Let There Be Light!
- David M. Jones The Old Schoolhouse
- 2009 5 May
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Beginning in Genesis and going through Revelation, light is mentioned in the Bible more than one hundred times. “Let there be light” (Genesis 1:3), “Light of the world” (Matthew 5:14), and “I [Jesus] am the light of the world” (John 8:12, 9:5) are all from familiar verses of the Bible. In the Word of God, light is used to illustrate important points such as wisdom, illumination, and good vs. evil. But what is “light” from a scientist’s perspective?
From the ancient Greeks and Egyptians to Newton to Einstein to modern-day engineers, the quest to understand light and use it is rich with history. While this history is worthy of study in and of itself, this article will briefly touch on the fascinating science of light and color and describe a few inexpensive hands-on experiments that you can try at home.
What Is Light?
Like all other topics in science, knowledge builds upon knowledge. The ancient Greeks and Egyptians thought that light was something that emanated from our eyes. Today we know that light is a form of electromagnetic energy. Some light we can see; some we cannot. The light that we can see is called visible light. It is a small portion of the broad spectrum of electromagnetic radiation.
Light is unique in that it exhibits the properties of both a wave and a particle. If two people were holding a jump rope, and one of them snapped her wrist, the rope would move in “waves.” This motion gives us a good illustration of the way light travels in waves. That action adds energy to the rope in the form of a wave that travels along the length of the jump rope to the other person. Each time the jump rope holder snaps her wrist, another wave is generated.
The distance between one peak of the wave to the next peak is referred to as the wavelength. The wavelengths in the visible spectrum are measured in nanometers and range approximately from 380 to 740 nanometers. A nanometer is one-billionth of a meter. (A human hair is about 100,000 nanometers wide.)
The number of waves passing through our jump rope (or in a light wave) over a given time period is referred to as the frequency. Frequency is measured in cycles per second and the unit of measurement is a hertz. The higher the frequency is, the higher the energy in the wave.
In the visible part of the spectrum, color depends on the wavelength. The colors we can see range from dark red to deep violet and can be remembered by the fictitious man’s name: Roy G. Biv, which is a mnemonic for Red, Orange, Yellow, Green, Blue, Indigo, Violet. Of these, the primary colors are red, green, and blue (RGB).
All colors in the visible spectrum can be created by mixing different proportions of the three primary colors. This is the basis by which nature and our PCs, televisions, and other electronic display devices operate. Given this, what three wavelengths of color do you think our eyes are designed to be sensitive to? You guessed it: red, green, and blue. In fact, our eyes are most sensitive to yellow-green wavelengths. Is it any wonder why tennis balls and safety vests are this color?
But what about white? Isn’t it a color of light? Why isn’t it a part of the spectrum? Our eyes see white, but as Sir Isaac Newton (1643–1727) proved in 1665, the combination of all three primary colors of the spectrum actually produces white light. White is the presence of all colors, and black is the absence of all colors.
The combination of different wavelengths of light is referred to as additive color mixing. It is called “additive” because black is the base, and light is “added” to get to white. The combination of any two primary colors results in the secondary colors of cyan, magenta, and yellow. For example, equal intensities of red and green light combine to produce yellow. You can demonstrate this at home with colored lights or by using flashlights with colored cellophane over the lenses (you may need a couple layers of cellophane to get the desired result) shining on a white surface.
If this sounds different than what you’ve learned about mixing colors of paint, that’s because it is different! The mixing of dyes, paints, inks, and pigments is known as subtractive color mixing. It is called “subtractive” because the color we see is actually the result of either the absorption or reflection of colors by the ink or pigment. The three primary colors for subtractive color mixing are cyan, magenta, and yellow (CMY), and the secondary colors are red, green, and blue. Sound familiar? Subtractive color mixing is the basis for producing printed material, including this magazine. If you have a color printer at home, check the ink or toner colors; odds are you’ll find cyan, magenta, and yellow, plus black.
Healthy leaves on a tree in the summer look green. That is because they absorb (i.e., subtract) most of the red and blue wavelengths in the sunlight and reflect the green wavelengths. If the light shining on the green leaves has no green in it, then no green light can be reflected and the leaves will look black. In a dark room, shine a red light (or a flashlight with red cellophane over the lens) on a green leaf. What do you see?
Try This at Home
Among his many scientific contributions, Newton used lenses and prisms to show that a prism splits sunlight into its constituent colors. The conventional thinking of the day was that the prism somehow created the colors. Newton proved that the prism actually separated the colors, which added up to produce white.
Beginning science students may want to follow in Newton’s footsteps and experiment with prisms and different colors of incident light. For best results, use a good quality glass prism. You should be able to find one at a hobby shop for about $10.
Following Newton’s work with prisms, Sir Frederick William Herschel (1738–1822) made an important discovery about a kind of light that we cannot see. Herschel was one of the world’s foremost astronomers of his day and is perhaps best known for his discovery of the planet Uranus in 1781.
In 1800, while experimenting with filters for his telescopes, Herschel noticed that different color filters seemed to pass different amounts of heat. So he devised a setup for measuring the varying temperatures. This experiment can be repeated today and makes for a good science fair project.
You’ll need a good quality prism, a cardboard box, some white paper, some tape, black paint (or a black magic marker), and three thermometers. You can get the thermometers at any hardware store for less than $10 total. (Be sure to choose three thermometers that are all reading the same temperature in the store.)
First, blacken the bulbs of the thermometers so they are able to better absorb the heat from the light. Next, attach the prism to the outside edge of the box. I used a printer paper box standing on end and used the lid of the box as a base. I taped the prism in the handle hole, and I lined the lid with white paper.
Adjust the prism so that the spectrum lands on the paper in the shaded portion of the box. You may have to experiment to find the right height for the prism, depending on the angle of the sun.
Record the ambient air temperature. Then place the thermometers side by side under the spectrum and record the color and temperature readings after a minute or two. (You’ll also learn something about how fast the sun moves across our sky!) It’s best to get at least three sets of temperature readings to ensure accuracy.
Next, move the thermometers such that one is just beyond the red end of the spectrum; record its temperature. Much to his amazement, Herschel found that this area where there was no visible light was the hottest part of the spectrum. He was the first to realize that there is light that we cannot see. The invisible light that he discovered was later named infrared (which means “below the red”) light.
Television remotes use infrared light to communicate with TVs. Herschel and others found that infrared light has all the other properties of light except that we cannot detect it with our eyes. You can prove this by using a mirror to bounce a remote signal toward your television.
Through these experiments, you’ve begun gaining knowledge about the fascinating topics of light and color. Don’t stop here. There are numerous resources in the library and on the web, and there are a number of hands-on science kits that can further illuminate your knowledge of light and color.
Published on May 5, 2009
The son of a son of an engineer, David M. Jones has long been fascinated by science and technology. With two engineering degrees and more than twenty-six years of experience, he recently co-founded Edamar, Inc. and has turned his energies toward helping others learn the fundamentals of science using KitBooks™. Learn about Edamar’s exciting new approach to hands-on science at www.KitBook.com.
Copyright 2008. This article originally appeared in The Old Schoolhouse Magazine, Winter 2008/09
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