Chemistry for Everyone
Letters Liquid Crystals Activity A Classroom Activity featured in this Journal relates to liquid crystals (LC). The article states that such materials are used in watches and calculators (1). The classroom activity focuses on diffraction explanations for the optical behavior of the substances used. The “Especially for High School Teachers” article in the same edition mentions the activity by introducing the subject with a question: “How does it (work)…(2)?” We suggest that devices employing a liquid crystal display (LCD) primarily take advantage of changes in the polarizing angle accomplished by applying a potential across a small element of the display. That surmise is buttressed by reports of an increase in production of polarizing film and a new production process that leads to a less expensive means of making “color filters” used in LCD television monitors (3, 4). We suggest that the LC in the LCD primarily functions in response to an applied electric field somewhat in the manner of a Kerr cell (5). We propose a short activity that makes use of devices that may be available in a high school setting: watches, cell phones, calculators, and flat screen computer monitors. The activity can serve as an introduction to the concept of polarization—an important property of light that has chemical significance with respect to optically active chiral substances. We use a calculator for the activity, but other LC devices can be used. Fill the display screen with numbers, then cover most of the numbers with a polarizing disk and rotate the disk so the numbers disappear. Rotating the polarizing disk will make the numbers reappear (see the Supplemental MaterialW for photos). Plastic polarizing discs used in the activity can often be obtained from the high school physics laboratory. The effect is observed in two kinds of display: those that use reflected ambient light (calculator and watch) and those that are backlighted (cell phone and flat screen computer monitor). Temperature effects that might indicate a phase change in the LC may also be demonstrated. Enter a calculation such as 1 + 2 + 3 but do not press “=”. Hold the display area
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near, but not touching, a hot 40 W light for ∼200 s. The screen will turn black. While the screen is black, enter “=”. When the screen cools, the correct sum is displayed (see the Supplemental MaterialW). W
Supplemental Material
Images of a liquid crystal display of a calculator showing the two demonstrations are available in this issue of JCE Online. Literature Cited 1. Lisensky, G; Boatman, E. J. Chem. Educ. 2005, 82, 1360A— 1360B. 2. Jacobsen, E. K. J. Chem. Educ. 2005, 82, 1273. 3. Business Concentrates. Chem. Eng. News 2005, Sep 5, 18. 4. Business Concentrates. Chem. Eng. News 2005, Sep 12, 14. 5. Kerr effect. http://en.wikipedia.org/wiki/Kerr_effect (accessed Sep 2006) Don L. Lewis Box 486 Bandera, TX 78003
[email protected] Mark Warren Bandera High School Bandera, TX 78003
[email protected] Editor’s Note: Additional information about the effects of electric fields on LCDs can be found in the article: Campbell, D. J.; Lorenz, J. K.; Ellis, A. B.; Kuech, T. F.; Lisensky, G. C.; Whittingham, M. S. J. Chem. Educ. 1998, 75, 297–312. Movies demonstrating liquid crystal melting are also available at http:// mrsec.wisc.edu/edetc/cineplex/LC/ (accessed Sep 2006).
Vol. 83 No. 11 November 2006
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