark
the exact positions of the back (reflecting) surface of the mirror and of the nail in front of the
mirror.
OPTIONAL: If you do this experiment on a cork board, you can use pins instead of sighting
along the ruler.
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P31220 Lab
Name: ________________________________
Lab Section: ________________ T.A.’s ____________________________________________
Today’s Lab Partners: ___________________________________________________________
Put the nail at the
location of the image
This line records
the position of the
mirror. Put the
BACK surface of the
mirror on the line.
Sight the image
along the top of
the ruler
Draw the line
pointing to the image
This line points
to the image
Circle the nail
to record its
position
Fig. 10: Geometry for finding the image created by a plane mirror

Place your head down so that you are at eye level with the mirror. Close one eye and sight
along the top edge of the ruler. Point the ruler towards the center of the image of the nail.
(Alternatively, stick pins in a cork board.) Mark the position of the ruler (or pins) on the
paper.

Repeat this on the other side of the nail so that you have at least two lines pointing towards
the center of the image of the nail.

Remove the mirror and the nail. Use the ruler to carefully extend the two ruler lines until
they cross. The point where the lines cross will be the location of the image. Note that the
lines point to the center of the front outside surface of the nail, not the interior center of the
nail!

Take the ruler and draw a line connecting the position of the nail with the position of its
image.
MEASURE the following distances:

Distance between the nail and the mirror line: ___________________
Distance between the image of the nail and the mirror line: __________________
These two distances should be nearly the same. If they are not, check your work. You must
measure carefully!
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P31220 Lab
Name: ________________________________
Lab Section: ________________ T.A.’s ____________________________________________
Today’s Lab Partners: ___________________________________________________________

Now replace the nail and the mirror in their original positions. Put a second nail at the
position of the image of the first nail which you found by drawing the lines. If everything is
perfectly correct, you'll see a whole nail behind the mirror! The top of the back nail will line
up perfectly with the image of the front nail and will stay that way no matter where you are
when you look into the mirror. You may have to move the back nail slightly to get this to
work. TRY IT!
image
A
B
C
D
nail
Fig. 11: Position of the image from a plane mirror

Remove the mirror and nails from the paper. Draw the rays showing how light goes from the
nail to where your eye was located when you sighted along the rulers. Measure angles A, B,
C, and D.
Angle A: ______ Angle B: ______
Angle C: _____
Angle D: _____
24. What general principle did you discover about these angles and the position of the image?
Real Images:
You have a “Mirascope,” shown in Fig. 11. This is a device consisting of two parabolic mirrors.
The mirrors produce a real image of any object placed on the center of the bottom mirror.
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P31220 Lab
Name: ________________________________
Lab Section: ________________ T.A.’s ____________________________________________
Today’s Lab Partners: ___________________________________________________________
How it works: The object is placed near the focus of the top mirror. The rays reflecting from
the top mirror are nearly parallel. These rays then strike the bottom mirror, which brings the
nearly parallel rays to a focus at the location of the image.
CAUTION: Do NOT touch the mirrored surfaces inside the Mirascope. It’s nearly
impossible to clean fingerprints off without scratching the surface. Please do not put your own
objects into the Mirascope, as they are liable to scratch it.
Fig. 11: Mirascope, assembled and apart
Observe the image carefully.
25. Describe what you see.
26. What is the magnification of the Mirascope’s image? ______________________
Hint: Measure the image and object with a ruler. Magnification = (image size)/(object size)
27. What is the orientation of the image, compared to the object?
28. Measure the focal length of the Mirascope’s
mirrors. The distance between the center of the
(single) mirror and the point where the observed
image of the finger (or a pen) changes from erect
to inverted is the focal length. Take care not to
touch or scratch the surfaces of the mirrors when
making this measurement. It’s OK if your
measurement is approximate.
Focal length of bottom mirror: _______ cm
Focal length of top mirror: _______ cm
Fig. 12: Measuring focal length
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P31220 Lab
Name: ________________________________
Lab Section: ________________ T.A.’s ____________________________________________
Today’s Lab Partners: ___________________________________________________________
Displays:
Several displays are placed on the counter at the back of the room. No writeup is necessary.
Simply take a look and discuss what you see with your group. If you wish to learn more, please
look these phenomena up in your textbook or on the Internet.
 Newton’s Rings and Air Wedge:
Alignment is tricky, so please do not touch the apparatus.
As you know from observing soap bubbles and oil slicks, thin films can cause pretty colors to
appear. In the case of Newton’s Rings, the thin film is a bit of air sandwiched between a flat
glass plate and a slightly curved glass plate. As with slits and gratings, the spread of colors
comes from constructive and destructive interference between waves. If the thickness of the air
film is half a wavelength of light, constructive interference for that color will occur. The number
of fringes per inch indicates how rapidly the thickness of the air film is changing.
The display table contains two examples of this phenomenon: the classic “Newton’s Ring”
apparatus, and the air wedge, which consists of two optically flat plates of glass that are wedged
apart slightly at one end.

