"The Colors of Life"
by: G. O. Miller, Sierra, January 1984
Directions: Read the entire article first, then answer the following questions.
Color
1. Who found that white light actually contains all colors of light? ______
2. Colors of light physically differ in their ______.
3. How does a prism "break up" white light into its component colors?
4. How is a rainbow made?
Hues In Nature
5. How do we explain the colors of the blue sky, a red sunset, or the deep blue ocean?
(Same process accounts for all three.)
6. What are pigments?
Plant and Animal Colors
7. What does chlorophyll, the most important of all pigments, do?
8. Name two other pigments found in plants or plant parts:
9. What happens in the fall when the leaves of some kinds of trees change color?
10. Iridescent colors are not produced by pigments. How are they made?
11. Explain the ability of a small lizard called an anole ("chameleon") to change its skin color.
12. Explain the quote from the end of the article, "With sunlight as a brush, Nature has painted a beautiful world…" (Write your answer on the back).
by: George Oxford Miller
Why is the sky blue and the sunset so often red? Why do a peacocks feather sometimes boast beautiful, glimmering shades of blue and green, while at other times they can appear a dull gray? How can there be so many different colors of flowers? Scientists learned the answers to these and many other questions as they discovered more about light.
Three hundred years ago, one of the greatest scientists of all time, Sir Isaac Newton, discovered that red, yellow, green, blue, and all other colors are contained in every sunbeam. He used a specially cut and shaped block of glass called a prism to prove that there is a rainbow in every ray of white light, which is one of the kinds of visible light produced by the sun. White light is actually a combination of many different colors, or frequencies, of light.
We can use a prism to help us understand how sunlight causes the colorful arch we call a rainbow to display itself across the sky during a rain. To do so, we first need to see how light behaves when it travels through some material other than the air.
Light travels more slowly through water or glass than it does through a vacuum or the atmosphere. In fact, through water it travels only three fourths as fast as it does through a vacuum, and through glass two-thirds as fast. This is because the air is not nearly as dense as these substances are. When a ray of light enters a raindrop at an angle to the drop's surface, it bends as it slows down. The different colors do not all bend at the same angle because each is traveling at a different frequency.
This effect is easy to see if we use a triangular prism. (See the illustration on the next page.) A beam of white light projected through the prism is separated into a spectrum, a band of colors that contains all the hues of the rainbow: red, orange, yellow, green, blue, indigo, and violet. These spectral colors are the visible impressions we receive of the different wavelengths that make up white light. Although light waves are almost unbelievably short, the subtle differences among their lengths are interpreted by our eyes in such a way that we perceive them as being of different colors. Wavelengths of light are either shorter of longer than one another; they travel through the prism at different speeds, and so enter and depart the prism at slightly different times and angles. This is why a prism "breaks up" visible light into its spectral colors the way it does. Short wavelengths, which appear to us as blue or violet, are refracted (bent) the most; these are the ones that travel most slowly through a prism. Longer, reddish wavelengths, which travels most rapidly, are refracted the least.
Once people understood that white light contains all of the colors of the spectrum, they began to study the causes of the many beautiful shades and hues in nature.
Some things in nature reflect light, just as mirrors do, while others may either scatter (that is, disperse) or absorb it. Atoms or molecules of gas and particles in the atmosphere will scatter light. Their electrons may first absorb energy from the light and later begin themselves to disperse light as though they were tiny flashlights. Different kinds of atoms, molecules, or particles will disperse, or scatter, various colors differently. The smallest particles scatter blue light the most. When we look into the sky, it is the blue light being scattered that we see. If there were no atmosphere to scatter the light, the sky would appear black!
Larger particles in the air, such as dust, scatter more red light that blue/violet light. At sunset the sunlight follows a longer path through the atmosphere because it is on the horizon, not overhead. Blue light is scattered out, and red light remains to cause a colorful sunset by illuminating local clouds. If there is much dust, or pollution, in the air, this effect is seen much more dramatically.
