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Magic Patch
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Selective Reflection (Liquid Crystals)
Optics Suitcase

Remind the students that you have demonstrated how to see the colors in white light through diffraction and polarized transmission. Making sure that everyone is looking at you, take the large sheet of microencapsulated liquid crystal and place it against your face, shiny side out. [If you wear glasses, remove them first.]

  • Wait a bit for the “oohs” and “ahs” to die out, then ask... “Where do the colors come from?” [If some students say the heat from your face, answer no.]
  • Explain that the colors come from the white room lights reflecting off the black “paper”.
  • As the teacher passes out the Magic Patch Theme Packets, explain that the paper has liquid crystal fluid trapped, or encapsulated, against the shiny side. [Note: The large demo sheet and the give-away patches are not made from the same liquid crystal material. The patch material goes through a complete color change with a temperature change of 71° F to73° F. If the room is warm, you may have to open the ice pack to enable students to cool their patches down. Ice cubes also work well.]
  • Suggest that each student determine if she (he) is a vampire by placing the magic patch on the inside of her (his) wrist. [Vampires are the living dead and give off no heat.]
  • Ask if anyone can “see a vein”. [This would be characterized by a blue line.] To explain selective reflection in liquid crystals and “mood rings”, you may use the set of transparencies included as follows:

Liquid Crystals in Education Transparency

Students are laying on the grass, disguised as liquid crystal mole cules. In liquid crystals, the molecules show a high degree of organization and alignment in some regular order.

Liquid Crystals

Selective Reflection in Cholesteric Liquid Crystals Transparency

  • If we try to stack students (or liquid crystal molecules) in a series of layers on top of each other, they tend to twist as shown on the left of this picture. The heads of the students point in a spiral direction as we move up through the structure, like a spring.
  • Think of the liquid crystals in the black sheet as springs. When they are cool, the springs are relaxed with a long pitch (or twist ). [Refer to the lower right hand side of the picture.]
  • Most of the colors in white light pass right through the cool liquid crystal film and are absorbed in the black paper. The patch appears black.
  • As body temperature heats up the patch, the liquid crystal “springs” get tighter and they “selectively” reflect the red light part of the white room lights. As the temperature climbs, yellow, then green, and final- ly blue light is reflected. This is as tight as this liquid crystal gets.
  • The effect is reversible. Students may put their patches on an ice cube or under a mug of hot cocoa to see the effect of selective reflec- tion. Give a warning that the sheet should not be folded or marked with a sharp object like a pencil.

Selective Refection

A Subtle Message About Smoking Transparency

Mention that doctors may study circulation in the body with liquid crystals. On the left side of the picture, the blue color of the man's hand shows good blood circulation to the fingers. The picture on the right side shows that, after smoking one cigarette, the man's fingers reflect green because of reduced circulation. Nicotine constricts the blood vessels.

End by reminding the students that liquid crystals are found in every thing from cell phones to watches to laptop computers to GameBoys. What would the information age be like without the engineers and scientists who developed liquid crystals?

Blue warm, green cooler

Copyright by Stephen D. Jacobs, Rebecca L. Coppens and Christine Andrews-Angelo
December 24, 2001

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This site was last updated June 3, 2005.
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