Light Absorption by Liquid
How does light intensity
change as you go below the surface of a liquid like water?
* 60-W Light Bulb
* Clear, Clean Cylinder
* Water w/ food coloring
* Black Construction Paper
* Ruler or Meter Stick
* Light Sensor
* Interface or LabQuest
1. Add food coloring to water so it passes light but is readily
2. Mount the cylinder securely using ring stands to hold it in
position. Note that the light sensor will need to be directly
below it taking measurements, so allowance needs to be made for
that. One option would be to place the cylinder on a ring above
the light sensor.
3. Position the light bulb above the column and hold it steady
with a ring stand or other means.
4. Position the light sensor below the cylinder so it can record
the light that passes through the column. Hold it steady with a
ring stand and clamp.
5. Cover the sides of the cylinder with black construction paper
so the main light source visible to the light sensor will be the
light bulb. Extend the construction paper so it covers the space
around the light sensor, too, blocking ambient light from the
room. Note that you will need to move the paper aside to take
measurements periodically, so allowance needs to be made for that,
6. With the light bulb turned off, zero the light sensor.
7. Turn the light bulb on and record the light sensor reading and
the depth of the water, 0.
8. Add water to the cylinder until it is approximately 5 mL deep.
It is not necessary to make the depth precisely 5.0 mL as long as
you measure it carefully. Record the light sensor reading and this
9. Continue adding water in approximately 5-cm increments until
the cylinder is almost filled, recording your readings along the
10. Return the water carefully to the location your instructor
indicates. Dry the cylinder and return it and the light sensor and
light bulb, also.
1. Construct a graph of Light Intensity versus Depth for your
water. What general pattern exists for the light that passes
through as the column of water increases in depth?
2. Conduct an analysis to determine the mathematical pattern
between light intensity and depth of the water. Express this in
3. How does the amount of light passing through twice the depth of
water compare to the amount going through a single depth? (Compare
20 cm depth with 10 cm depth.) Is the ratio of light intensities
similar to the ratio of depths? Explain.
4. Is there a point at which the amount of light passing through
is approximately half the original amount without any water at
all? What depth produced this value? Is there similarly a point at
which the light intensity dropped to a fourth of its original
value? What depth produced this? How do the two values compare?
5. If each group did their own addition of food coloring, how
would the results from one group compare with those from another
group? Would they be identical? What would be the same and what
would be different?
1. If you could take measurements as you descended into the ocean,
would you expect to see a similar pattern? Explain any
similarities and any differences.
2. Conduct the Light Filters experiment* where you use colored
plastic pieces instead of water. Would you expect similar results
in that situation? How might they be different?
I have not given specific instructions that take advantage of
features within Logger Pro computer software or the LabQuest app.
Certainly using "Events with Entry" would allow students to record
their data and graph it with a minimum of effort. Check the
instruction manual for how to set up Events with Entry. In Logger
Pro there is a file in the Tutorials folder on this feature.