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Light, Color, Optics: Exploring Visible and Invisible Light

  • 2009 20 May
Light, Color, Optics: Exploring Visible and Invisible Light

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 Imaging

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.


Now that we have a basic understanding of light and the color spectrum of visible light, how do we actually see light? One could easily write several books about the intricacies of how the eye functions. But let’s take a quick look at the basics. Remember, light travels in waves. These light waves enter our eyes through the cornea and lens. The cornea and lens work together to focus the light onto the retina. The retina is full of cells (rods and cones) that change the received light waves into nerve impulses that are then collected by the optic nerve and sent to the brain to be interpreted. It is important to understand that a person’s eyes simply help deliver information to his brain. The brain has to then sort out the information and interpret it. We generally take this capability for granted, but try to do the following experiment to understand how our brains are involved in the seeing process.

Are You Seeing This?

Take a look at the picture below. What do you see?

Most people see a white circle. But look carefully; there really isn’t a circle anywhere—just straight lines. Your eyes automatically filled in the missing information in order to see the circle. This is actually an amazing part of how our brain can interpret the light that is sent to it. Without this capability, we would be unable to identify objects that are only partially visible.

Also, there is a still image of a very unique optical illusion. To experience this motion machine, log onto; click on “activities” and then click on “optics.”

Practical Applications

The practical applications of thermal imaging are endless. Following is just a brief list.

  • Search and rescue
  • Detecting fires in a home or forest
  • Diagnosing disease
  • Identifying heat loss in a house so it can be insulated better
  • Law enforcement—search for and identify suspects in buildings
  • Weather forecasting
  • Military—locating targets, gathering intelligence, surveillance
  • Personal Application

Many people wish they could see God and the spiritual world. Some people feel that just because you cannot see something, then it must not be there. But that would be like saying, “I cannot see ultraviolet light, so it must not exist.” Indeed, often what we don’t see affects us much more than what we do. Second Corinthians 4:18 says it this way: “While we look not at the things which are seen, but at the things which are not seen: for the things which are seen are temporal; but the things which are not seen are eternal.” Remember, it is the eternal things that are the most important.

Enjoy your science!

Published on May 20, 2009

John Grunder and his wife Susan began homeschooling in 1993 and recently graduated their oldest of four. For fifteen years John taught science, math, and technology at Christian and public schools. In 2002 he started Exploration Education™ and has authored both elementary and intermediate homeschool science curricula. For more information and additional science activities, visit their website:

Copyright 2008. Originally appeared in The ld Schoolhouse Magazine, Winter 2008/09.
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