The ocean, and some very deep lakes, seem blue for a similar reason. Light shining in water can be absorbed or scattered by the water molecules. The molecules scatter more blue light than red light to the surface, and absorb red light. This is why blue is the dominant color that reaches our eyes. Like a mirror, water also reflects the blue color of the sky.
Most of the colors we see in nature are caused by pigments. Pigments are chemicals that absorb certain colors (as a sponge absorbs water) and disperse others. The color that is dispersed by a pigment is the color we see. Pigments in certain plant and animal cells are what give these organisms their colors. Let's look at a few plant and animal species to better understand how this happens.
Plants produce green, red, yellow, orange, blue, and purple pigments. If a plant appears to us to be a certain color, it means that even though it is absorbing all the colors contained in visible light, it is absorbing less of the color we see. (Red for an apple, as an example.)
Chlorophylls, which make plants green, are the most important of all pigments. They trap the energy in light so that the plant can produce food and grow. The process is called photosynthesis. Animals cannot make their own food, and must get all their food from plants or other animals.
Carrots get their orange color from the pigment carotene. Carotenoid pigments give color to oranges, tomatoes, and other fruits as well. Beets and red cabbage get their deep red color from another group of pigments, called anthocyanins.
Pigments make sunflowers yellow, roses red, and violets blue. Although some pigments help absorb the sun's energy, as we have seen, these colorful pigments have a totally different function. The brilliant colors attract insects that will then pollinate the flowers and enable them to produce seeds.
A tree leaf has many pigments that are hidden by the green chlorophyll. In the fall, when the weather turns cold, a special set of cells around the leaf stem squeezes shut. The leaf begins to die, and its cellular "food factories" stop producing. As the green chlorophyll decomposes and fades, brilliant reds, yellows, and oranges are revealed, because these pigments do not decompose as fast. These anthocyanin pigments do eventually fade, however, leaving only brown. (Tannic acid, which breaks down very slowly in nature, is what makes the leaves brown; it is also what gives tea its rich color.)
Animals, as well as flowers, come in almost every color of the rainbow. Their colors are caused by granules of pigment in skin and hair cells, and by reflected light.
The blue and green in some bird feathers or insect wings are caused by the way the light is dispersed. Transparent cells in the feathers or scales are positioned so that light reflects off their front and back surfaces. Different colors are seen, depending on the angle at which light strikes these cell surfaces. A peacock, butterfly, or dragonfly appears brilliantly colored in bright light, but fades to gray in the shade. These variations, called iridescent colors, are caused by reflected light, not by pigments.
Special cells in the feathers of parrots and blue jays scatter blue light in the same way that air or water does. The blue light seen against a background of dark pigments makes the feathers blue. If you hold a blue feather up to a light and look through it, it appears gray, because you are not seeing the scattered blue light.
Some animals, such as frogs and lizards, have a background pigment of yellow along with specialized cells that scatter blue light. The blue light mixed with the yellow background makes the animal appear green. The anole lizard has cells that can open and close over the pigment. In just a few seconds the lizard can change from bright green to brown. As if the anole had pulled down a shade, the cells close and cover up the color. Fear, hunger, and changes in temperature are three factors that cause anoles to change colors.
The beautiful colors of plants and animals all have a purpose. A little while ago we saw that many flowers have colorful, sweet-smelling blossoms to attract insects that will pollinate them. Flowers that bloom at night, when colors are not visible, are usually white, to reflect moonlight; it's their way of attracting nocturnal insects. The colors of animals often help them blend in with their surroundings and hide from their enemies or prey. Brightly colored animals, like male peacocks, use colors both to attract females and to warn rivals to stay away.
Sometimes, even if we know how a color is formed, it is a real challenge to understand why a plant or animal has its particular coloration. With sunlight as a brush, Nature has painted a beautiful world and used colors in the most imaginative ways.