Optical Activity of a Sugar Solution
We demonstrate the rotation of the plane of linearly polarized light after passing through a sugar solution in monochromatic and white light.
Optical activity is the ability of substances to rotate the plane of polarized light. This property can be interpreted as birefringence of the substance with respect to left- and right-handed circularly polarized light. The substance behaves as a material with different refractive indices depending on the left- or right-handedness of the passing light. Linearly polarized light can be described as a superposition of left- and right-handed circularly polarized light. The resulting phase shift between the individual circular polarizations manifests in the superposition as a rotation of the plane of linear polarization.
We let linearly polarized monochromatic or white light pass through a cuvette with a sugar solution that exhibits optical activity. Behind this cuvette, we place a second polarizer (analyzer), and by rotating it, we can observe changes in the image of the cuvette projected onto the screen.
If we rotate the analyzer in monochromatic light by about 35°, the light that has passed through the sugar solution is absorbed. Thus, the plane of polarization has rotated by about 35°. If we rotate the analyzer by 90°, so that crossed polarizers are formed, the light that has not passed through the sugar solution is absorbed.
White light consists of many wavelengths. For each of them, the phase shift between the circular polarizations is different, and therefore the angle of rotation of the plane of polarization is different for each color. For various analyzer rotations, different wavelengths of light are absorbed differently. As a result, by rotating the analyzer, we observe various colors on the screen.
The apparatus consists in sequence of: halogen lamp with an attached filter, polarizer, cuvette with sugar solution, second polarizer (analyzer), lens f = +50 mm, lens f = -100 mm, screen.
Detail of the cuvette with sugar solution.
Image on the screen in monochromatic light. Deviation of the second polarizer (analyzer) 0°.
Image on the screen in monochromatic light. Analyzer deviation 35°. A dark field is observed at the location of the sugar solution. The direction of polarization was thus rotated by about 35° when passing through the sugar solution.
Image on the screen in monochromatic light. Analyzer deviation 90°. A dark field is observed in the area outside the sugar solution. Crossed polarizers do not transmit light.
Image on the screen in white light. Analyzer deviation 0°.
Image on the screen in white light. Analyzer deviation 20°.
Image on the screen in white light. Analyzer deviation 30°.
Image on the screen in white light. Analyzer deviation 45°.
Image on the screen in white light. Analyzer deviation 70°.
Image on the screen in white light. Analyzer deviation 90°. A dark field is observed in the area outside the sugar solution. Crossed polarizers do not transmit light.