2ndSem.-Physics – LO Lab-02
Sunglasses and Other Optical Filters
LO - Lab #02 - Sunglasses and Other Optical Filters

You may find it helpful to review the reference pages on linear and exponential
functional models and on using Excel before coming to lab!
Task #1 - Effect of Sunglasses as Perceived by Your Eyes
Your instructor will pass out a pair of plastic sunglasses for your group to exam. Using
your eyes as light detectors, conduct some brief experiments to examine how these sunglasses
affect the light that passes through them. As you conduct these experiments, discuss the
following questions with your partners.



1.
How do the sunglasses affect the intensity of light perceived by your eyes?
How do they affect the color of the light perceived by your eyes?
Is there any difference in this affect if you look at something white, red, yellow, or
blue?
Based on your discussions, record a physical description of how sunglasses seem to affect
light.
Your instructor will lead a brief discussion of your results and to introduce some
important terms and concepts.
Task #2 - Effect of Sunglasses as Perceived by MBL Light Sensors
Locate the following equipment at your lab station:
 Vernier MBL light sensor (sensor at end of a long tube – Channel A – 6,000 range)
 Pasco MBL light sensor (sensor in small black box – Channel B – gain = 1)
Turn on the computer and interface and start the file Lab #07 – MBL – Light Sensors.
There are two provided light sensors – one is identical to what you used in the last lab and one
is different. Using the software, you will be able to collect light intensity data with each of the
two provided light sensors.
At your lab station you should also find an incandescent light source attached to a
power supply. This power supply sends out a constant voltage to the light, which will reduce
the “noise” in your data (you may have noticed some noise in the data you collected last week).
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2ndSem.-Physics – LO Lab-02
Sunglasses and Other Optical Filters
To further reduce noise, make sure that the overhead lights are turned off. However, the
window blinds should be open to let in a little light as you work. (Why do you think this light
doesn’t create “noise” in the data?)
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2ndSem.-Physics – LO Lab-02
Sunglasses and Other Optical Filters
Turn on the light and monitor the data to test that both of the light sensors are working.
For example, move the sensors toward and away from the light, cover the light with your hand,
etc., and record the corresponding values. Note: These sensors have been calibrated to relative
units so it is ok if you get a value greater than 100%.
2.
If the sensors appear to each be working, briefly state your evidence. If one is not
working, have your lab instructor correct the situation and then answer the question.
Data Collection
Turn on the incandescent light source and press the Monitor button in ScienceWorkshop.
Place the Vernier light sensor in front of the light source so that it reads approximately 50%.
Slowly move the sensor toward the light until the intensity measurement has a value of 94-95%.
The intensity will fluctuate some so you will have to give your best estimate for the "true" value.
Make a note of this intensity reading in your logbook and label it the initial intensity.
Measure and record the distance of the light sensor from the light’s filament and keep this
distance fixed during the following experiment.
You should create a data table with the following columns for your data:
Sensor
Distance
(
)
Initial Intensity
(
)
Intensity with Sunglasses
(
)
Once you have recorded the initial intensity, place one pair of sunglasses directly in
front of the light sensor. That is, you can think of the sensor as wearing sunglasses. Record the
resulting intensity in your data table.
Repeat this experiment for the Pasco light sensor and add the data to your table. Be
careful to follow the same procedure for positioning this new sensor.
Data Analysis
Scientists and technicians in many fields often refer to the optical density, or absorbance,
of a sample. This quantity is defined as:
Absorbance = Optical Density = log 10
I0
I
where I0 is the initial light intensity before a sample is added and
I is the light intensity after a sample added.
Using your data, calculate the optical density of one pair of sunglasses using the data
measured with the two different sensors.
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2ndSem.-Physics – LO Lab-02
Sunglasses and Other Optical Filters
You should compare your values to those found by at least one other lab group.
3.
How consistent are the values for the optical density? Does it vary much between
sensors? Does it vary much between groups?
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2ndSem.-Physics – LO Lab-02
Sunglasses and Other Optical Filters
As you found last week when comparing the MBL sensor to your eyes, results can be
very dependent on what you choose to use as your detector. For example, these two light
sensors are made to respond differently to different kinds of light. Here are the spectral
response curves for these two sensors. These curves can tell you how sensitive the sensor is to
different kinds of light.
4.
Based on these detector response curves and your study of an incandescent light last
week, why do you think you obtained different values for the optical density of the same
pair of sunglasses?
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2ndSem.-Physics – LO Lab-02
Sunglasses and Other Optical Filters
Here is a similar spectral response graph for the typical human eye.
5.
Since these sunglasses are made to work with human eyes (like the one used to make the
graph above), which light sensor is a better model of the detecting abilities of the human
eye? Explain your choice.
6.
What value would you estimate for the optical density of sunglasses for human eyes? Do
you think these sunglasses are of a typical optical density?

Please return the sunglasses to your instructor before you continue.
Task #3 - Effect of Colored Filters as Perceived by Your Eyes
Obtain a packet of colored filters from your lab instructor. It should contain some red,
blue, and yellow filters. Using your eyes as light detectors, conduct some brief experiments to
examine how these filters affect the light that passes through them. As you conduct these
experiments, discuss the following question with your partners.

