You may not be aware of it, but polarized light is all around you. Do you have an 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 optics or 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?
Some Background Information on Polarization
Shaking the end of a Super Springy 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 Super Springy 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
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 polarizing filter should absorb 50 percent of incident light.
Try This: Observe a source of light such as a light bulb through a polarizing 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.
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.
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.
Two polarizing filters with their polarizing axes crossed, that is, at right angles, will pass no light.
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 share a polarized plane. Place a small ball in the box. Replace the lid. Provide another piece of polarizing film at the station for student investigation.
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. 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
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.
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. In modern optics, Brewster's angle is important for laser emissions and photography.
Polarization by Scattering
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 polarizing filter will convince you of this. The figure below illustrates the scattering and polarization processes.
If the weather permits, go outside and investigate skylight with a polarizing 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 polarizing 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."
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).
Using a polarizing 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).
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 polarizing 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.)
Did you know you can share the science of Polarization with the art department!
How would you like your students to produce polarization 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
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.
Optically Active Substances
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.
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
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.