With white light you cannot tell this is happening. That is because the light is vibrating in all directions to begin with, so when it rotates it's still vibrating in all directions. Consequently, this property only becomes apparent when you work with polarized light, so let me take a moment to describe polarized light.
Rotating Polarized Light
When a chemical containing asymmetric carbon atoms (such as glucose or fructose) is placed between the polarizing filters, it is possible to observe that light polarized by the first filter is rotated before passing through the second filter.
Something else that you might be able to pick up and might not is that there is a color change through there. The light coming through the glucose between the crossed filters has a blue color. This is because different colors of light are rotated different amounts. At 90o there is a little bit of blue light getting through but the other colors are blocked out.
We have a demonstration of this ability to rotate polarized light set up in the lab for you to take a look at when you are in the lab. When you work with this demonstration, you will be able to see a variety of colors at various degrees of rotation of the top polarizing filter.
The reason that this is so important, however, is not because these particular compounds can rotate light. Instead, it has to do with the way that biological reactions take place. Biological reactions generally are catalyzed by enzymes and the enzymes are set up to specifically handle a particular orientation of atoms within a molecule. An enzyme that would be able to take D-glucose, and hydrolyze it, or oxidize it, or do something else to it would not be able to carry out that same reaction on the mirror image of D-glucose. That is because it wouldn't fit properly up against the enzyme as is necessary to carry out the reaction. As a consequence, different arrangements of the atoms in molecules, even just one or two of the OH's switched in position in the molecules shown above will make a biological difference. Therefore, it's very important to be very familiar with the orientations of the functional groups within a particular molecule.
E-mail instructor: Sue Eggling
Clackamas Community College