Although Newton could be fiercely possessive about his work, to the public he usually tried to present a becomingly modest appearance. When praised about his towering achievements, he replied, “If I have seen farther than other men, it is because I stood on the shoulders of giants.” He also wrote:
I do not know what I may appear to the world; but to myself I seem to have been only like a boy playing on the seashore, and diverting myself now and then in finding a smoother pebble or a prettier shell than ordinary, whilst the great ocean of truth lay all undiscovered before me.
An interesting metaphor. When little children play on the seashore, they often throw the pebbles or shells they find into the water. And when a pebble hits the water, it creates little waves that spread out in a circle all around it.
There were scientists who, even before Newton, believed that light is not a flow of tiny particles, but a stream of waves, like the waves of surf rolling in on a beach or the waves of sound that our ears detect.
In 1672, Newton was elected to the Royal Society, one of the oldest and most prestigious scientific societies in the world. When he reported on his work in optics (Opticks would not be published for another thirty-two years), he was immediately challenged by Robert Hooke.
Hooke passionately believed that light was a wave phenomenon, like sound waves. One problem with that idea is that waves need a medium in which to propagate. Water waves travel through water; sound waves travel through all sorts of media, including air. Since it was obvious that light traveled through interplanetary space, Hooke believed that space was not empty, but was filled with an invisible substance he called ether. In later generations, the “luminiferous ether” became a firm part of established physical theories. Belief in the ether had world-shaking consequences-even though it never existed in reality.
Belief is one thing. Proof is another. Hooke was not the careful experimenter that Newton was, nor was Hooke as deep a thinker (who was?). Yet he attacked Newton so vehemently that a lifelong enmity developed between the two of them.
The Dutch astronomer Christiaan Huygens took up Hooke’s ideas and developed a mathematical treatment that showed how light waves are bent by a prism or a lens. Huygens was a many-faceted man: he developed an improved method for grinding lenses; he observed that the planet Saturn is surrounded by bright rings and discovered Saturn’s largest moon, Titan; he improved on Galileo’s work with pendulums and built the first grandfather clock, which ushered in the age of accurate time-keeping.
Huygens believed that light was a wave phenomenon, like the ripples on a pond when a pebble is dropped into the water. But whereas the waves on a pond’s surface are two-dimensional, the waves of light coming from a candle (for example) are threedimensional: they expand outward spherically, like swelling balloons. Huygens’ mathematics, based on the wave theory, explained refraction at least as well as Newton’s particle concept.
Newton himself realized that there were certain things about the behavior of light that could not be easily explained by the particle theory. Indeed, in Opticks he carefully stated that the concept of light being composed of submicroscopic particles was not proved; it was merely an attempt at an explanation based on the available evidence.
While Huygens was a giant among the era’s scientists, Newton was a supergiant. His reputation had become unassailable, and—as happens so often—his supporters took a much stronger stand against the wave theory than Newton himself did.
It must be added here that the latter part of Newton’s life was marked by a turn toward mysticism, and some modern investigators believe that he may have suffered brain damage from heavy metal poisoning. Newton spent years working with large amounts of mercury, in part for his studies of optics, and it is possible that he breathed in enough mercury vapor to damage his brain. Eventually it killed him.
Nevertheless, Newton’s followers backed the particle theory so strongly that it seemed unassailable. This is not to say that everyone accepted Newton’s work without criticism. Even after both Hooke and Newton died, there was discussion and even argument. But gradually the particle theory, with Newton’s enormous reputation attached to it, became the established theory.
Still, there were doubts. In 1810, the German writer and philosopher Johann Wolfgang von Goethe published his Theory of Color, in which he attempted to prove Newton wrong. He failed, principally because he reverted to philosophical analogies instead of conducting experiments that could provide data that contradicted Newton’s findings. The author of Faust, a man who dramatized the concept that the thirst for knowledge is a path to damnation, tried to undo experimental evidence with words. The evidence prevailed. However, Goethe’s words about how human beings perceive colors did serve as a forerunner of the modern science of perceptual psychology.
In science, no matter how well established an idea may be, no matter how prestigious the people who support it, it can be overthrown. Science is very different from other areas of human endeavor in this regard; the establishment can be blown away on the strength of a new piece of information. There is no permanent priesthood in science. Science is an ongoing process, a constant search for truth. Every idea in science must be testable, must be capable of being proven either wrong or right. Only the ideas that survive the tests are accepted as valid.
This is what makes many people, and particularly politicians, so uneasy about science and scientists. Nothing is certain forever. Nothing is set so firmly in concrete that it cannot be uprooted and supplanted by later, better evidence. Most people find that unsettling. Most politicians find it unnerving. They want eternal verities. Hitler provided eternal verities. So did the Spanish Inquisition and Josef Stalin’s Soviet Russia.
