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Polarized Light

Estimated Time of Duration

1-2 hours

Guiding Question

What is polarized light?

Objectives

For grades K-3
- Introduce the concept of polarization
- Observe evidence of polarized light in the world around us
For older students
- Relate a visual picture of how polarization arises
- Introduce the concept of birefringence and observe its effects(?)
- Introduce the concept of optical activity and observe its effects

Concepts

Energy. Energy is the capacity for work or vigorous activity. You can experience energy through the mechanical energy required to throw a ball, the electrical energy required to operate a flashlight or the chemical energy required to digest your food. Heat energy causes your ice cream to melt and water to boil.
Light. Light is a form of energy. It enables us to see. It comes in many different colors, red, orange, yellow, green, blue, indigo, violet, white and black.
Polarization. The alignment or orientation of light through rotation.
Polarized light. Light that has been filtered to pass energy that vibrates only in a certain direction. The light is screened through the use of sunglasses just as a vertical coinslot of a parking meter will only accept coins inserted vertically.

Principles

Vibrate. To move or to cause to move back and forth rapidly
Reflect. To throw or bend back from a surface.
Refract. To deflect from a straight path.
Angle of Incidence The angle which light makes with the surface it strikes compared to a line that hits the surface head on, at 90 degrees.
Angle of Refraction The angle at which light passes through a material compared to a line that passes through the surface at 90 degrees.
Angle of Reflection The angle which light bends back after striking a surface compared to a line that hits the surface head on 90 degrees.
Rotation. The process or turning an object such as a door knob or a beam of light.
Photon. A form of energy that is a particle of energy that behaves like a wave. A photon is much like a single drop of water that makes up an ocean wave.
Brightness. A measure of the intensity of light refracted or reflected from a surface. The more photons deflected or bent by a surface, the greater the intensity.

Facts

Light emitted from the sun or fluorescent lamps is unpolarized. This means that the photons vibrate differently and travel different in directions.
Paper can polarize light. For example, photographs developed from polaroid cameras use the polarizing quality of the special film paper to absorb light parallel to the fine structures within the paper. Light that travels perpendicular to the paper passes through and is detected by the film, producing an instantaneous image.
British television sets are built differently than American television sets. British TV's have vertical antennas, where Americans have horizontal TV antennas. The different direction of their antennas is due to the manner in which the signals are emitted. American television stations emit their signals to vibrate in a horizontal plane. British television stations emit their signals to vibrate in a vertical plane.

Observing Polarized Light

Although we cannot see it happening, light vibrates as it travels. Light from the sun or from a light bulb vibrates in all directions at once. When light is reflected from water, asphalt, or other non-metallic surfaces, it becomes polarized. That means the light that is reflected from the surface will now vibrate more in one direction than in other directions. A polarizer will only allow light that is vibrating along one direction to pass through.

Materials

2 pieces of polarizing material per student (2" x 2" squares)
1 clear light bulb and small lamp for each work station
1 piece of shiny opaque plastic per student

Assembly

None required.

Experiment

Place the light bulb with its filament parallel to the surface of the plastic.
Look at the reflection through a piece of polarizer.
Vary the angle at which you look at the plastic until you get the dimmest reflection. (Do this by moving the plastic up and down with respect to your eye.) The dimmest reflection should be evident at about a 35° angle.
Rotate the polarizer by 90° as you watch the reflection. The reflection should become much brighter.
Observe reflections in the room around you (e.g. off of windows, etc.)
Rotate the polarizer and adjust the viewing angle to vary the brightness.

Other things to observe with the polarizer

Reflected light from a mirror: no difference in brightness using the polarizer
LCD display on a calculator: the numbers will disappear at some angles of the polarizer
The sky: the brightness changes as you rotate the polarizer the surface of a pond on a bright, sunny day: at one orientation of the polarizer, the surface reflections are reduced and you can see below the surface of the water.
Snow when skiing
Glare from asphalt while driving

Since it won't be possible to observe some of these things in the classroom, it is best if the polarizers can be given to the students for future observations.

