CoolStuff Newsletter Vol. 28 August 2007
Polarization
You may not be aware of it, but
polarized light is all around you. Do you have a LCD flat screen
television, a laptop computer, a calculator, or other liquid crystal
display? Well then, you have been exposed to polarized light. Light
reflected from non-metallic surfaces such as water, a polished table
top, and glass is also partially polarized as is the light scattered
by the earth’s atmosphere.
Generally speaking, the human visual system is not particularly
adept at perceiving polarized light. While some animals, bees in
particular, are quite sensitive to polarized light, humans usually
find it difficult to detect it with the unaided eye. (see Haidinger’s
brush in Interesting Links)
If you are not familiar with the properties of polarized light, then
you may have some questions such as…What exactly is polarized light?
How does light become polarized? What are some practical uses of
polarized light?
~ Chris Chiaverina
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Austine Wood Comarow invented the art of Polage and has been creating it since 1967.
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Polage is the use of polarized materials in creating works of art. To learn more about her fascinating work visit: www.austine.com
Some Background Information on Polarization
Shaking the end of a Slinky up and down will produce a train of crests and troughs. As the wave moves down the Slinky, all segments of the spring will eventually be set into vibration in a vertical plane (see figure). If the Slinky is shaken side to side, or for that matter in any one direction, once again waves vibrating in a single direction will be produced. These vibrations are said to be plane polarized.
Light emanating from a common source, such as a candle, light bulb, or the sun, consists of vibrating electric and magnetic fields that are randomly oriented in the plane perpendicular to the direction in which the wave is traveling. Such light is unpolarized (see left side of the grid in the figure below). The light shown to the right of the grid is plane polarized in the vertical direction.
Polarization by Selective Absorption
Key Concept: Some materials selectively absorb light with the electric field vibrations in a certain direction but pass light with electric field vibrations perpendicular to this direction. The mineral tourmaline is an example of such a material. A well-know, synthetic polarizing material is Polaroid. An ideal Polaroid filter should absorb 50 percent of incident light.
Try This: Observe a source of light such as a light bulb through a Polaroid filter. What effect does the filter have on the light passing through it? Observe what happens as you rotate the filter. Do you detect any change in intensity of the light viewed as the filter rotates? If you have an LCD screen available, observe what happens when you view the screen while rotating the filter.
Crossed Polarizers
Key Concept: As the figure shows, a transverse wave on a rope will pass through both fences when the slots in the fences are aligned, but will be blocked when the slots are at right angles to each other.
When light passes through a device called a polarizer, such as a Polaroid filter, only the waves vibrating in one direction pass through; all other light waves are absorbed. When a second polarizer, often referred to as an analyzer, is placed over the first, and slowly rotated, it is possible to totally block the light. Just as with the fence analogy, a wave that passes through one polarizer is absorbed by the second polarizer (see figure). This occurs because the transmission axes of the polarizers are “crossed,” that is, at right angles to each other.
Try This: Overlap two sheets of Polaroid material. Look at a source of light through the filters while rotating one of the filters. Describe what you see. When the light is completely blocked by the overlapping filters, rotate them through 90 degrees. What do you observe now? Rotate the filters through an additional 90 degrees. Explain what is happening as you rotate the filters.
Bogus Barrier
Key Concept: Two
polarizing filters with their polarizing axes crossed, that is, at
right angles, will pass no light.
Setup Instructions: Cut a wide “window” in opposite
sides of a shoebox. Cut a piece of polarizing film (Polaroid) into
four pieces, keeping track of the directions of polarization. Attach
the film to the windows as shown, so that the two pieces in each
window are polarized differently, but films directly across on the
opposite window are polarized alike. Place a small ball in the box.
Replace the lid. Provide another piece of polarizing film at the
station for student investigation.
Try This: Examine the inside of the shoe box by looking through the tinted windows on either side. DO NOT OPEN THE BOX! Note that a wall divides the inside of the box into two regions. Tilt the box so that the ball rolls back and forth.
Does the ball pass through or bounce off the wall?
Can you explain this mysterious behavior?
Notice the direction of the polarized film panels.
If you are totally baffled, you may take the lid off the box. To discover why the “bogus barrier” exists, look through each of the windows with one of the square Polaroid filters provided at this station. You may find rotating the filter while looking through each window quite revealing.
