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NO DOUBT you've heard the old philosophical riddle: "If a tree falls in a forest and no one is around to hear it, does it make a sound?" But what about this one: "If a ripe Macintosh apple is hanging on a tree in the sun and no one is around to see it, is it still red?"
If you're like most people, you think the apple is red no matter who is around. You probably think of color as a property of an object. An apple is red; an orange is orange; a banana is yellow.
We disagree. In fact, Paul begins his classes on color by telling his students, "Without a human eye and brain there is no color." Color, Paul says, is a human perception of what physicists call the "spectrum" of a light source. If there's no one there to see that apple, it isn't red. In this column, we're going to examine the reasons behind Paul's assertion.
A few science fiction writers have considered the topic of color in a fictional way, creating aliens who perceive color differently from humans. In Vernor Vinge's novel A Deepness in the Sky, for example, the aliens that humans call "Spiders" can see more many more colors than humans, having nine times as many different kinds of color sensors. Their perception includes light outside the range of human vision.
In C. J. Cherryh's Chanur Saga, the aliens known as the Stsho create art using infinite different shades of the color white, which mostly look the same to other species.
But we don't have to consider extraterrestrials to challenge our understanding of color. We can do that right here on Earth.
RAINBOWS AND WAVES
Most school kids can name the colors of the rainbow using the memory aid mnemonic "ROY G BIV." Each letter in that peculiar name stands for a color: Red Orange Yellow Green Blue Indigo Violet.
Isaac Newton coined this description of the colors of the rainbow or the spectrum of visible light. Back in 1672, he darkened a room, allowing only a thin beam of sunlight to enter through a slit. He placed a prism of glass in the beam of light. The prism bent the light and spread the spectrum across a wall of the room.
When Newton first looked at the spectrum, he proclaimed that there were five colors: red, yellow, green, blue and violet. Later on, influenced by numerology and a belief in the power of the number seven, Newton added orange and indigo to make the number of colors in the spectrum equal to seven, the number of notes in the musical scale.
In 1801, nearly one hundred years after Newton, Thomas Young looked at the spectrum and noted there were three broad bands of color—red, green, and blue—with narrow color regions of orange/yellow and blue/green between them. He then hypothesized that the human eye had three different types of color absorbers. One had peak sensitivities in the red region of the spectrum, another in the green region, and the third in the blue region.
Young also discovered something very strange about light. When he passed light through two slits, light passing through one slit interacted with light passing through the other slit and created a pattern of light and darkness. This "interference pattern" could be explained by saying that light is a wave. Where the crests of light waves overlap, you get brightness; where one wave's crest overlaps with another wave's trough, you get darkness.
Every wave has certain measurable characteristics. It has a wavelength that's measured from crest to crest. It has a frequency—number of times a wave crest passes a given point in a given period of time. It has a speed. And it has an amplitude—the height of the wave.
These characteristics, applied to light waves, are very important to understanding light and our ability to see color. The wavelength of light waves is usually measured in nanometers. (A nanometer is a billionth of a meter.) What our eyes perceive as color relates to the wavelength of light. The light that makes up the visible spectrum of the rainbow ranges in wavelength from 620–750 nanometers (red light) to 380–450 nm (violet light), with the other colors of the spectrum in between.
Light waves also have a frequency—a measurement of how rapidly the wave oscillates—which correlates with its wavelength. The light our eyes perceive as red is oscillating at 4 x 1014 times per second (4 x 1014 hertz or 400 terahertz).
And as every sf reader (and fan of faster-than-light travel) knows, light has a speed. In a vacuum, light of any color travels at the speed of light, about 3 x 108 m/s. (Paul, as a physicist, categorizes FTL travel as fantasy. He knows nothing that carries information travels faster than light. The speed of light (186,000 miles per seccond) is not just a good idea, It Is The Law!) In the wave model of light, amplitude controls the brightness.
WHAT YOU SEE (AND WHY)
What does all this have to do with what you see? You see because light gets into your eye and makes an image on the retina, the layer of light-sensitive cells at the back of your eye. If you think of your eye as a camera, the retina is the light-detecting film.