Inside Information

The light bulb produces unpolarized light--each photon is vibrating in its own different direction. Non-metallic surfaces tend to reflect light that is vibrating parallel to the surface and to transmit or absorb light vibrating in all other directions. (What happened in our case with the plastic? Was the rest of the light transmitted or absorbed? What could you base your answer on?) In our case, if the plastic is horizontal, then it reflects light that is vibrating horizontally, i.e. horizontally polarized light. The plastic reflects less light that is vibrating vertically. It isn't perfect at separating the two--if it were, we would see no light at all when the polarizer was oriented in one direction. Point out that this is what is behind polarized sunglasses.

String Analogy

A plucked string serves as a mechanical analogy to the polarization of light by reflection. This demonstration explores the analogy between the transverse vibrations in string and the transverse wave motion of light. In this activity, strings represent a ray of light striking water (the incident ray), a ray of light reflecting from the surface, and a ray of light transmitted through the surface. When the "incident ray" is plucked, you can see that the vibration in the "reflected ray" becomes polarized.

Materials

A board (at least 12" by 12")
Three nails
One sturdy rubber band
String
Colored tape

Assembly

Tie the sting into a Y-shaped configuration as shown in the diagram. Attach rubber band to the end of the reflected ray, as shown.
Place the nails and fix the strings in place as indicated.
Make sure the strings are taut when you fix them in place.
See diagram for string lengths and dimensions.

Note: Since the behavior of strings and light waves are not identical but merely analogous, the angles used here do not exactly correspond to the polarization angles of light reflecting off water. Label the top part of the board "AIR" and the bottom part "WATER." Use a colored piece of tape to mark the interface. Place the tape so that it passes right below the point of reflection.

Demonstration

Stand the board on edge on a tabletop so that the tape which marks the water surface is horizontal and the water side is down. Pluck the incident ray horizontally, vertically, and diagonally, in turn. Notice that only the horizontal component of the vibration appears in the reflected ray. This matches the behavior of light reflecting from a non-metallic surface.

Polarized Light Mosaic

Using transparent tape and polarizing material, you can make and project colored patterns that are reminiscent of abstract or geometric stained glass windows. Rotating the polarizer as you view the patterns causes the colors to change. With some creativity, you can also create colorful objects or scenes.

Materials

Two sheets of polarizing material (about 8" x 11")
Transparent tape (inexpensive cellophane--Scotch brand will not work)
A roll for each student, if possible.
A piece of glass or Plexiglas for each student (plastic will not work)
Overhead projector

Assembly

Before buying large quantities of cellophane tape, test the brand of tape by placing a strip between two pieces of polarizing material.
Rotate the polarizers with respect to each other. If the tape changes from dark to light, then the tape will work in this activity.
The reason sheets of plastic should be avoided is that plastic will add color to the design. Glass and Plexiglas will not.
Distribute tape and Plexiglas sheets to the students.

Experiment

Show the students the idea by putting strips of tape on a piece of Plexiglas in a criss-cross, random pattern.
Be sure there are regions where two or three strips of tape overlap.
Sandwich the glass between the polarizers and place the whole thing on the overhead projector.
Rotate the top polarizer and observe the color changes. If the students are advanced enough, they can cut the tape with scissors so that it makes desire d shapes or letters.

Inside Information

The colors that you see result from differences in the speed of polarized light as it travels through the cellophane tape. In cellophane tape, the long polymer molecules are stretched parallel to the length of the tape. Light polarized parallel to the stretch of the molecules travels more slowly than light polarized perpendicular to the stretch.Every material has an index of refraction -- that is, the ratio of the speed of light in a vacuum to the speed of light in the material.

Light travels through the tape used in this demonstration at two different speeds. Materials with this property are called birefringent, which means "doubly refracting. When polarized light enters the tape, its direction of polarization will probably not line up with the length of tape. If the light is polarized in a direction that does not line up, its direction of polarization will be split into two perpendicular components. One of these components will be parallel to the length of the tape, and one will be perpendicular.

The waves which make up the two components are initially in sync with each other. But as they travel at different speeds through the tape, they become out of sync. In other words, the crest of one wave no longer lies up with the crest of the other. When these out-of-sync light waves emerge on the other side of the tape, they recombine, making light with a different polarization than the original light.The thicker the tape is, the more out of step the components will get, and the greater the change in polarization will be. If, for example, the two waves recombine after one has been delayed by one-half a wavelength, the direction of polarization will be rotated by 90°.

