Expt 004 -- Kaleidoscoptical Sugar Rotation

Description

A radially polarized filter is placed on an overhead projector stage and a regular (parallel) polarized filter is positioned above it. As expected, the image produced shows four quadrants, alternating light-dark-light-dark. As an optically active solution (corn-syrup) is poured between them, the image rotates and separates into a beautiful spectrum of colors.

Chemical Concepts

  1. When a molecule contains a carbon atom bonded tetrahedrally to four different atoms (or groups), then two different versions of that molecule exist. These versions are mirror images of one another, but they are not the same -- that is, they cannot be superimposed. They represent a special type of isomer known as enantiomers.
  2. These molecules have the unusual property of being able to twist light waves passing by them: one isomer (designated D-) twists the light clockwise; its mirror-image isomer (L-) twists light counter-clockwise. Solutions containing such D- or L-molecules are said to be "optically active."
  3. The degree to which light is rotated as it passes through such a solution depends on several factors: the specific enantiomer, its concentration, the percent D- and L- present, the distance through the solution that the light is traveling, and the frequency of the specific light waves.
  4. Regular light is comprised of waves of all different orientations, and so when it gets twisted, it is impossible to tell. Thus this light-twisting effect is only detectable when using polarized light (light with only one orientation).

Applications

Polarimeters which measure the rotation observed in this demonstration are used to measure the concentration of sugar in many food processing operations. Wine makers routinely use a specially calibrated polarimeter to measure the sugar remaining as the wine ages.

Safety

Use ordinary laboratory safety procedures. Do not look directly into the overhead projector.

Procedure

  1. Turn the overhead projector on, place the cardboard mask on your overhead to block peripheral light, place the radially polarized filter directly on the stage, and show the spoke-like configuration of the wedges. Rotate the filter wheel to show the effect.
    !!!Click here to See Picture.

  2. Use a ring stand to suspend the regular polarized filter horizontally about 20-30 cm above the radial filter, or simply hang it over the lens that directs the light onto the screen. Rotate the filter wheel to show the effect.

    !!!Click here to See Movie. Click |> or <| to step the slides forward or back.

  3. Place a red plastic sheet under the radially polarized filter. This allows you to examine the rotational effect with just one frequency of light first.

  4. Center a modified plastic tennis ball container (see Lab Hints) or a 600-mL tall-form beaker on top of the radial filter. Gradually add a few cm of corn syrup. Note the rotation as the syrup is added.

    !!!Click here to See Movie. Overhead image from a wall. Movie is accelerated 5 times the actual pace of the demonstration.

    !!!Click here to See Picture.

  5. Remove the red sheet. Center a second beaker or container on the radial filter. Add the corn syrup slowly. Pause to allow the syrup to clear after a few centimeters. Continue adding syrup until the container is nearly full.

    !!!Click here to See Movie. Movie is accelerated 5 times the actual pace of the demonstration.

    !!!Click here to See Picture.

    Note to shorten the time: use one container; add only a few centimeters with the red filter; then remove the filter and continue adding.

Questions

  1. Summarize the principal finding of this experiment.

  2. Suggest other experiments with this apparatus to test properties of the sugar solution.

Handout Makeup

Name ___________________________ Class ________

Teacher __________________________

BeckerDemos 004 Kaleidoscoptical Activity

Watch the movies.

  1. Describe the changes when the red filter is present.
  2. Describe the changes when the red filter is removed.
  3. Answer the questions.

Curriculum-

Use when studying stereo isomers (or enantiomers). This experiment is best placed late in a first year course or in a second year chemistry course.

Activity-

Demonstration - Student or Teacher

Time-

Teacher Preparation: 5 min (Preparing the radial filter which may be used many times requires an additional 30-40 minutes.)

Class Time: 10 - 15 minutes

Materials-

See Lab Hints for instructions about building the filter wheel and container.

Disposal-

Store all materials including syrup for reuse.

Lab Hints-

A tennis ball can with plastic wrap has a flat bottom which displays less distortion than a tall form beaker.

Construction of Container:

  1. Cut off and discard the bottom one third of a plastic tennis ball container. Use sharp scissors.
  2. Fasten a piece of plastic wrap over the metal rim with a strong rubber band. Pull the plastic wrap firmly in all directions until a smooth surface is viewed. Any wrinkles distort the image on the overhead. Trim the excess protruding plastic wrap to a few mm with scissors.

!!!Click here to See Movie. Click |> or <| to step the slides forward or back.

Construction of Filter:

Use a zigzag pattern to cut the rectangular filter into 20 to 30 sharp wedges -- isosceles triangles with altitudes all running parallel to one another.

Each wedge changes the amount of light transmitted as the axis is rotate with respect to the axis of a second polarized filter.

!!!Click here to See Movie.

The specifications are not critical, but a possible template for cutting the wedges is shown below.

Use a paper cutter to cut the wedges. The more wedges made, the more acute each one must be; the top angle, of course, should approximate 360º/n, where n is the total number of wedges cut. Clean off any pen lines that may have been used as cutting guides, then rearrange the wedges into a pie configuration. Make sure that no pieces get flipped over. If they don't fit perfectly into a pie pattern, trim a few of the angles down, or cut one wedge to fit the gap. Test your wheel by placing a second filter a small distance apart and turning. You should see a butterfly image that turns as the second filter is rotated. Then, tape them together with 2-cm lengths of transparent tape along the outer seams.

!!!Click here to See Movie. Click |> or <| to step the slides forward or back.

Cut Blackout Paper:

Use a piece of cardboard or heavy dark paper with a large circular hole cut in the center to block out any peripheral light. Use your completed wheel as pattern, but cut the hole a bit smaller.

Observations-

The bright butterfly image on the screen appears to rotate as the corn syrup is added. That is, each wedge's image becomes progressively brighter and then darker in sequence around the circle as the plane polarized light passing through the wedge is brought into and then passed parallel alignment with the regular polarized filter above it. Furthermore, the leading edge of the rotating image turns red, and the trailing edge turns violet. This is a result of the fact that different frequencies of light have different rates of rotation through an optically active solution. At a corn syrup depth of 6-8 cm, with the bright butterfly image rotated approximately 90º, an entire spectrum of colors is clearly visible two-fold around the circular image. Most intriguing, the colors are not those of the rainbow, but instead are those of the additive light color wheel: magenta, cyan, etc. -- a beautiful kaleidoscopic display.

Answers-

Q1. Summarize the principal finding of this experiment.
A1. The light is rotated by the sugar solution. The degree of rotation depends on the amount of sugar in the path.
Q2. Suggest other experiments with this apparatus to test properties of the sugar solution.
A2. This question is open-ended and may lead to some new variations if asked during the demonstration. One obvious suggestion is to try different concentrations of sugar.

Reference-

Acknowledgements:

This is a variation on a demonstration shown to me by John Ihde of Wausau, WI. He uses two regular polarized filters oriented perpendicular to one another so that the image is completely dark. Then, as the corn syrup is added, a light patch gradually appears on the screen.

Further readings:

1. Noller, C. R. J. Chem. Educ. 1949, 26, 269-270.

2. Hill, J. W. J. Chem. Educ. 1973, 50, 574.

3. Kolb, D. J. J. Chem. Educ. 1987, 64, 805

4. Hambly, G. F. J. Chem. Educ. 1988, 65, 623.

5. Shakhashiri, B. Z. "Rotating Rainbows: A Solution in Polarized Light", Chemical Demonstrations, Vol 3, The University of Wisconsin Press, 1989, 386-389.

Key Words 1-

enantiomers, isomers, polarized light, optically active, stereoisomers, sugar, fructose