Despite the fact that the authority of Sir Isaac Newton’s reputation was protecting the particle theory of light, that theory was eventually supplanted. One of the major actors in the overthrow was a failed physician.
Light Can Bend Around Corners!
There was a chink in the armor of the particle theory, and it had actually been noticed by a relatively obscure Italian physicist, Francesco Maria Grimaldi, at just about the time that Newton was first presenting his work to the Royal Society and stirring the wrath of Hooke. Grimaldi noticed that light does not always travel in strictly straight lines. Under certain conditions, light rays bend slightly around an obstacle, a phenomenon called diffraction.
For more than a hundred years, this uncomfortable little fact was ignored. It was uncomfortable because it could not be explained by the particle theory. If light is composed of particles, and if those particles travel in straight lines, how come they can bend around corners? The phenomenon of diffraction could be ignored because the evidence for it was very slight; the bending that Grimaldi had noticed was quite small, and his work was not widely read.
Then came Thomas Young, an English physician who was more interested in research than treating patients. A child prodigy who could read by the age of two and had read the entire Bible twice before he was six, Young discovered in 1801 that astigmatism is caused by a roughening of the surface of the cornea and, more than a decade later, helped to translate the Rosetta Stone, which opened the way to understanding the writings of ancient Middle Eastern civilizations.
In between those two accomplishments, he demolished the particle theory of light.
Young’s interest in the human eye led him to an interest in light itself. In 1803, he performed a classic experiment. He passed a beam of light through two closely spaced pinholes and then allowed the light to fall on a screen.
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Young’s classic “two-slit” experiment, which showed that light creates interference patterns like ripples in a pool of water, and therefore light must be a form of wave motion rather than a stream of particles.
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Instead of two spots of light, the screen showed bands of alternating light and darkness. These bands are called interference patterns. There is no way that streams of particles could produce such patterns. Only waves can. Young made the comparison between sound waves produced by tuning forks and waves of light.
You can duplicate Young’s experiment for yourself. Get a flashlight and a pin. Then take three pieces of white cardboard, the sort of board that laundries place in men’s shirts. Place the first cardboard close enough to the flashlight so that it blocks the entire beam of light. Set up the second cardboard about a yard away. Now carefully put a pinhole into the first cardboard. Turn off all the room lights, leaving only the flashlight on. The second cardboard will be illuminated by the tiny shaft of light coming through the pinhole. It will be dimly lit, but the light will be evenly distributed across the cardboard. So far, the particle theory explains what can be seen. Streams of particles coming through the pinhole move outward in straight lines to strike the second cardboard.
Now put two pinholes in the third cardboard, close together enough so that the light beam shines on them both simultaneously. Place the third cardboard between the first two. The light now travels from the flashlight, through the first cardboard with its single pinhole, then through the next cardboard with the two pinholes, and finally falls on the rearmost cardboard.
What do you see on that rear board? The light no longer falls evenly across it. Instead, it creates a checkerboard pattern of bright areas and dark areas. You have created an interference pattern, just as Young did in 1803.
You can see interference patterns on a sunny day at the beach. Go into the water up to your ankles and watch the interplay of light against the sand when a wave runs back toward the sea. The ripples of water create patterns of light and dark against the sand.
Young’s experiments and his conclusion that light is a wave phenomenon did not sit well with those who backed the particle theory. They dismissed Young’s work as unscientific and even “un-English.” Yet about a decade later, a French physicist, Augustin Jean Fresnel, not only repeated Young’s work but extended it so solidly that the particle theory ultimately collapsed.
It collapsed because the wave theory worked. The wave theory explained observable phenomena that the particle theory could not explain, in addition to explaining all the phenomena that the particle theory did explain. Science is ultimately a democratic endeavor, where the “votes” are factual observations of nature. If your “candidate” theory explains more of nature than another candidate, then your theory wins the election. But there is always the chance of a new election, based on new observations of nature or reinterpretations of existing observations.
The wave theory outvoted the particle theory, thanks mainly to Young and Fresnel. (But there was to be another “election” in the twentieth century.)
All waves have certain physical properties to them, whether they are waves of sound, water, light, or something else. Waves have crests (their high points) and troughs (their low points). They also have:
1. Wavelength: the distance between crests (or between troughs, if you’re a pessimist); either measurement is fine, as long as you are consistent and don’t mix crests with troughs.
2. Frequency: the number of waves that pass a given point within a fixed period of time.
3. Amplitude: this one is a little tricky. Amplitude is measured as half the distance between crest and trough. If you imagine a line of waves washing up on a beach, their amplitude would be the distance between their crests and the level of the undisturbed water.
