By middle school, most students are aware that prisms can bend light. It isn’t until high school physics that polarization usually gets discussed, and even then only in basic terms of polarizing filters and intensity. This is a chance to explore how certain chemicals interact with polarized light in a phenomenon called “optical rotation”.
THEORY
For a little bit of background, polarized light is light in which all the electric fields of the light waves are oscillating in the same orientation – up and down for instance. If nothing interferes, the light waves will continue to travel on, always oscillating in the same direction (called the plane of polarization). This is true whether the light is traveling through space, through the air, through a glass window, or through a glass of water.
Certain chemicals are special though – when light passes through them, they cause the direction of oscillation to change slightly. Some make the plane of polarization rotate slightly to the right (clockwise) and are traditionally called dextrorotary chemicals. Others cause rotation to the left (counterclockwise) and are traditionally called levorotary.
Certain crystals (like calcite) cause optical rotation, but more frequently of interest are chemicals that cause rotation while dissolved in solution. These are usually chiral compounds – chemicals that have a carbon with four different chemical groups bonded to it. This creates an asymmetry that leads to a tiny bit of light rotation.
Polarized light passing through a solution of an optically active chemical will rotate ever so slightly each time it encounters a molecule of the chemical. If the path through the solution is long enough, and the concentration of the chemical high enough, the rotation of light becomes measurable.
DEFINE YOUR PURPOSE
You can pursue this experiment in a few different ways. Here are a few options to consider.
Advanced
You already know about chiral chemicals, so use that knowledge to choose chemicals that you expect to be optically active. Use the experiment to check your predictions and to measure the optical activity of each. (Compare concentration, pathlength, and angle of rotation.)
Intermediate
You know chemicals, but aren’t familiar enough with structure to identify which ones are chiral. No problem – this is a great test for chirality. (It isn’t foolproof though, some chiral compounds aren’t optically active.) Choose a selection of chemicals and test to see which ones are optically active. Use this to identify which molecules have a chiral carbon in them.
Basic
You’ve never even set foot in a chemistry lab, and words like “chiral” blow your mind. No problem, stick to household chemicals, and just test to see which ones can rotate light. It’ll still be the same experimental set-up, and you still get the same neat effect, you just don’t have to research as far. If you want to, look up a picture of the molecular structure of the chemicals that do rotate light afterwards.
EXPERIMENT
Materials
Professional chemists spend $5000 or more on an instrument called a polarimeter to do this same experiment. Show that you’re smarter by making your own for much much less. This list is only a suggestion of what materials to use to make your polarimenter, you can improvise all you like.
Two polarizing filters. These can be sheet, film, or glass filters. (You can buy these at places like Edmund Scientific or American Science & Surplus).
Monochromatic light source. That’s a light that only gives off one color of light. You have options here.
– A bright flashlight or spotlight with one color filter works. You’ll find that yellow or green are the most visible colors to work with. (Again, science stores have these filters.)
– Similarly a light source with a colored bulb will do the trick.
– If you have a laser, you can use it, but you’ll probably need a couple lenses to make the beam wide enough to work with.
A sample chamber. You have to be able to put the test solution in something. The requirements are that it must have flat, glass sides for the light to pass through, and it has to be able to hold water. Try to get something that’s about ten to twenty centimeters long. One suggestion would be a small fish tank. (Remove the fish first.)
A balance.
Protractor, tape, pencil, ruler, white paper.
Water, and the chemicals you want to test.
Procedure
Note – all steps involving the light beam should be performed in a darkened room.
1. Align your polarizing filters. If they’re already marked with the polarizing direction, that’s great. Cheap ones may not be. Take one and mark opposite ends with tape to be your “top” and “bottom”. This defines a point reference that you will be able to call zero degrees. Stack the two polarizing filters.
Darken the room and shine your light source through the two filters and onto the paper. Rotate the second filter until you find the orientation that lets the least light through. Where the second filter lines up with your reference marks on the first filter, label both ends “90”. This is the point at which the polarizing directions of the two filters are exactly perpendicular to one another. Use your protractor to mark off other angle measures around the second filter. Ten degree increments should be good.
2. Set up your optical pathway. Tape the first polarizing filter to the front of your sample chamber (fish tank) so that your markings are up and down. Mount the light source so that it shines straight through the first filter and doesn’t go beyond the outer edges. (You may need to use some creative cardboard cut-outs to block extra light.) If you are using a color filter, you can also tape that to the front of the sample chamber.
Again, make sure that all of the light beam goes through both filters. Place your white paper somewhere beyond the sample chamber (about four inches back should be fine) so that you can see the light beam shining on it. Fill the sample chamber with water. Now hold the second filter between the sample chamber and the white paper. As you turn the filter, you should be able to see the light beam at its brightest when “0” is at the top, and the beam should be darkest when “90” is at the top. If this is all true, you’re good to go.
3. Measure and record the length of your sample chamber. (From inside the front wall to the back wall.)
4. Make your first test solution. Dissolve a weighed amount of your first chemical in a measured amount of water that will fill the sample chamber. If you’re a practiced chemist, you can try starting with a 0.001 M solution. If you aren’t, you can use trial and error. Remember to record the amount of chemical and water used in each mixture.
5. Place your test solution into the sample chamber. Once the bubbles have all cleared, turn your polarizing filter until the light beam on the paper is the brightest. Read the angle at the top of the filter to get your measurement. Remember to record whether it was to the right or to the left, too.
– If you get a reading of “0”, there are two possibilities. Either that chemical is not optically active, or your solution is not concentrated enough to se a change. Check by making a stronger test solution and seeing what happens.
– If you get a very small rotation, you can increase the concentration of the solution to get an easier to measure value. Always remember to record the angle along with the concentration (the recorded amount of chemical in the recorded amount of water) so that you can compare optical activity later.
6. If you want to calculate how much each chemical causes light to rotate, you can calculate a value using the concentration, angle of rotation and pathlength (length of the sample chamber.)
Any units of concentration can be used. For non-chemists, simply divide the mass (or weight) of the chemical you dissolved by the volume of water you used. Keep the units, so you’ll have an idea of what the number means.
Here’s a quick example:
Dissolved 50 grams of chemical X in 3 liters of water.
Sample chamber has a length of 15 cm.
The light beam was rotated by 5 to the right.
You can calculate how many degrees a light beam would rotate if it passed through a solution of 1 gram of the chemical in 1 liter of water with a 1 cm pathlength.
5 x 3 liters = ? x 1 liter
50 grams x 15 cm 1 gram x 1 cm
? = 0.02
If you do this calculation for every chemical, you’ll have an easy way to compare which ones cause the most rotation.
Note for chemistry students – using moles for your concentration will give more meaningful results than grams.
If you’re at a loss for chemicals that cause rotation, here’s a hint to get you started. When you try this one, it’ll make you say “That’s sweet!”