Polarization by Reflection
Key Concept: When light is reflected from a non-metallic surface, such as a tabletop, snow, or water, it becomes polarized in a plane parallel to the surface. In Polaroid sunglasses, the axes of polarization are vertical. The reflected glare, which is at least partially polarized in the horizontal direction, is reduced by the Polaroid sunglasses.
Try This: Find a surface
(e.g., tabletop, blackboard, etc.) from which the reflected light
produces glare. While viewing the glare through a Polaroid filter,
rotate the filter until you no longer see the reflected light. In
this position, the filter’s axis of polarization is vertical. Place
a small piece of masking tape along the edge of the filter and
indicate the axis of polarization with an arrow. Rotate the
filter to pass the maximum amount of light. Describe the orientation
of the axis of polarization now.
Now view reflected light from a sheet of metal such as a piece of
aluminum foil. Describe what happens this time as you rotate the
filter. Compare the reflected light from a metallic surface to that
reflected from a non-metallic surface.
Getting Quantitative: Finding
Brewster’s Angle
View the glare from a surface through a Polaroid filter held close
to one eye. When the axis of polarization of the filter is vertical,
you will notice that the reflected light is dim for a variety of
viewing angles but completely dark for only one. The angle of
reflection that produces completely polarized light is called
Brewster's angle and depends on the reflecting surface.
Measure Brewster's angle for one or two different reflecting
surfaces. To do this, have your partner extend a string from the
filter to the spot on the surface where the light is totally
extinguished. Use a protractor to determine the angle formed by a
normal (perpendicular line) to the surface and the taut string.
Brewster's angle for water is approximately 53 degrees; for glass, 56
degrees. You
may wish to find Brewster's angle for plastic or floor wax by using
them as your reflecting surfaces.
Polarization by Scattering
Key Concept: When light interacts with an object whose
size is comparable to the wavelength of light, it shakes the charges
in the object. These charges then radiate in all directions. This
phenomenon is called scattering. The light filling the daytime sky
is due to scattering that occurs as sunlight passes through the
atmosphere. Blue light is scattered more than red light. That is why
the sky is blue. Atmospheric scattering also polarizes light.
Looking at the sky with a Polaroid filter will convince you of this.
The figure below illustrates the scattering and polarization
processes.
Try This: If the weather permits, go outside and investigate skylight with a Polaroid filter. DO NOT LOOK DIRECTLY AT THE SUN! Slowly rotate the filter as you view a portion of the sky.
Is it possible to reduce the brightness of the sky for certain orientations of the filter?
Now examine other areas of the sky. Does the light in certain portions of the sky seem to be more polarized than others?
Estimate the angle formed by imaginary
lines drawn between your head and the sun and your head and the
portion of the sky with the highest degree of polarization.
Look at the portion of the sky with the greatest polarization. If
clouds are present in this region, observe what happens as you
rotate your filter while viewing the clouds.
Do the clouds seem to stand out for certain orientations of the filter?
This occurs because the light scattered by the atmosphere is polarized, but the light scattered many times by water droplets in the cloud is not.
Simulating Atmospheric Scattering
Atmospheric scattering may be simulated by adding a few drops of
milk to a container of water. This simulation works because the
solid particles in milk are much smaller than the wavelength of
visible light. So if you can’t go outside, you may wish to try
performing the following demonstration of scattering.
First fill a transparent container with water. Shine a flashlight
beam through the water and observe the scattered beam from the side
of the container. Also observe the transmitted beam by projecting it
onto a piece of paper. Describe the scattered and transmitted light.
Now add a little milk to the water, a drop or two at a time.
Carefully note the color of the mixture as the milk is stirred into
the water. What color is the scattered light now? Note also the
color of the transmitted light that is projected on the paper. What
color do you observe on the paper? Continue adding milk in small
amounts and notice changes in the scattered and transmitted light.
Describe these changes. View the blue light scattered by the milk
particles through a Polaroid filter. What do you observe as you
slowly rotate the filter? Can you explain your observation? Also
look at the top of the water through the rotating filter. What do
you observe?
Finally, use a Polaroid filter to examine the transmitted beam. Is
this light polarized? Additionally, place the polarizing filter
between the flashlight and the milky water. Look from the side and
notice what happens when you rotate the polarizer. Also notice what
happens to the transmitted red “sunset.”