In 1959, scientists looked at the wavelengths of light absorbed by chemicals on the retina. There they found the three color absorbers that Young had suggested. They found pigments that absorbed three different colors or wavelengths of light. The pigments were contained in cone-shaped cells, which the researchers named cones.
If you have average color vision, each of your retinas has about 3 million cone cells of three different types. (You also have 100 million rod cells that detect dim light. Since these cells don't help us discriminate colors, we'll ignore them here.) When you see color, what you see is dictated largely by the types of cone cells in your retina and by the wavelengths of light to which these cells are most sensitive.
Each type of cone cell contains a different light-sensitive pigment molecule. Each of the three pigment molecules responds to a different set of frequencies of incoming light. One pigment is most sensitive to the long wavelengths of red light, one to the midrange wavelengths of green light, and one to the short wavelengths of blue/violet light.
Each of these different pigments changes form when exposed to light of its particular frequency. The light of that frequency makes certain electrons in the pigment start moving, and that changes the shape of the molecule.
The molecule crosses the cone cell membrane seven times. (Maybe Newton was on to something with his obsession with the number seven.) When the molecule changes form, it allows ions to flow across a cell membrane. The flow of ions triggers a nerve impulse that moves through layers of cells in the retina to the optic nerve and through the optic nerve to the visual cortex of the brain, where the perception of color is created.
Many people identify the cone cells by colors: the red cones, green cones, and blue cones. But you can get yourself in trouble that way since the so-called "red cones" are actually most sensitive to light in the wavelength associated with yellow. To avoid confusion in naming, most researchers now refer to these cones as short-wavelength, medium-wavelength, and long-wavelength cones.
So when light with wavelength of around 700 nanometers shines into an average human eye, the long wavelength cones fire, sending a signal through other cells to the brain. The brain registers this signal as the color red. Since humans are fundamentally lazy, we get around this long explanation by calling the light that entered the eye "red light." But the light itself isn't red. Your perception of the light is red.
DO YOU SEE WHAT I SEE?
Have you ever wondered if other people look at the same colors you look at and see the same thing? The Exploratorium has an exhibit named "Comparing Yellows" that answers this very question.
In the center of a circular disk is a color that Paul sees as yellow. Around the edge of the circle (where the numbers would normally be if this were a clock) are other colors. At seven o'clock there's a color Paul sees as green, at noon is a color Paul sees as yellowish, and at five o'clock is a color Paul sees as red. At one o'clock Paul sees a color that looks to him like a pretty close match to the yellow in the center.
But if you ask a group of people which color matches the central spot, not everyone agrees. Most will choose the color at one o'clock, but some choose three o'clock, others choose noon, and a few choose seven o'clock, which looks very green to Paul. Yet the person who points to seven o'clock color is sure it is the same as the central yellow. Two people look at the same colors and see them differently.
Remember a few paragraphs back, when we said, "The light itself isn't red. Your perception of the light is red." Maybe you thought we were being semantically picky.
Not so. This exhibit is an example of why we distinguished between the color of the light and the color you see.
How does it work? The light in the center comes from a light-emitting diode or LED that emits a very narrow band of wavelengths—between 570–580 nm. Most people perceive this as yellow. Each of the twelve spots around the face of the clock is lit by two LEDs: one green and one red. Now here's something you may find startling. (We did, when we first learned it.) The right mixture of green light and red light looks yellow to most people.
That's where this exhibit comes in. What constitutes the "right mixture" depends on the idiosyncrasies of an individual's eyes.
The spots around the clock face are made with light from red and green LEDs. The comparative brightness of the two LEDs varies as you move around the clock face, making it likely that you will see some combination of these two lights as identical to the central color. People don't agree on which combination matches the central yellow because small differences in numbers of cone cells and the chemicals that detect light in these cells lead to different color perception. As a result, one person can see two colors as identical while another sees them as different.
As a visit to the Exploratorium exhibit Comparing Yellows demonstrates, variations in color vision are fairly common. Differences in human perception explain why physicists characterize light by measuring it with a spectrometer, an instrument that objectively measures the energy at different wavelengths. They can't rely on their eyes, since different human eyes give different answers.
People who see color a little differently from most of the population are said to have anomalous color vision. People who see color very differently—possibly because they lack one type of cone—may be said to be color blind.
