Light, Color, Optics: Exploring Visible and Invisible Light
- Monday, May 11, 2009
One cold November afternoon in Nebraska, a young boy named Trevor Lewin became lost in a heavily wooded ravine. After unsuccessfully searching for her son, Trevor’s mom contacted the Sheriff’s Department, where a search team was immediately dispatched. Still not finding the boy by 10 p.m., the search and rescue team requested assistance from the National Guard, who sent a helicopter equipped with a thermal imager (heat sensing camera). At midnight, one of the crew members saw a thermal image of what appeared to be a small person next to a tree. The rescue team was alerted, and the cold but otherwise healthy boy was found. Had the helicopter crew not been able to “see” in the infrared range of light, the boy may not have been found. So how could the National Guard “see” the boy, but the search and rescue team couldn’t? To answer that question let’s first take a look at the nature of light.
What do we see?
When we think of light, we naturally think of what we can see. This visible light allows us to identify colors, objects, and our surroundings. But is visible light all there is? As our sense of sound is limited to a certain range, preventing us from hearing the high-pitched noises that dogs can hear, so our sense of sight is also limited to a certain range. We call it the visible range of light. However, there is also light all around us that we can’t see.
What is light?
Light is radiant energy. The entire spectrum of radiant energy is called the “electromagnetic spectrum.” Light travels in waves. At one end of the spectrum is ultraviolet light. Ultra means “beyond.” Our eyes can see the short wavelength of violet but nothing shorter or beyond that level. At the other end of the spectrum is infrared light. Infra means “below.” Our eyes can see light waves as long as red, but nothing longer or below that level. Some of the long infrared light waves can be felt as heat. A hot sidewalk can give off infrared light even at night. We cannot see it, but we can feel it. Shiny surfaces, like aluminum foil, reflect light. This is true of infrared (invisible) light as well. Many times take-out food, such as a burrito, is wrapped in aluminum foil. The infrared light (heat) coming from the hot food hits the aluminum foil wrapping and is reflected back into the food, keeping it warm longer.
Thermal means “heat.” So “thermal imager” is a fancy name for a heat sensing camera. These cameras detect the amount of radiant heat energy (infrared light) in an object or person and display it in a visible range of light that we can see. Getting back to the story of the boy, because it was dark there was no visible light reflecting off the boy to help the searchers find him. However, the boy was generating a lot of heat energy that the thermal imager could detect. The amount of infrared light (heat) coming from the boy was a lot greater than the amount of infrared light (heat) that was coming from his surroundings (trees, dirt, rocks), and so a thermal imager could detect him easily. This technology is so good that you can even see recent footprints left behind on the ground because they leave a heat signature that the thermal imager can pick up.
Humans cannot see infrared light, but there are animals that can. Pit vipers, like rattlesnakes, have sensory pits that allow them to image infrared light. This helps them detect warm-blooded animals at night or animals that are concealed in burrows.
As we have learned, the light spectrum is quite large. The small section in the middle is what we call “visible light.” Visible light has all the colors of the rainbow in it. When we see all these colors at once, like the light coming from the sun, it appears to be white. But if you break the light up into its different wavelengths using a prism, or like the clouds break up the light when we see a rainbow, then we can see all the colors in visible light. Here are some interesting questions: If sunlight has all the colors in it, what colors do objects have? Is a red ball really red? What happens if you take a red ball into a completely dark room? Is it still red? The reason we see colors is not because an object has color in it but because it reflects certain colors (wavelengths) and absorbs others. A red ball reflects the longer wavelength color of red and absorbs the rest of the wavelengths. A white ball reflects all the wavelengths of visible light. A black ball absorbs all wavelengths of light. Wearing black out in the sun can be hot, because the black clothing is absorbing the sun’s radiant energy.
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