Liquid crystal sheet (available at
museum, nature stores, and science
supply catalogs)
Table top
Procedure:
1. Have a student touch his or her fingertips on a table top for 30 seconds. Make sure the student has warm hands.
2. While handling the liquid crystal sheet only by its edges, place it where the fingertips touched the table. Observe what happens over the next several seconds.
Discussion:
Infrared telescopes have a detector sensitive to infrared light. The telescope is placed as high up in the atmosphere as possible on a mountain top, in an aircraft or balloon, or flown in space because water vapor in the atmosphere absorbs some of the infrared radiation from space. The human eye is not sensitive to infrared light, but our bodies are. We sense infrared radiation as heat. Because of this association with heat, telescopes and infrared detectors must be kept as cool as possible. Any heat from the surroundings will create lots of extra infrared signals that interfere with the real signal from space. Astronomers use cryogens such as liquid nitrogen, liquid helium, or dry ice to cool infrared instruments.
This activity uses a liquid crystal detector that senses heat. Also known as cholesteric liquid crystals, the liquid inside the sheet
exhibits dramatic changes in colors when exposed to slight differences in temperature within the range of 25 to 32 degrees Celsius. When a student placed his or her fingers on a table top, heat from the fingertips transferred to the table's surface. The liquid crystal sheet detects the slight heat remaining after the student's hand is removed. A visible image of the placement of the fingertips emerges on the sheet. In the case of an infrared telescope in space, the energy is detected directly by instruments sensitive to infrared radiation. Usually, the data is recorded on computers and transmitted to Earth as a radio signal. Ground-based computers reassemble the image.
For Further Research:
- How was infrared radiation discovered?
- Why do infrared detectors have to be kept cold?
- Learn about cholesteric liquid crystals. An Austrian botanist Freidrich Reinitfer discovered them in 1888.