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

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


(5 minutes or less)

Use the marking pens to color one half of the back of the liquid crystal postcard black and the other half silver.

Hold the side of the card that you colored with the marking pens toward the light source. Hold the card a few inches away from the slide projector or the lamp. If you are using a slide projector, remove the lens. If the sun is your light source, just hold the card in the sunlight.

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 side is black again. Then heat the card up by touching it to your hand 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.

Dark-colored materials absorb visible and infrared light better than light-colored materials. That is 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 infrared radiation more readily than lightcolored materials, and so they cool faster.

You may be tempted to skip coating one half of the card with the silver marker. After all, that half of the card is white, which indicates that it reflects light in the visible portion of the electromagnetic spectrum. But although the white paper reflects visible light, it 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 postcard, 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 due to radiation.

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 longrange disorder, like a liquid.

These temperature-sensitive liquid crystals are made from cholesterol. The long, rod-shaped molecules of the liquid crystal are arranged side-byside in layers, with each layer at a slight angle from the layer 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. Thus, 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.