2ndSem.-Physics – LO Lab-01
Making Light and Detecting Light
LO - Lab #01 - Making Light and Detecting Light
Task #1 - Comparing Light by Using Your Eyes as Your Detector
Your lab instructor will show you two different lights – one is an incandescent light and
the other is a mercury gas discharge tube. Here is a brief description of how these two lights
work.

Incandescent Light – This light works like the miniature bulbs you used in the earlier
circuit labs. When a voltage is applied, the resulting current through the filament wire
heats up this wire, producing light.

Mercury Gas Discharge Tube – The tube inside this light has been evacuated and filled
with mercury (Hg) vapor. When a voltage is applied across the tube, mercury atoms are
excited. As they decay from their excited state, the excess energy is given off as light.
1.
Carefully observe these two light sources. Record a description of how each light looks
to your eyes. You should include a description of the color of each light and its
brightness.
Task #2 - Analyzing Light with a Diffraction Grating and Using Your Eyes as Your
Detector
Your lab instructor will now give each group a diffraction grating to assist in the
analysis of these two light sources. A diffraction grating is made from a material that has been
etched with many, many tiny scratches. When light passes through the area between the
scratches, it is bent through a certain angle. The value of the angle depends upon the color of
the light. Therefore, light of different colors will be bent through different angles, causing
them to separate.
Carefully observe these two light sources when looking through the diffraction grating.
Look for the different colors of light you see, their relative brightness, and their relative
position.
2.
a) Record a precise description of how each light source now looks to your eyes,
including the colors and brightness.
b) Using crayons or color pencils, make a drawing of what your eyes see when looking
through the diffraction grating. Your drawing should depict the relative position of the
different visible colors.
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2ndSem.-Physics – LO Lab-01
Making Light and Detecting Light
3.
Based on your visual observations, how are these two light sources similar and how are
they different? Explain your ideas for why these two kinds of sources behave so
differently.
4.
When using a diffraction grating, which color(s) of light seem to be bent the most
(through the largest angle)? The least? Explain your reasoning.

Please return the diffraction grating to your instructor before you continue.
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2ndSem.-Physics – LO Lab-01
Making Light and Detecting Light
Task #3 - Analyzing Light with a Prism and Using Your Eyes as Your Detector
Locate the following equipment at your lab station: Incandescent light source mounted
on optics bench, black cloth, spectrophotometer (including prism, collimating slit, lenses,
rotating platform, and optics bench).


Please do not touch the glass prism. The oils from your fingers can damage it.
Turn off the overhead lights if you have not done so already.
Equipment Setup:
Light sensor, screen, focusing lens, and
glass prism on the rotating platform
Focusing lens, collimating slits, and
incandescent light source
Turn on the light source by plugging it in to a power strip. It should be setup so that it
is shining through slit #1 of the collimating slits. If not, you can ask your instructor for
assistance in setting it up.
Adjust the alignment if necessary so that the light passes through the collimating slit,
lens and the glass prism. You can rotate the white screen of the spectrophotometer by using
the handle located underneath the light sensor. Rotate it so that you can view the resulting
rainbow in the center of the screen (where there is a second small #1 slit opening). The colors
should be visible and look similar to what you saw in Task #2.
5.
When using the glass prism, which color(s) of light seem to be bent the most (through the
largest angle)? The least? How does this compare to the diffraction grating?
6.
a) Using crayons, draw what you see in your logbook. Try to accurately depict the
relative width of each color band.
b) Which color seems the brightest to your eyes? Identify this brightest part in your
drawing.
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2ndSem.-Physics – LO Lab-01
Making Light and Detecting Light
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2ndSem.-Physics – LO Lab-01
Making Light and Detecting Light
For the remainder of the lab, you will need to work in very low-light conditions. Keep
in mind that the light from your station may interfere with the data collection of another
station. Try to coordinate your activities to minimize disruptions. To reduce the interference
of stray lights, there are black cloths that you can use to cover the backside of the light sources.
You will now measure the position of each relevant color. Use the handle to position
the rotating platform to the far counter-clockwise position. Make a note of this current angular
position (around 120.0, as measured on the scale printed on the spectrophotometer) and
record this angle in your logbook. You should call it the "starting angle."
Rotate the platform so that the center slit is located in the middle of the red band.
Measure and record this angle. Create a data table and record an angle for the center of each
color (red, orange, yellow, green, blue, and purple). Calculate the angular displacement by
subtracting the initial starting angle from each position.
Sample data table:
Color
Start
Angular Position
()
Angular
Displacement (  )
0
Red
Orange
…
Brightest
Based on your own eyes, record the angle of the light that seems to be the brightest.
Each person should do this measurement for his/herself since your eyes may not perceive the
same location. Record this angle in your data table as well.
7.
Compare your angle measurements with another group. Do the values seem consistent?
Should they be? Explain why you think why or why not.
Task #4 - Analyzing Light with a Prism and Using an MBL Light Sensor as Your
Detector
You will now use the power of the MBL data acquisition system to analyze this light in
more detail. Turn on the computer and start up the Lab #06 MBL – Spectrophotometer file. You
should see a graph that will plot data of light intensity vs. angular position (as measured in
degrees.)
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2ndSem.-Physics – LO Lab-01

Making Light and Detecting Light
The light from the computer monitors may interfere with some of the experiments. Try
to keep your monitor facing away from the experimental area. In addition, you can use a
black cloth to cover it and reduce the stray light.
Setting up the MBL Equipment

Find the light sensor on the spectrophotometer and verify that the Gain switch is set to 1.