The white light shining from the overhead projector is made up of light of all different colors, or wavelengths. Since the index of refraction of the tape is different for each color of light, each color has its own unique pair of speeds as it passes through the tape. The result is that the polarization of each color is changed by a different amount for a given thickness of tape. When a second piece of polarizer is placed over the tape and rotated, it transmits different colors at different angles. This accounts for the color combinations that you see at a given angle, and for the changes in color as the polarizer is rotated.

Rotating Light

In this experiment, there are some strong similarities with the polarized light mosaic experiment. Both the Karo syrup and the cellophane tape change the direction of the light's polarization as it passes through. However, while the cellophane tape exhibited birefringence, the sugar solution exhibits optical activity. This distinction can be omitted when dealing with younger grades. When polarized white light passes through a sugar solution, each color's direction of polarization is changed by a different amount.It is possible to see the colors change as the depth of the solution changes, or the Polaroid filter is rotated.

Materials

A tall thin beaker or graduated cylinder (100 ml size works well)
Two sheets of polarizing material
Karo syrup
Light source (flashlight or overhead projector)
Clear plastic cylinder 1" in diameter with one closed end (solid or hollow)
Colored filters (red and green are especially helpful)

Assembly

Fill the beaker or graduated cylinder with several inches of Karo syrup.
Put one piece of polarizing material under the bottom of the base and hold the other above the top of the tube.
Put a light source, such as the overhead projector, below the bottom polarizer.
Make a dual-color filter: Take a white piece of paper (the size of notebook paper) and cut two holes in it. One should be the size of the base of the cylinder holding the syrup, the other can be smaller. Cover half of the larger hole with a piece of red filter, and the other half with a piece of green filter.

Experiment

Look down the tube at the light source.
Slowly rotate the top polarizer and notice the color changes in the syrup. (Note: if you are doing this on an overhead projector, the image of the light from the beaker can be put on the screen. Alternatively, each student could look directly into the beaker.) Now hold the polarizer still and change the depth of the syrup. You can do this by pouring more syrup into the tube, or by pushing the smaller cylinder into the syrup to change the height of the liquid. You can use colored filters to show that different colors of light travel at different speeds through the syrup.
Take the dual color filter and place it between the base of the cylinder and the first polarizer.
Now when you rotate the top polarizer, you will see that the red and green light is extinguished at different times. You can perform a quantitative experiment by placing a solid colored filter under the cylinder. Suppose you use a blue filter.
In order to see the blue as you add syrup, you must slowly rotate the upper polarizer. Determine the depth of the syrup required to rotate the direction of the polarization of the blue light by one full turn.
Try this experiment again for a red filter. With red light, a greater depth of syrup is required.

Inside Information

This experiment builds on concepts introduced in Polarized Light Mosaic. Light from the initial light source (the flashlight or overhead projector) vibrates in all directions. The first polarizer allows only light polarized along one direction to pass through. So as the light enters the Karo syrup, it is all polarized in the same direction.When polarized light passes through the Karo syrup, the direction of its polarization is changed. It is rotated to a new angle. The amount of rotation depends on how much syrup the light passes though. The more syrup it passes through,the more it is rotated. Also, the more concentrated the sugar solution, the greater the rotation.

In addition, the amount of rotation depends on the color (wavelength) of the light. Blue light(shorter wavelength) rotates more than red light (longer wavelength).When the light emerges from the sugar solution, each color in the light has its own direction of polarization. When viewed without a polarizing filter, the light will appear white. This is because our eyes cannot detect the direction of polarization of light, and we see only the colors of the light. Since all colors are present, we detect it as white light.

When you view the light through a polarizing filter, you are only permitting light of one polarization, and hence of one color, to enter you eye. So in this case, we can distinguish individual colors as the polarizer is rotated. Materials which change the orientation of the polarization of light are called optically active. Optically active materials may either rotate the polarization clockwise or counter-clockwise.