Light travels very fast: slightly more than 186,000 miles per second in a vacuum. Nothing in the universe is faster. In glass, light travels “only” two-thirds as fast, or nearly 124,000 miles per second.
Wavelengths of light, as we have seen, are extremely small. Visible light ranges from about four hundred to seven hundred nanometers.
Frequency is expressed in the unit called hertz, which is equivalent to one cycle per second, or one wave passing the measuring point each second. The current used in lighting and other electrical equipment in the United States oscillates at sixty cycles per second, or sixty hertz (abbreviated Hz). Humans can hear sound waves, which propagate through air, from about sixteen Hz to twenty thousand Hz, or twenty kilohertz (abbreviated kHz). Light waves, as you might expect, have enormously higher frequencies, on the order of one hundred million million (1014) Hz.
The wave theory of light made sense of colors. It was already known that when white light goes through a prism it comes out spread into a spectrum of colors (see Plate 1). Now it became clear that each different color represents a slightly different wavelength of light, and each wavelength is bent (refracted) at a slightly different angle in the prism. Red light, for example, lies in the wavelengths from roughly 650 to seven hundred nanometers. Blue light is from 450 to five hundred nanometers.
Leaving aside the particle versus wave argument for a moment, let us take a look at some of Newton’s painstaking experiments dealing with color.
We will be dealing here with pure colors and their mixing. By a pure color, I mean a ray of light of one single color, as it comes from a prism. Newton called such a ray homogeneous light. Physicists today call it monochromatic light; that is, light of one color only. Instead of the broad mixture of wavelengths from four hundred to seven hundred nanometers that make up white light, these experiments dealt with narrower wavelengths, the fifty to one hundred nanometers spreads between one individual color and another.
Newton found that when he mixed two beams of monochromatic light of different colors, he got a third color. Mixing pure red and pure green produced yellow, for example. If he passed the yellow light thus produced through a prism, it broke up into red and green again. (You can do the same experiment for yourself.)
We know now that the color receptors in our retinas are sensitive to red, blue, and green. Little wonder that Newton and the other scientists who studied color phenomena found that there are three “primary” colors—red, blue, and green—and all the other colors of the rainbow could be obtained by mixtures of these three.7
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7Newton dealt with light. As we shall see in Chapter 12, things are different when we deal with pigments.
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The science of color mixing began. Artists, of course, had been mixing pigments since the Stone Age. They worked with what they could find, colored clays and bits of crumbling earth, bright juices squeezed from berries. By the time of the Renaissance, the painter’s palette had become sophisticated enough to yield the glowing colors of Titian and Raphael and the somber hues of Rembrandt.
Now scientists began unraveling the reasons behind color mixing. And technicians began adapting their knowledge for practical use. Color printing arose and was improved to the point where today even cheap newspapers can be printed in a rainbow of hues. Color motion pictures and television broadcasts, even colorization of films that were originally shot in black and white, are now commonplace.
While Newton pioneered the scientific understanding of color mixing, the wave theory of light gave his empirical studies a firm theoretical underpinning.
A word about that word theory.
In everyday usage, theory has come to mean about the same as hypothesis: that is, an informed guess, an unproved set of assumptions. In science, theory means something very different.
To a scientist, a theory is an organized set of ideas, a structure of thought that explains a wide range of observations—and also predicts new observations. If you think of an individual observation of nature as one of Newton’s seaside pebbles, a theory is a detailed map that shows where the pebbles were found, what their relationship is to the ground around them, and where new pebbles can be located.
For example, Charles Darwin gathered a vast array of observations about plants and animals-observations that ranged from the songbirds of the Galapagos Islands to the facial muscles that allow us to smile. Out of that multitude of observations he derived the idea of evolution based on natural selection: organisms change in response to the environments in which they exist. This is called the theory of evolution. It is one of the greatest products of human thought. It made sense out of the entire study of biology and allowed scientists to understand how their different observations are related to one another. Moreover, it allowed scientists to predict where and how they could make new observations, learn new facts about nature.
Yet, because of that word “theory,” many people believe that scientists themselves are not certain of Darwin’s concept. “Evolution is only a theory,” they say, meaning that it is an unproved hypothesis. Not so. Darwin’s concept of evolution is the central guiding path for the entire science of biology and has been useful in other sciences as well.
We have seen that the particle theory of light was supplanted by the wave theory. In the next few pages, we will see that the wave theory itself has been outmoded. Some theories are replaced by better ones, just as an acorn is “replaced” by a mighty oak.
But some theories stand the test of time and remain intact despite all the new observations and experiments that are made. The tests verify the theory, rather than contradict it. Evolution is one of those powerful theories. Einstein’s concept of relativity, as we shall soon see, is another.