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.
Temperature-sensitive liquid crystal material
Exploratorium Store).
Desk lamp, slide projector, or sunlight.
A black marking pen.
A metallic silver marking pen.
(5 minutes or less)
(available as postcards from the
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, rodshaped 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.