|
In the November 2006 issue of this Journal, Lewis and Warren published an interesting letter describing some activities regarding liquid crystals (1). The idea was to shed some more light on the mechanism of functioning of LC devices. As the authors say: 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.…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. While the stand-alone quoted text is surely important and relevant, together with the first proposed activity it may be misleading. Lewis and Warren propose an activity using a calculator to introduce the concept of polarization. In the light of what has been quoted above, the reader may get the wrong impression that the activity has something to do with the applied electric field while the calculator is in use. The effect they demonstrate is, in fact, due to the polarizing film that the screen of the calculator is covered with. To prove this, dismount the screen from a cheap calculator, remove the thin reflective (aluminum?) surface and place it over a text typed on a white paper. Then, by means of another polarizer one can make the printed text visible (parallel polarizers, see Figure 1, left image) or completely dark (crossed polarizers, see Figure 1, right image). What the authors presented could be simply accomplished by two pieces of a polarizing film. It is not related to the response of the LC display under the applied electric field. 
Figure 1. The screen of a calculator viewed through: (left) parallel polarizers; (right) crossed polarizers.. Literature Cited- Lewis, D. L; Warren, M. J. Chem. Educ. 2006, 83, 1602.
|