Birefringence
Key Concept: Crystals, such as calcite and quartz, are said to be birefringent, meaning two indices of refraction. When unpolarized light enters a birefringent material, it divides into two components. These components have different speeds and are polarized at right angles to each other (see below).
Figure 1
Using a Polaroid filter to view the two rays as they emerge from the crystal, either ray can be extinguished while the other remains visible. Students are fascinated by the formation of a double image when an object is viewed through a birefringent crystal (see figure 2).
Try This: Place a calcite crystal on some printed material. How many images do you see? Now view the printed material after you have placed a Polaroid filter on top of the crystal.
What do you see now? Can you extinguish one image at a time by rotating the filter? Why do you think this occurs?
When stressed, plastic and glass become birefringent. Viewed between crossed Polaroid filters, this birefringence appears as colored contours. Place a plastic fork, or other plastic object, between your filters to make the stress lines visible. If you are using a fork, squeeze the tines together.
What happens to the colored stress lines?
Due
to their birefringent nature, some transparent tapes produce
brilliant colors when viewed between polarizing filters. Using only
transparent tape and a pair of polarizing filters, it's possible to
create beautiful colored designs reminiscent of cubist art and
stained glass windows.
(Note: Not all clear tapes (e.g. Magic Tape) are birefringent. You
may wish to try transparent packing tape or old fashion cellophane
tape.)
You can share the science of Polarization with the art department! How would you like your students to produce Polage-like art in your classroom? I can say from experience that it's an activity capable of engaging your students beyond your wildest dreams! To make the experience even more meaningful, consider collaborating with your school's art department. At New Trier High School an art teacher and I would often bring art and science students together to introduce them to polarization, color, and artistic composition.
After students perform exploratory activities relating to polarization and color, the art teacher can give them an overview of what makes for interesting composition. Students then set to work applying this knowledge to create their works of polarization art.
They begin the process by layering tape on microscope slides in a step-like fashion. Placing the layered strips between crossed Polaroid filters helps them determine the exact relationship between color and tape thickness.
Petri dish art. Get Polarized film here
Once they have created their color keys, students produced polarization art by placing carefully cut pieces of birefringent tape, such as clear packing tape, on a clear substrate (a sheet of overhead transparency material or a glass or plastic Petri dish works well).
When sandwiched between Polaroid filters, the resulting creations have the appearance of luminous stained glass.
Trick of the Trade: Students can use an laptop LCD screen as a source of polarized light. Placing their art work between the screen and a Polaroid filter will enable them to see their work as it evolves. Taking a cue from Austine Wood Comarow, students can don Polaroid glasses, thereby freeing their hands. Using a laptop computer as a light source
Optically Active Substances
Key Concept: Optically active materials are capable of rotating the
plane of polarization of transmitted light. These materials include
sugar solutions, corn syrup, turpentine, amino acids, and some
crystals. When polarized white light passes through an optically
active liquid, each color's direction of polarization is changed by
a different amount. Changes in color are observed when the light
transmitted through the liquid is viewed through a rotating
polarizing filter.
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The degree of rotation of the plane of polarization depends on the depth of the liquid. Therefore, different depths of solutions will exhibit different colors when viewed through a stationary polarizing filter. In the photo below, pieces of glass placed in Karo syrup create a variety of depths, and hence different colors.
The Barber Pole Demo
Key Concept: Molecular antennae, called dipoles, constituting an optically active liquid absorb and reradiate light. This process is the result of electric field vibrations acting on electrons within the molecules. Known as scattering, re-radiation occurs most strongly in the plane perpendicular to each dipole. As polarized, monochromatic light passes through an optically active liquid, its plane of polarization rotates, and with it, the direction of scattering. The figure below shows how the plane of polarization “corkscrews” as it passes through the optically active liquid.
When polarized white light passes through an optically active liquid, the plane of polarization of each of its constituent colors changes by a different amount. Thus each color is scattered in a different direction, producing effect shown in the photo below.
Click here to see a video of this demonstration.
Next time...
According to Webster’s Online Dictionary, “a toy is something to play with, for children, adults or both.” While this is certainly true, in the hands of a science teacher a toy can be so much more. Toys have the power to delight and engage the learner while illustrating the laws of nature. Join us next time for “Toying with Science” when we will examine some of the seemingly endless possibilities for using toys in the science classroom.
~Chris Chiaverina
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