By John Palka — Posted December 13, 2015
Exploring the origins of color in our world, in the post Our Brightly Colored World, we uncovered a fundamental physical principle: There are molecules that absorb certain wavelengths of light—certain portions of the visible spectrum—but do not absorb others. These molecules are called pigments. The wavelengths that are not absorbed are available to our eyes to be seen as colors. When the entirety of the visible spectrum is present and no selective absorption has taken place, we see white or colorless light. When all visible wavelengths are absorbed, we see black. Were it not for the selective absorption of visible light by pigment molecules, we would not see our world as being colored.
Suppose we take a walk in the springtime woods. In the wettest places, the green leaves of the spectacular swamp lanterns blend in well with the surrounding plants, including the mosses, ferns, gymnosperms, and angiosperms we met in a previous post. In contrast, the flowering heads—the lanterns—seem to truly glow a brilliant yellow. Why are the two parts of a single plant so different from one another?
The leaves of all green plants, no matter what group they belong to, are packed with the pigment chlorophyll. Later we will learn a great deal about chlorophyll, because it is the foundation of our own life here in Earth. For now, however, the important point is that chlorophyll looks green to us because it absorbs blue and red wavelengths but not green ones. In full accord with our principle of selective absorption, the unabsorbed green light is free to enter our eyes, so we see the leaves of the swamp lanterns as green, just like all other leaves. The flowering parts contain little or no chlorophyll, but they do contain pigments belonging to the group called carotenoids. Carotenoids do not absorb yellow and related colors, though they do absorb quite a lot of green. The yellow portion of the sun’s spectrum passes through these pigments and into our eyes, so we see yellow lanterns glowing amongst the dark green of the sword ferns, thimbleberries, mosses, and the lanterns’ own leaves.
In mountain meadows, like these surrounding Hurricane Hill in the Olympic Mountains of Washington, we can revel in a multitude of yet other colors, including the intense blue of the gentians or of these bluebells of Scotland.
And why are the bluebells so blue? The same principle is in operation: the dominant pigments in their flowers, called anthocyanins, absorb many portions of the spectrum but they fail to absorb blue, so we get to see and appreciate it.
One more set of pigments is of central importance to our experience of color. We will encounter them in some detail later in Nature’s Depths, but we need the core idea now to start to appreciate how our experience of color arises. This core idea can be expressed in three parts: (1) The cells in the retinas at the back of our eyes contain pigments. (2) The absorption of light by the pigment of a retinal cell results in that cell’s production of an electrical signal. Ultimately, electrical signals from the retina make their way to the brain, where they deliver the information that light has been absorbed. In this way, retinal cells and their pigments are the foundation of our visual experience. (3) Different retinal cells contain different pigments, and each pigment selectively absorbs some wavelengths and fails to absorb others. The fact that particular retinal cells signal the absorption of particular wavelengths of light is what underlies our ability to distinguish one color from another.
Isn’t there a wonderful symmetry here? Both plant flowers and human retinas contain pigments, and pigments have the property of selectively absorbing specific wavelengths of light while allowing other wavelengths to pass through. The wavelengths of light that are not absorbed by flowers are available to our eyes, where they are absorbed by retinal cells that ultimately tell our brain that, say, there are red Indian paintbrush on Yellow Island or blue bluebells on Hurricane Hill!
With this kind of understanding, I have often had a strong sensation of actually being immersed in a world of light and color. I can sometimes evoke this sensation by going outside and imagining beams of light streaming in from the sun, being absorbed or reflected by the plants and other objects that surround me, reaching and entering my eye, and by being absorbed there allowing me to appreciate the wondrous world through which I am walking. I invite you to try this practice. Focus your mind on the light and its journey through the world around you, and see how this affects your sense of connection with everything you see, living and non-living!