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Science Behind the Scenes: Digging Deeper How do certain types of liquid crystals change color when exposed to infrared energy? 1. How do we see the colors of the liquid crystals in the film? We see light reflected from the crystals that are warmed by our hands or the hair dryer. All objects that we see emit light like the Sun or reflect light from their surfaces to our eyes.. 2. What are liquid crystals? Much of matter exists as solid, liquid or gas. These are the most common experiences. Matter also exists as plasma, like the Sun. Matter can also exist in a form called a liquid crystal in which the particles are arranged something like a spiral staircase. Figure 1, on the right, shows how liquid crystals are organized somewhere between solids and liquids.
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4. What causes liquid crystals to change their orientation? When liquid crystals warm up they twist more. Remember when you turned the hair dryer on? You could feel the warm air.
Imagine the liquid crystals as a spiral “staircase” (the oval shapes.) The crystals are the “steps”. The spiral has to rise some distance before the particles twist around parallel to their original direction as shown in Figure 2, above. The distance the liquid crystals twist over is the pitch. This determines the color of light we see reflected from the liquid crystals. Outside the sensitive temperature range of this IR film (between 35-‐40° C) there is no reflected visible light, and the film looks black. Different IR films have different active temperature ranges.
Fig. 2
Fig. 1
Colors we see reflected from liquid crystals
less twist more twist As temperature changes, spirals, which are layers of liquid crystals, rotate and change in pitch (angle of twisting). Fig. 3
As particles begin to move more quickly, the film changes from a more solid to liquid crystal form. If the crystals are cold enough, they are very orderly and closely spaced. As the crystals get warmer, the spacing between the crystal particles increases. The color change observed is primarily due to an increase in the angle of twisting and a increase in tightening of the spiral staircase. (See Figure 3.) credits: figure 2 -‐ Nanotechnology Education: http://www.nnin.org/sites/default/files/files/Liquid_Crystal_Thermometer_Sidhu_TG.pdf figures 1 & 3 Uchicago: http://kicp-‐yerkes.uchicago.edu/2003-‐winter/pdf/ywi2003-‐liquid_crystals.pdf 5. What is the electromagnetic spectrum? (credit: NASA Introduction to the Electromagnetic Spectrum http://missionscience.nasa.gov/ems/01_intro.html When you tune your radio, watch TV, send a text message, or make popcorn in a microwave oven, you are using electromagnetic (EM) energy. Electromagnetic energy travels in waves and spans a broad spectrum from very long radio waves to very short gamma rays. Sometimes the electromagnetic spectrum is just called light. The human eye can only detect visible light, a small part of the EM spectrum. A radio detects a different portion of the spectrum, and an x-‐ray machine uses yet another portion. Locate infrared energy on the chart and notice that it is a range of wavelengths. 6. Infrared Astronomy http://missionscience.nasa.gov/ems/07_infraredwaves.html figure credits We can sense some infrared energy as heat and some objects as so hot they also produce visible light. Fire is a good example. Other objects, such as humans are not as hot and only produce infrared energy. Our eyes cannot see infrared energy waves, but we have detectors that can make that energy visible such as the infrared sensitive film, night-‐vision goggles or infrared cameras.
Many objects in the universe are not hot enough to give off visible light but can be detected in the infrared. Examples of cooler objects are planets, cool stars, interstellar dust and nebulae. Infrared waves can pass through dense regions of gas and dust in space with less scattering absorption. This allows infrared instruments to reveal objects that visible light optical telescopes may not detect
Constellation Orion shown in Visible Light, left and Infrared Light, right. In the infrared image on the right, the areas that are shown in yellow have more infrared energy than the darker areas.