All organically-made materials which are optically active rotate polarizations in a clockwise fashion. All the proteins in all organisms on earth are composed of amino acids which are clockwise optically active. But amino acids synthesized in the laboratory, and those found on meteorites, are clockwise optically active about half the time, and counter-clockwise optically active the other half. The reason for this is a mystery. One advantage of this state of affairs is that counter-clockwise optically active sugar molecules can be made synthetically. These taste just as sweet as natural, clockwise optically active sugars; but since our bodies can't process them, we can eat them without adding calories to our diets!

Why is the Sky Blue?

This experiment will illustrate two effects that the atmosphere has on sunlight. Both effects are the result of scattering of the sun's light by the particles and gas molecules in the atmosphere. First, in keeping with the theme of this lesson plan, the scattering causes the light to become polarized. (This is why the sky appears brighter at certain orientations of a polarizer, as mentioned in the "Observing Polarization" activity.) Second, because blue light is preferentially scattered, the blue-colored appearance of the sky results.

Materials

Transparent plastic box, or a large beaker, jar, or aquarium
Flashlight or projector (slide or film)
Powdered milk or Maalox
A sheet of polarizer
Blank white cards for image screens

Assembly

Fill the container with water.
Place the light source so that the beam traverses the container.
Add the powdered milk or Maalox a little at a time and mix it in, until you can clearly see the beam shining through the liquid.

Experiment

Look at the beam from the side of the tank and then from the end of the tank.
Use a white card so that you can see the light more clearly.
Ask the students to describe the light that the see in the two positions.
They should see that the light at the end appears yellow-orange, while the light from the side appears bluish.
If you have added enough milk or Maalox to the water, you will be able to see the color of the beam change from blue-white to yellow orange along the length of the beam.

Inside Information

As previous activities in this lesson plan have noted, the sun's white light contains light of all different colors. Each color of light corresponds to a wave of a certain wavelength and frequency. In order of shortest frequency to longest, the colors are: violet, indigo, blue, green, yellow, orange, and red. When white light from the sun passes through the earth's atmosphere, the light is hit by the particles and gas molecules that make up the atmosphere. The shorter the wavelength of the light, the more it is scattered by the atmosphere. So blue light and violet light are scattered more than red light.

The light that is not scattered as much as the blue continues on its original path. Because the scattered light is now traveling in all directions and the higher frequency light is only going in one direction. So the blue light reaches your eye from all directions in the sky, and we see the sky as blue. Some students may ask why the sky does not appear violet, since violet has a frequency even shorter than blue and hence is scattered more than blue. The reason is that the sun's spectrum contains much more blue than it does violet, so the effect of the violet is overwhelmed by the blue. The second important point to consider is that the scattered light is polarized.

Scattering polarizes the light because light is a transverse wave.

Evaluation

After completing theses lessons the students should understand:

The light spectrum is composed of different types of light that differ in oscillation or frequency and wavelength.
Polarizers filter light. The physical properties of the polarizer determine which frequencies of light are absorbed and which are reflected.

Stress Detection

Polarized light reveals stress patterns in clear plastic. This is very closely elated to the previous activity using cellophane tape. The same principles are involved.

Materials

Overhead projector
Two sheets of polarizer
Transparent plastic objects: picnic silverware, CD holders

Assembly

Adjust the overhead projector so that the light is shining on the screen. Place one of the polarizers on the stage of the overhead projector. Either tape the other polarizer over the lens of the projector, or be ready to hold it over the objects you place on the screen.If you have a thin sheet of plastic, you can cut shapes, such as letters, out of the plastic to use in your observations.

Experiment

Hold a clear plastic object above the first polarizer and below the second polarizer. Carefully bend or flex the object. Note the colored pattern that appears in the image of the object on the screen. Rotate one of the polarizers and note the effect on the colors.

Inside Information

By flexing or bending the object, you induce a stress on it. Since the different parts of the plastic undergo different amounts of stress, the structure of the plastic is altered more in some areas than in others. The portions of the plastic that exhibit rapid changes in color are areas of high stress. Likewise, in areas of low stress, the color patterns will change more slowly. Areas that are likely to be high stress areas are usually corners, or areas which have been cut or stamped.