Owens 1
The Invisible Test Tube
Source: “Investigating change using the invisible test-tube demonstration.” Patrick
Brown. Science Scope. January 2013. Volume 36. p. 76.
Materials:
Basic experiment - 2 beakers, 2 test tubes, water, vegetable oil
Dig Deeper – listed materials, metal spoon, plastic test tube
8.5
Safety:
1. All participants must wear safety goggles or glasses.
2. All spills must be cleaned up immediately to prevent slip-and-fall accidents.
Set-Up:
Part One:
1. Fill one test tube nearly to the top with water.
2. Submerge the test tube filled with water into a beaker filled approximately half
full with water.
3. Observe the appearance of the submerged portion of the test tube in comparison
to the portion above water.
Part Two:
4. Fill the second test tube nearly to the top with vegetable oil.
5. Submerge the test tube filled with vegetable oil into a beaker filled approximately
half full with vegetable oil.
6. Observe the appearance of the submerged portion of the test tube in comparison
to the portion above oil.
What will happen?:
Part One: The submerged portion of the test tube will appear magnified in comparison to
the portion of the test tube above water.
Part Two: The submerged portion of the test tube will appear invisible.
Why it happens: It is helpful to use a table of refractive indexes when explaining this
experiment.
Refractive Indexes indices
Material
Approximate Refractive Index
Air
1.00
Water
1.33
Pyrex Glass
1.47 ???
Vegetable Oil
1.47
As the refractive index of a substance deviates from a value of 1.00, a bending of light
occurs. It can be understood by looking at the refractive indexes of the materials used in
this experiment that as the refractive index deviates from 1.00, the amount that light
bends changes. In part one of the experiment, the water (with a refractive index of 1.33)
Owens 2
and the Pyrex glass of the test tube (with a refractive index of 1.47) bend light more than
air, which has a refractive index of 1.00. Thus, the bending of light causes the submerged
portion of the test tube to appear magnified. Curvature of beaker glass?
However, in the second part of the experiment, when a Pyrex test tube is filled with
vegetable oil (with a refractive index of 1.47) and submerged in a beaker of vegetable oil,
the submerged portion of the test tube appears invisible. It can be understood that the
approximate refractive indexes of both vegetable oil and Pyrex glass are the same, at
1.47. As a result, the matching refractive indexes remove reflections and refractions
therefore there is no visible boundary between the test tube and oil. The test tube appears
invisible because light is being transmitted directly through it without bending, bouncing
back, or being absorbed.
A demonstration of a similar experiment and explanation can be viewed at:
http://www.youtube.com/watch?v=Wif_yGZcK18
Further explanation of the concepts of refraction and refraction index can be found at:
http://www.optics4kids.org/home/teachersparents/articles/the-refraction-of-light/
Curriculum Fit:
Saskatchewan:
Grade 8 Science – Outcome OP8.1: Identify and describe, through experimentation,
sources and properties of visible light, including: rectilinear propagation, reflection,
refraction.
Physics 20
Unit II: Waves – B. Wave Phenomena
Learning Outcome 11: Demonstrate an understanding of wave-transmission, reflection,
and refraction by relating these phenomena to practical and common experiences.
Learning Outcome 12: Interpret the relationship between speed and wavelength for
waves undergoing refraction.
Unit III: Light – A. Characteristics of Light
Learning Outcome 6: Recognize that the index of refraction for a particular medium can
be used to determine the speed of light in that medium.
Learning Outcome 10: Solve problems to determine the relative index of refraction
between any two given media.
Manitoba:
Senior 3 Physics 30S
S3P-1-11: Describe, demonstrate, and diagram the refraction of plane (straight) waves.
S3P-2-08: Experiment to show the particle model of light predicts that the velocity of
light in a refractive medium is greater than the velocity of light in an incident medium
(vr>vi).
S3P-2-11: Summarize the evidence for the wave model of light. Include: propagation
reflection, refraction, partial reflection/refraction, diffraction, dispersion.
Alberta:
Owens 3
Physics 20 – D2.5K: Describe how the speed of a wave depends on the characteristics of
the medium.
British Columbia:
Physics 11 – B1: Analyze the behaviour of light and other waves under various
conditions, with reference to the properties of waves and using the universal wave
equation.
B3: Analyze situations in which light is refracted.
References:
“Investigating change using the invisible test-tube demonstration.” Patrick Brown.
Science Scope. January 2013. Volume 36. p. 76.
http://www.youtube.com/watch?v=Wif_yGZcK18
http://www.optics4kids.org/home/teachersparents/articles/the-refraction-of-light/
www.progetudes.gov.sk.ca
www.edu.gov.mb.ca
www.education.alberta.ca
www.bced.gov.bc.ca
Reflection
Overall, I felt as though my POE went very well. I thought it was great to hear my
colleagues’ comments as they worked through my POE, and tried to develop an
explanation of what had happened. Almost everyone who attempted my POE was
amazed at the “invisible test tube” portion of the experiment. This reaction of amazement
was surprising to me as I had assumed, being in a class full of science people, that most
had seen something similar to this experiment done before. I think that my POE
generated excitement around the science involved simply due to the “wow-factor” of the
experiment.
One thing I would change with my POE would be to include more digging deeper options
to help those conducting the experiment to come to an accurate explanation of the science
involved. I had posted on my POE board that you could try putting a metal spoon in the
oil, or attempt the experiment using plastic test tubes, however I did not include the
refractive indices of these objects. If I were to use this POE again I would provide this
information on the board, as well as the steps to complete the digging deeper portion of
the experiment. I think this would provide more tools and information for those
conducting the experiment to come to an accurate explanation of what had occurred.
Another POE that I thought was super interesting was one where Shreddies cereal was
blended with water and placed in a plastic cup. Then, you used a strong magnet, holding
it against the side of the cup of blended Shreddies, and stirring the mixture slowly to
attract the iron in the cereal to the magnet. I found this POE fascinating, as I had never
seen anything like it before. I liked that the procedure was very simple, and would be
easy to do in a classroom. I also thought that students would find this POE intriguing, as
Owens 4
they would get to experiment with food that many of them may consume at home.
I would like to use the POE activities in my own classroom as I think these are a great
way of getting students involved in hands-on science. In my own classroom, I would like
to come up with multiple POE stations that connect to the same topic, and have students
circulate between the stations, conducting the experiments. I could use these POE
activities throughout the school year, helping students to connect the science they are
learning to practical experiences. I think this would be great in a classroom setting as the
experiments take relatively short periods of time to complete, and allow students to
partake in the nature of science. POE’s could be used at any point throughout a science
unit or for concept development - as an introductory activity or a knowledge-building
activity.