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Give and Take

Science Snack
Give and Take
Dark-colored materials both absorb and emit energy more readily than light-colored materials.
Give and Take
Dark-colored materials both absorb and emit energy more readily than light-colored materials.

Using a card or strip made of temperature-sensitive liquid crystal material, you can monitor temperature changes. By observing these changes, you can show that dark-colored materials absorb and re-emit the energy contained in light more readily than light-colored materials.

Tools and Materials
  • A black marking pen
  • A metallic silver marking pen
  • Temperature-sensitive liquid crystal material strip or card
  • Desk lamp with an incandescent bulb or sunlight

Use the marking pens to color one half of the back of the liquid crystal card black (if it isn’t already black) and the other half silver (click to enlarge image below).

To Do and Notice

Hold up the card so the silver-and-black side that you colored with the marking pens is facing the light source. Hold the card a few inches away from the lamp. Or, if the sun is your light source, just hold the card in the sunlight. (Note: this Snack does not work with LED or fluorescent lights. In sunlight, it works best when it's bright out but not too hot or too cold.)

Watch the liquid crystal side of the card. Notice that the side with black on the back changes color faster than the side with silver on the back. This color change indicates that the blackened side is changing temperature faster than the silvered side.

Let the card cool until the liquid crystal is black again. Then heat up the card by touching it to your hand or forehead, or by shining light onto the liquid crystal side. Remove the card from the heat and watch the liquid crystal as it cools. The black side should cool faster than the silver side.

What’s Going On?

Dark-colored materials absorb visible light better than light-colored materials. That’s why the dark side of the card heats up first. The lighter side absorbs less of the incident light, reflecting some of the energy. Darker materials also emit radiation more readily than light-colored materials, so they cool faster.

You may be tempted to skip coating half of the card with the silver marker. After all, that half is probably white, which indicates that it reflects light in the visible portion of the electromagnetic spectrum. But, although the white paper reflects visible light, it also absorbs infrared light. If you could see infrared light, the white paper would look black when illuminated with infrared.

Unlike plain white paper, silver paint reflects infrared light as well as visible light. The white paper is an infrared absorber, and so it is also a good infrared emitter: It will cool almost as fast as the blackened paper. The silver is a good infrared reflector and a poor infrared emitter: It will cool more slowly than the blackened side. Therefore, the heating experiment with visible light will work with black-and-white halves of the card, but the cooling experiment will not!

Even with the silvered coating, the cooling effect is harder to observe because the card is cooled by conduction and convection in addition to radiation. This is in contrast to the heating experiment, where the only heating is from radiation.

Going Further

The reflective power of a given surface is called the albedo ("whiteness" in Latin), which is the ratio of reflected energy compared to the amount hitting the surface. An opaque object that reflects 80% of the electromagnetic energy has an 80% albedo and absorbs 20%.

Albedo is an important concept related to climate change. Snow and ice have a high albedo and reflect most of the sun's energy back into space. Water, soil, and plants have a lower albedo, and absorb more solar energy, which contributes to warming and heating in the atmosphere. This, in turn, leads to more melting of snow and ice, which leads to more warming, especially in polar regions. This cycle is an example of a positive feedback loop.

The term liquid crystal sounds like an oxymoron. However, if you examine molecules of a liquid crystal that are close to one another, they will be arranged in an orderly structure, like a crystal. If you examine molecules that are separated by longer distances, the molecules will be disordered, as they would be in a liquid. Liquid crystals, therefore, have a short-range order, like a crystal, and a long-range disorder, like a liquid.

Temperature-sensitive liquid crystal material is cholesteric—that is, arranged so that the long, rod-shaped molecules sit side by side in layers, with each layer at a slight angle from the layer above and below it. You could picture this arrangement as a spiral staircase, with the molecules as the risers in the stairs.

When the liquid crystal is heated, two things happen. First, the layers move farther apart. Second, the angle between the molecules in each layer increases. These two changes combine to cause the spiral staircase of molecules to wind up tighter as the crystal heats up. As a result, at lower temperatures, the spiral matches and reflects visible light with longer wavelengths—red light. At higher temperatures, the spiral matches and reflects light with shorter wavelengths—blue light.