Rotating Light

1. Fill the cylinder with several inches of Karo syrup.
2. Use tape to hold one piece of polarizing material under the bottom of the cylinder and hold the other over the top.
3. Place the light source below the bottom polarizer. (In the diagram below, the light source is the illuminated piece of white paper. Click to enlarge the image.) 
   

Look down through the polarizing filter into the cylinder at the light source. Slowly rotate the top filter and notice the color changes in the syrup.

As you continue looking through the top filter, pour more syrup into the cylinder. Notice that the color changes as the depth of the syrup changes. See if you can keep the color constant by rotating the filter as the syrup level gradually rises.

You can perform a quantitative experiment by adding a 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 polarizing filter. Determine the depth of syrup required to rotate the polarization of blue light by one full turn. Then try the same experiment with a red filter. With red light, a greater depth of syrup is needed.

Try a variety of transparent liquids and solutions such as honey and sugar syrup. Some are better than others at changing the direction of polarization.

Light from most ordinary light sources wiggles up and down, left and right, and diagonally. Your polarizing filter lets through only the light that is vibrating in one particular direction. In this polarized light, the light waves all wiggle in the same direction.

To understand what this means, picture waves traveling along a rope. If the waves vibrate up and down, they are vertically polarized. Vertically polarized rope waves can pass through the slots between the vertical slats in a fence; waves vibrating in other directions are blocked by the slats. If you orient a polarizing filter properly, vertically polarized light waves can pass through the filter, while waves vibrating in other directions are blocked.

The light emerging from the light source at the bottom of the tube is unpolarized. That means it vibrates in all directions perpendicular to the light’s direction of motion. The polarizing filter under the sugar solution polarizes this light so it vibrates in one direction only.

When polarized light passes through the Karo syrup, the direction of its polarization is changed. Light vibrating from to side to side, for example, might end up vibrating at a 45-degree angle. The amount of rotation depends on the depth of the syrup: The angle of rotation is proportional to the depth. It also depends on the concentration of the syrup: The more concentrated the syrup, the greater the rotation. Finally, the angle of rotation depends on the wavelength or color of the light. Blue light, with its shorter wavelength, rotates more than longer-wavelength red light.

When the white light emerges from the sugar solution, each color in the light has its own direction of polarization. When viewed without a polarizing filter, this light still appears white because our unaided eyes cannot detect the direction of polarization of light. However, when you look through a second polarizing filter, you see only the light that is vibrating in a direction that can pass through the filter. Only certain wavelengths or colors of light have the appropriate polarization. The intensity of the other colors in the light, which have different directions of vibration, is diminished. If a certain color of light has its polarization perpendicular to the axis of the polarizing filter, it is blocked out completely. (Think about the fence again. The rope waves won’t get through if they are vibrating perpendicular to the slats.) As you rotate the filter, each orientation of the rotated filter produces a different dominant color, as does each different depth of sugar solution.

Materials that change the orientation of polarized light are called optically active materials. Some optically active solutions rotate the direction of polarization clockwise, to the right; ­others rotate it counterclockwise, to the left.

All organically produced glucose rotates the direction of polarization of light clockwise. This sugar is called d-glucose. Another sugar, called l-glucose, rotates the direction of polarization counterclockwise. It can only be made by inorganic chemical synthesis. Both d-glucose and l-glucose have the same chemical formula: C₆H₁₂O₆. However, the atoms in each of these isomers are arranged in a different pattern. The left-handed sugar (l-glucose) tastes just as sweet as the right-handed one (d-glucose), but your body can’t use it as an energy source. That’s how left-handed sugars can produce sweetness without calories.

All the proteins in your body and in all organisms on Earth are made from amino acids that rotate the direction of polarization of light counterclockwise. On the other hand, laboratory-­synthesized amino acids, and amino acids found on meteorites, are made up of equal numbers of amino acids that rotate light to the right and amino acids that rotate light to the left. No one knows why this is so.