7.
How do the different filters affect the color of the light perceived by your eyes?
Based on your discussions and what you saw with your own eyes, which of the filters do
you think has the greatest optical density? The least? Explain your predictions.
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2ndSem.-Physics – LO Lab-02
Sunglasses and Other Optical Filters
Task #4 - Effect of Many Colored Filters as Perceived by an MBL Light Sensor
For the rest of the lab you will only use the light sensor that is most similar to the optical
behavior of the human eye (as you identified in question #5). Be sure you can identify which
numerical display in the software corresponds to this sensor and use this display for the
remainder of the lab.
Describing the Colored Filters
Your packet should include 15 pieces of one color of filter (red, yellow, or blue) that you
will investigate. Identify this color and record a brief description of the filters you will be
investigating in your logbook.
Studying the Effect of Multiple Filters
Turn on the incandescent light source. Reposition the light sensor again by starting at a
value of 50% and moving the sensor forward so that it has an intensity reading of about 95%.
Measure and record the distance of the light sensor from the light’s filament and keep this
distance fixed during the following experiment.
Place one of the filters directly in front of the light sensor (like it was wearing colored
glasses). Make a note of this intensity reading for n = 1 (where n = the number of filters).
Add another filter back to back to the first filter and put them directly in front of the
sensor. Take a new intensity reading (this is n = 2). Continue to stack back-to-back layers of
these colored filters between the detector and the light source, adding one filter at a time.
Record the light intensity reading for each number of filters. Add filters one by one until n = 12
filters. Record your data in a table.
Data Analysis Using Excel
We could continue to work with this data by hand, but it turns out that there is an easier
way. There are computer spreadsheet programs such as Excel that have been designed to assist
in the analysis of data. You will use the following steps to learn to use this program on the lab
computers.

Make sure that you quit the ScienceWorkshop program.

Start the Excel file named Lab #07 – Excel– Absorption data.

Enter your data into the two columns: # of filters and Intensity. You can enter data by
clicking on a cell once with the mouse and then typing the appropriate number.
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Sunglasses and Other Optical Filters
Once your data has been entered, you should save the file by clicking on the save button. Be
sure to give the file a unique name and record this name in your logbook. You should save
your work often to keep from losing any information if you have computer problems!!
Now that your data is in Excel, you are ready to continue your analysis by having Excel create a
graph for you by using the following steps.

Click once on the Chart Wizard button.

Step 1 – Select XY Scatter for the Chart Type.

Step 2 – Click on the Series window.
Enter a Name for your data (e.g., red filter).
Click on the X values box and then highlight the number-of-filters data.
Click on the Y values box and then highlight
the corrected intensity data.

Step 3 – Enter a Title for your graph and the name and units of each axis.

Step 4 – Place chart as object in Sheet 1. Then click on Finish.
Your instructor can assist you with completing these tasks if you have problems.
Creating Functional Models with Excel
Excel is able to fit your data with a linear mathematical model (of the form, y = m x +b ).
You can try this kind of fit by using the following steps:

Select the data by clicking once on one of the data points.

Select "Add Trendline…" from the "Chart" menu.

Select "Linear" from the "Type" window.

Select "Display equation on chart" from the "Options" window.

Select "Display r-squared value on chart" from the "Options"
window.

Select "ok".
8.
a) In words and with a sketch, describe the trend of the data on this XY Scatter graph.
b) Record the linear fit found by Excel. Does this seem like a "good" fit? Explain why.
Hint, a very good fit will give an R2 value of almost 1.000.
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2ndSem.-Physics – LO Lab-02
Sunglasses and Other Optical Filters
Excel is also able to fit your data with an exponential mathematical model (of the form,
y = A e -bx). You can try this kind of fit by using the steps listed above except choose
"Exponential" from the "Type" window.
9.
a) Record the exponential fit found by Excel. Does this seem like a "good" fit?
b) How does the exponential fit compare to the linear fit? Which seems to best represent
the trend of the data?
Adjust your graph as necessary so you can see the data, trend line, and equation of the
best mathematical model. Prepare to print a copy of the graph. Select “Print…” from the “File”
menu. Choose “Microsoft Excel” under the pull-down menu marked “General.” Verify that the
option “Print Selected Chart” has been selected. Press the
button.
You should now have printed one copy of the graph. If it worked, then print another
copy for the remaining members of the group. If it did not work, then ask you instructor for
assistance before trying to print again. Tape the printed graph in your logbook. On this graph,
rewrite the equation to include the appropriate variables (I, n) and all relevant units.
10.
Which functional model appears to best represent your data? If you believe the
exponential model gives the best fit, then use your graphical model (the smooth curve) to
estimate how many filters it takes to cut the intensity of the light by 50%. Make this
estimate at least two different times, as you did in lab #03.
Calculating the Optical Density
Using the ability of Excel to make calculations, calculate the optical density of your
colored filter from your data. You should make a calculation for each time that you added a
single filter.
Hint! Each time you added a single filter, you had a new value of I0.
Hint! You will want to use an Excel formula of the form: =log(C7/C8)
Record the individual values for the optical density as well as an average value.
11.
From your data, what would you say is the best value of the optical density of this filter
color? How does this value compare to what you found for the sunglasses?
Task #5 - Effect of Different Colors
On one of the white boards in the room, record the following information for the filter
color you investigated:



color
your mathematical model of the absorption of light by this filter color
your best estimate of the optical density of this filter color
Make a note of the colors and information obtained by all other groups. Consider all of
this information in answering the following questions.
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2ndSem.-Physics – LO Lab-02
Sunglasses and Other Optical Filters
12.
Consider the equation, I(n) = A e  n. What is the physical meaning of the variable A?
What do you think the physical meaning of the variable  is?
13.
How did the value of A change for the different colors? Is that change meaningful? Why
or why not? How did the value of  change for the different colors? Is that change
meaningful? Why or why not?
14.
Which color of filter seems to be the most efficient absorber of visible light? Which color
seems to be the least efficient? Explain your evidence.
15.
In words, describe the significance of the optical density of a color filter.

Please return all filters to your instructor before you leave.
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