What it shows:

Certain materials (sugar in this experiment) are optically active because the molecules themselves have a twist in them. When linearly polarized light passes through an optically active material, its direction of polarization is rotated. The angle of rotation depends on the thickness of the material and the wavelength of the light.

How it works:

To show that a given thickness of optically active material will rotate the polarization of different colors by different amounts, we use a polarimeter on the OHP. Although the one described in the Malus' Law demonstration could very well be used here, we prefer a dedicated setup which allows for a more quantitative analysis. A linear polarizer is cemented into the center of a 360° plastic protractor. The sugar solution (or other liquid ¹ ) is contained in a 400 ml beaker and is placed on the polarizer. A plastic cylinder, with an analyzer polarizing filter glued on its top, is placed over the beaker and the entire ensemble sits on the OHP. A marker on the cylinder shows the relative orientation of the polarizing and analyzing filters on the protractor scale.

To understand how it works, think of the linearly polarized light (produced by the polarizer) as a superposition of equal amounts of right and left circularly polarized light. The optically active solution has a net handedness, like that of its individual molecules, and thus the solution has different indices of refraction for right and left circularly polarized light. Therefore, depending on which handedness, one of the circular components will get ahead of the other in phase and this changes the direction of the linear polarization--the direction of rotation being the same as the direction of rotation of the faster circular component. As with birefringence, the effect is frequency (color) dependent.

As an example, suppose that the thickness and optical activity of the substance is such that the polarization direction of blue light is rotated by 90° while red light is hardly rotated at all. If the analyzer and polarizer are oriented parallel, the blue light will be blocked and the transmitted light will look yellowish (white light from projector minus blue light). The color will change as the analyzer is rotated. At the other extreme, when the analyzer and polarizer are perpendicular, the red light will be blocked and the resulting transmitted light would be greenish blue (cyan = white - red).

Setting it up:

An OHP with a screen or a light box with a color camera provides the light source and means of observation. Have the liquid in a separate beaker to be transferred to the polarimeter beaker during the course of the demo in the lecture.

Comments:

A few reminders about some sugars: Glucose (C₆H₁₂O₆) is also known as dextrose because solutions of glucose rotate the plane of polarized light to the right. ² Fructose (also C₆H₁₂O₆ but differs from glucose in the location of the doubly bonded oxygen atom) is also known as levulose because it behaves oppositely to glucose in that it rotates polarized light to the left. ³ Sucrose (C₁₂H₂₂O₁₁) is our common table sugar. ⁴ It is a molecule consisting of two glucose-like units; one unit is a six-member glucose ring and the other is a five-member fructose ring. When the enzyme invertase is added to a sucrose solution, the two parts of the sugar molecule are separated, giving a solution of equal parts of glucose and fructose known as invert sugar, the name stemming from the fact that it rotates polarized light in the opposite direction of sucrose.

Some food for thought: What happens if we send light through a sugar solution, reflect it from a mirror, and send it back through the solution in the opposite direction? Does the rotation get doubled or canceled?

References:

F.S. Crawford, Jr., Waves, Berkeley physics course - vol. 3, (McGraw-Hill, NY, 1968) chapter 8.

P.W Atkins, Molecules, (Scientific American Library, NY, 1987) pp 95 - 99.

1 Kerosene may also be used.
2 Glucose occurs in ripe fruits, the nectar of flowers, leaves, saps, and blood and is variously called starch sugar, blood sugar, grape sugar, and corn sugar.
3 Fructose is also known as fruit sugar since it is in many fruits and vegetables. It is the major sugar in most types of honey because nectar contains a high proportion of fructose.
4 Sucrose is abundant in sugar cane and sugar beet, which are the principal sources.