Rear-View Mirror:
If you own a car, you have a rear-view mirror. What does the lever do? How does it change the
mirror from day to night? Many people think that “there are two mirrors in there.” There are
“two mirrors,” but only one piece of glass. Examine the mirror closely. Can you figure out how it
works?

Invisible Spheres
Polyacrylate polymers can absorb a considerable amount of water. They are commonly used in
diapers and available as decorative media for arranging flowers. Fully hydrated polyacrylate
spheres have an index of refraction that is nearly equal to that of water. You have a sealed jar
containing spheres and water. What happens when you turn it over?

Moiré Patterns
You have some overhead transparencies and materials with various repeating patterns. Observe
what happens when you stack two transparencies and shift one of them slightly. Moiré patterns
are related to interference patterns and beats. They can be used to detect repeating patterns even
when the original pattern is too small to see.

Hologram
Holograms are created as interference patterns between laser light and laser light that has
reflected from an object. When they are illuminated properly, the interference pattern can be
used to reconstruct a three-dimensional image of the original object. Not all 3-D images are
holograms. For example, the image in the Mirascope is not a hologram. True holograms are
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P31220 Lab
Name: ________________________________
Lab Section: ________________ T.A.’s ____________________________________________
Today’s Lab Partners: ___________________________________________________________
difficult to counterfeit, so they’re often used as stickers on credit cards.
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P31220 Lab
Name: ________________________________
Lab Section: ________________ T.A.’s ____________________________________________
Today’s Lab Partners: ___________________________________________________________
Want more? Try these Optics Applets
Color Mixing:
http://users.hal-pc.org/~clement/Simulations/Mixing%20Colors/rgbColor.html
http://phet.colorado.edu/en/simulation/color-vision
http://www.exploratorium.edu/exhibits/mix_n_match/
Color mixing for light is different from color mixing for paint. Here are three web applets that let
you play with color mixing. You can find many more online. Many computer programs let you
mix colors, too. The key to color mixing is to realize that most colors consist of a mixture or a
range of wavelengths of light, not a single color. In addition, the responses of your eye and brain
to the light are just as important as the content of the light. If you want to know more, search for
“Color Theory” online. If you wish to learn about this topic in more depth, consult books on art,
graphics design, or printing.
Interference and Diffraction Applets
http://phet.colorado.edu/en/simulation/wave-interference
http://www.falstad.com/ripple/
A “ripple tank” is a shallow tray containing water. The tray has a glass bottom so that the
shadows of ripples can be seen beneath the tray. You have links to two virtual ripple tanks. The
PhET applet emphasizes that the same phenomena work with water, sound, and light. The
Falstad applet has more options for exploring interference and diffraction.
Experiment and explore! You will learn the most if you explore systematically. When you
experiment, try to focus on understanding one thing or one relationship at a time. Have fun!
Moiré Patterns:
http://lite.bu.edu/lite1/moire/index.html
Fun to play with! The radial patterns might give a familiar result. Click on the diagrams to get
going.
http://incompetech.com/graphpaper/
You can use this site to make your own graph paper and, if you work at it, Moiré patterns.
What to turn in:


Law of Malus graph
Pages 6-16 of this writeup
Clean Up, Please!




Please leave tables neat and tidy, with the proper equipment on them.
Turn off all DMM’s, desk lights, and lamps.
Report any missing or broken equipment to your TA.
Please put trash and recyclables in their proper containers.
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