If necessary, readjust the light source and covering materials at the end of the optics bench
so that the light leaves the bulb, travels through the slit and along the track, passes through
the prism, and you are able to see its resultant color spectrum. Try to block the extra light
from interfering with other groups.

Ask your instructor to verify your setup before you continue. The instructor will help you
focus the light from the source into the spectrophotometer.
Data Collection

Before collecting any data, be sure to always first position the light sensor at the "starting
angle." The computer will always set this initial position equal to zero degrees.

When you record data, the computer will measure the intensity of the light at the location
of the light sensor and plot it based on the angular position of the light sensor.

You can change the angular position of the light sensor by holding the handle underneath
the sensor and moving it slowly around. When collecting data, it is best to only move the
sensor in one direction. In addition, you should try to move it continuously at a
moderate speed (not too fast, not too slow).

Try collecting data by moving the sensor from the starting angle through at least 20, as
measured on the scale printed on the spectrophotometer. Once you have collected a data
set, you should stop collecting by choosing "Stop" from the "Experiment" menu.

Sometimes the sensor that measures the angular position of the light sensor slips and the
computer will think that the angular position is staying constant even when you are
moving the light sensor. When this happens, just stop taking data and try again! You can
delete "bad" data runs by selecting them with the mouse and typing the delete key.
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2ndSem.-Physics – LO Lab-01
Making Light and Detecting Light
Repeat your data collection until you have a nice clean run. If your intensity data
appears very low, ask your instructor about raising the gain setting.
Once you have a clean data run, then adjust the vertical axis so that the peak is as large
as possible. In addition, adjust the horizontal axis so you are looking at the relevant angles,
say from 6 to 16. Print a copy of this adjusted graph for each person in your lab group and
use this data to respond to the following items.
Data Analysis

Using the provided crayons and your earlier angle measurements, color the area
underneath the peak to match the colors that were visible to your eyes. If there is a region
where you did not see any light, then leave that part of the graph blank.
8.
What part of the light spectrum was brightest according to the MBL light detector? Give
the angle where this appeared. How does this peak location compare to the brightest
location that you identified earlier with your eyes as the detector?
9.
Based on the evidence of your data, do you believe that light exists that we cannot see
with our eyes? Explain why or why not.
10.
Do you think this MBL light sensor is a good model of human vision? Why or why not?
11.
What did the light sensor detect around 15-16? Could this be due to the characteristics
of the sensor or to the properties of the light? Can you think of an experiment that you
could conduct (if you had more time and resources) that would allow you to which it is?
Turn off the incandescent light source. Remove it from the setup by loosening the
screw at the base where it is held on to the optics bench.
Task #5 - Analyzing a Mercury Light with a Prism and Using an MBL Light Sensor
as Your Detector


Take care not to look directly at the light from the mercury light source. It can produce
ultraviolet light that may be harmful to your eyes.
To protect your eyes, do not turn on the mercury light until it is positioned next to the
collimating slits.
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2ndSem.-Physics – LO Lab-01
Making Light and Detecting Light
Next you will design and conduct an experiment to answer the question: How does the
light from a gas-tube light source differ from the light from an incandescent bulb with a
straight filament?
With the help of your lab instructor, set up the mercury gas-tube light source and the
collimating slits so you can examine its light with the spectrophotometer. Again, your
instructor can assist with focusing the light from this new source.
Place the light source as close to the collimating slits as possible and turn on the light.
Remember not to look at the light coming directly from the source. This keeps dangerous
ultraviolet light from reaching your eyes. Use a cloth to cover any stray light coming from the
setup.
In addition, this light source is not as bright as the incandescent light. Therefore, on the
light sensor, increase the GAIN switch to a setting of 100.
Data Collection
Using this new light source, collect data of the light intensity as a function of angular
position for the same range you used previously. Repeat the experiment until you have
obtained a good set of data. Print a copy of this good data and write an explanation of why
you think the data is good. (Hint, you could compare the graph to what you see with your
eyes.)
Using the ScienceWorkshop software cursor tool, measure the angular position of the
four to five brightest lines in the mercury spectrum. Record these positions in a table along
with what color the line appears to be as seen by your eyes.
Data Analysis
Scientists often use spectral lines to determine the composition of a gas. From previous
work, scientists have determined that the brightest spectral lines usually emitted by mercury
gas are lines of the following five wavelengths.
Wavelength
( nm )
Color
1026
Infrared
578
Yellow
546
Green
435
Violet
405
Violet
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2ndSem.-Physics – LO Lab-01
Making Light and Detecting Light
12.
If possible, identify (in words and with crayons) these five wavelengths on your data
graph. If it is not possible, explain why not.
13.
Based on your graphs from these two experiments and the wavelengths you should have
written on your graph (question #12), write a discussion about what kind of light can be
seen by human eyes.
Is all light visible to humans?
If not, estimate the range of wavelengths you think are visible to humans.
Does all light look the same to you?
Does any one light look the same to all people?
14.
a) How does your ability to detect light compare to the ability of the light sensor to
detect light? Explain.
b) If your eyes worked the same as the MBL light sensor, how do you think what you
see would be different? Explain your ideas.
Lab Cleanup:
1. Turn off the mercury light.
2. Return the incandescent light to the optics bench so it will be ready for the next
group.
3. Shut down the computer and turn off the interface box.
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