Men are ten times more likely to be color blind than women. That's because many of the genes that control color perception are on the X chromosome. Men only have one X chromosome; women have two.
If one of a woman's X chromosomes is missing the gene for the creation of a certain type of cone cell, the other X chromosome can supply the missing gene. On the other hand, if a man's X chromosome is missing the needed gene, he's just out of luck.
Having two chromosomes that code for cone cells make another sort of anomalous color vision much more likely among women. Recent research suggests that a substantial percentage of women may have more than three types of photopigments in their retinas. These women, known as tetramats (tetra for four), could have four different types of cone cells, expanding the range of colors that they see.
These individuals divide the spectrum into ten color bands, rather than the seven identified by Newton. Researchers estimate that people with three types of cones can see about a million different colors. What this means is that the people with normal vision can detect the differences between two color squares placed side by side for one million different colors.) But human tetramats could see one hundred times as many colors, distinguishing 100 million different colors.
A BIRD'S EYE VIEW?
Perhaps this is the place for Pat to mention that her favorite color is ultraviolet, a color that's just beyond the visible spectrum. The rainbow that Newton cast on his wall ranged from red to violet. Most human eyes can't detect infrared light, which has a wavelength a little bit longer than red light, or ultraviolet light, which has a wavelength just a little bit shorter than violet light.
Bees, on the other hand, can see ultraviolet, as can many birds, reptiles, and amphibians. To those of us with three types of color sensors, having four types may seem like an amazing extravagance of colors. But that's just the beginning. Swallowtail butterflies have five different types of color receptors, as do some species of fish.
But our absolute favorite critter in terms of over-the-top color vision is the mantis shrimp, a marine crustacean that isn't really a shrimp. Researchers who deal with mantis shrimp all seem to agree that these invertebrates are extremely aggressive and violent (they've been known to break aquarium glass) and have an amazing visual system.
The mantis shrimp has sixteen different types of light receptors. Eight of them are for portions of the spectrum we under-endowed humans regard as visible. Four of them are for ultraviolet light, and four are for analyzing polarized light.
As a biologist, Pat has to wonder why animals have the color sensors that they have—and why we humans don't have those color sensors. For some species, the reasons are easy to find. Bees and flowers have evolved together. Many flowers have ultraviolet markings that help bees find the nectar.
Ultraviolet vision may explain mate selection in some bird species. In some species of birds, the males and females appear (to a human observer) to be very similarly colored. But seen through ultraviolet filter, the males have very different plumage than the females. A bird with eyes that are sensitive to ultraviolet light sees differences that are invisible to humans.
Researchers are still discussing what advantages seeing polarized light gives the mantis shrimp. Professor Andrew White of the University of Queensland in Australia notes that animals use polarization vision for navigation, for finding food, for evading hunters, and for sex. In other words, for the four Fs: feeding, fighting, fleeing, and…flirting. Some animals on which the mantis shrimp preys are almost invisible in ordinary light—but easily seen in polarized light. Markings on the mantis shrimp that are visible in polarized light may be used in sexual signaling.
It's hard to imagine what the world looks like to a mantis shrimp or a bee or a butterfly. They see colors for which we have no names. But it's even stranger to imagine what the world looks like to an extraterrestrial species that evolved on a planet circling a star with a different spectral output from our sun.
Science fiction often deals with the outsider, the alien. In good science fiction, aliens aren't just humans in funny costumes. They have a different view of the world—seeing it through alien eyes. Considering the mantis shrimp has made us more aware of how very different that point of view could be.
So back to the question at the beginning. "If a ripe Macintosh apple is hanging on a tree in the sun and no one is around to see it, is it still red?"
In the end, we think the answer is simple: "You call that red?"
--------The Exploratorium is San Francisco's museum of science, art, and human perception—where science and science fiction meet. Paul Doherty works there. Pat Murphy used to work there, but now she works at Klutz Press (www.klutz.com), a publisher of how-to books for kids. Pat's latest novel is Invasion of the Bristlebots, which comes with two small robots that run on toothbrush bristles. To learn more about Pat Murphy's science fiction writing, visit her website at www.brazenhussies.net/murphy. For more on Paul Doherty's work and his latest adventures, visit www.exo.net/~pauld.
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