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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 itpasses through. However, while the cellophane tape exhibited birefringence, the sugar solutionexhibits 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 changedby a different amount.It is possible to see the colors change as the depth of the solution changes, orthe Polaroid filter is rotated.

Materials

y  A tall thin beaker or graduated cylinder (100 ml size works well)

y  Two sheets of polarizing material

y  Karo syrup

y  Light source (flashlight or overhead projector)

y  Clear plastic cylinder 1" in diameter with one closed end (solid or hollow)

y  Colored filters (red and green are especially helpful)

Assembly

1.  Fill the beaker or graduated cylinder with several inches of Karo syrup.2.  Put one piece of polarizing material under the bottom of the base and hold the other above the

top of the tube.3.  Put a light source, such as the overhead projector, below the bottom polarizer.4.  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 canbe smaller. Cover half of the larger hole with a piece of red filter, and the other half with apiece of green filter.

Experiment

1.  Look down the tube at the light source.2.  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 thescreen. Alternatively, each student could look directly into the beaker.) Now hold the polarizerstill 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 canuse colored filters to show that different colors of light travel at different speeds through thesyrup.

3.  Take the dual color filter and place it between the base of the cylinder and the first polarizer.4.  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 solidcolored filter under the cylinder. Suppose you use a blue filter.

5.  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.6.  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 lightsource (the flashlight or overhead projector) vibrates in all directions. The first polarizer allows onlylight polarized along one direction to pass through. So as the light enters the Karo syrup, it is allpolarized in the same direction.When polarized light passes through the Karo syrup, the direction of its

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polarization is changed. It is rotated to a new angle. The amount of rotation depends on how muchsyrup the light passes though. The more syrup it passes through,the more it is rotated. Also, themore concentrated the sugar solution, the greater the rotation.

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

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 thepolarizer is rotated. Materials which change the orientation of the polarization of light are calledoptically 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. Allthe proteins in all organisms on earth are composed of amino acids which are clockwise opticallyactive. 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 reasonfor this is a mystery. One advantage of this state of affairs is that counter-clockwise optically activesugar molecules can be made synthetically. These taste just as sweet as natural, clockwise opticallyactive sugars; but since our bodies can't process them, we can eat them without adding calories toour diets!