Seeing the Unseen—The Science of Optics
A SCENIC landscape, a brilliant sunset, a lovely flower—all beautiful sights that are a joy to behold. Though we seldom give thought to what is involved in seeing, we are certainly glad that we are able to see.
Marvelous as the eye is, what we are able to see with the unaided eye is but a fraction of what there is to be seen. By the use of optical instruments—from the simple magnifying glass to telescopes, microscopes, special cameras, spectroscopes, and so forth—the science of optics, as the study of light is called, has greatly expanded our knowledge of ourselves and of the world around us.
Though you may be familiar with some of these optical instruments, do you know how they work? Why, for instance, does a magnifying glass magnify? What makes one instrument bring up the world of the microorganisms and another bring into view the vast expanse of the universe? The science of optics has been an intriguing field of study for a long time.
The Basic Element
Have you ever used a hand-held magnifying glass to burn a hole in a piece of paper by focusing a beam of sunlight on it? What you had there was an optical instrument in its simplest form—a lens. That little spot on the paper was actually an image of the sun produced by the simple lens you had in your hand. Concentrating all the energy in that beam of sunlight into one little spot made it hot enough to burn the paper.
Another lens many are familiar with is the one in the front of a camera. You may know that it focuses the light from an object to form an image on the film for a picture to be taken. Essentially, that is what a lens does. It brings the light together to form an image of suitable size and intensity so that it can be observed or recorded. But how does the lens cause the light to be bent and brought together, or focused? The answer lies in an optical phenomenon called refraction.
When you dip a stick into a pool of water, what do you see? Doesn’t the stick appear to be bent at the point where it enters the water? This common but strange event illustrates that when a beam of light passes from one medium to another, such as from water to air, it does not continue on in a straight line; it is bent except when it hits the boundary perpendicularly. This is what scientists call refraction. The extent to which the light is refracted depends on the mediums—air, water, oil, glass, and so on—and on the angle of incidence, that is, the angle between the light ray and the vertical at the point of entry.
Take a look at the lens of a camera again. You will note that the surface of the lens is not flat but curved like the surface of a sphere, or convex. Now imagine a beam of light coming to it from a distance. At the center, the light is perpendicular to the surface of the lens; thus it travels straight through with no refraction taking place. The angle of incidence becomes progressively larger toward the edge of the lens. This means that the refraction caused by the lens is also greater the farther away from the center the light strikes. Because of this, all rays issuing from the same point on one side of a properly shaped lens will come together, or be focused, on the other side to form an image.
Designing an Optical System
To complicate matters, however, light of different colors, or wavelengths, is refracted in different degrees. This is why a prism spreads a beam of sunlight into its colors, forming a rainbow. This is exactly what happens with a simple lens; the image usually has colored, thus distorted, fringes.
This problem can be overcome by careful design. Scientists know, for example, that the chemical content of the glass used in a lens will alter its refractive properties. By creating a system of lenses made of different kinds of glass and having different curvatures, a designer can keep aberration and distortion to a minimum.
Designing such a system, however, is not simple. It used to involve many people performing laborious calculations for weeks and months to come up with a design. Today, computers are used to calculate all the possible variations in the angles of the light rays, the distances between lenses, the curvature of each lens, and a host of other factors. The computer is programmed to pick out the combination that will result in a system of the highest accuracy.
A good camera lens may have from four to seven, or more, individual elements, with surfaces accurate to a few millionths of an inch [ten thousandths of a millimeter]. Each element must be mounted in precise relationship to the others. To capture as much light as possible, the diameter of each element should be as large as practical. All these things are expensive to do, and that explains why a precision camera is so expensive. For example, one of the cameras used on the space shuttle can photograph details on earth 30 feet [10 m] across from over 150 miles [240 km] up in space. This camera has a lens with eight elements, and it cost nine million dollars!
Seeing the Invisible
Imagine what is involved in designing, producing, and testing an optical system for use in a telescope that will allow us to look out into our vast, awe-inspiring universe. Distant stars are so faint that most of them are invisible to the naked eye. A telescope will gather as much light from these remote stars as possible, focus it to a common point, and form a visible image.
Most optical telescopes use a concave mirror to collect the faint light rays. The famed Hale telescope on Mount Palomar, for example, has a mirror 200 inches [5 m] in diameter and can peer out to several thousand million light-years. Awesome as it is, the Hale telescope has now been eclipsed by one atop Hawaii’s Mauna Kea. This telescope has a 400-inch [10 m] mirror—four times the light-gathering capacity of the telescope on Palomar. It is so powerful, in fact, that “it will permit one to see the light of a single candle from the distance of the moon,” said Howard Keck, president of the foundation that donated 70 million dollars to support the project.
For some time the eyes of astronomers had been on a telescope of a different kind: the $1,600,000,000 HST (Hubble Space Telescope). Launched by the space shuttle, it circles the earth in an orbit 300 miles [500 km] out in space. Without the obstruction of earth’s atmosphere, it can see so well that, theoretically, its resolving power is “equivalent to distinguishing a car’s left and right headlights from a distance of 2,500 miles [4,000 km],” says the magazine Sky & Telescope. To achieve this degree of resolution, the surface of its modest 94-inch [2.4 m] mirror had to be accurate to within two millionths of an inch [five hundred-thousandths of a millimeter]. To everyone’s great disappointment, however, the first images the HST sent back from space were blurry, evidently the result of a manufacturing flaw. “A fragment of synthetic film the size of a grain of sand,” says a report in New Scientist, “broke off a calibrating device during the making of the telescope’s primary mirror. As a result the mirror was ground too flat.” Apparently, even the highest of high tech is vulnerable!
From seeing far with a telescope, we can turn to seeing close with a microscope. Early microscopes were no more than a magnifying glass. By the 17th century, compound microscopes came into use, in which the image formed by one lens was further magnified by another lens. The first lens is usually called the objective because it is directed toward the object to be viewed, and the second lens, the eyepiece.
For a microscope to do its job, it must be able to collect as many light rays as possible from a tiny object. To do so, the objective lens is shaped somewhat like a half sphere, something like the cap of a mushroom. Although only four hundredths of an inch [one millimeter] or less in diameter, its surfaces must be accurate to four hundred-thousandths of an inch [one thousandth of a millimeter].
Interestingly, the ability to see small objects is dependent not so much on the instrument as on the light used to illuminate the object. The smaller the object to be viewed, the shorter still the wavelength of the illuminating light must be. Optical microscopes use visible light, and this limits them to seeing objects no smaller than four millionths of an inch [ten thousandth of a millimeter] across. Early microscopes enabled scientists to discover that plants consist of innumerable cells—a revelation. Today, biology students can peer into the realm of bacteria and blood cells through their classroom microscopes.
To see still smaller objects, we have the electron microscope. As the name implies, instead of visible light, beams of high-energy electrons are directed at objects as small as four hundred-millionths of an inch [millionth of a millimeter]. This brings into view viruses and larger molecules.
What about the structure of the atom or its nucleus? To get a look at these things, scientists have to “smash” an atom and then use computers to construct a picture of the result. So, in a sense, the largest and most powerful “microscopes” are the particle accelerators—cyclotrons, synchrotrons, and others—the size of some being measured in miles. These instruments have given scientists a glimpse of the secrets of the forces holding the universe together.
The Marvel of Vision
Compared to these complicated instruments, the human eye, one might think, would be primitive indeed. Simple, perhaps; primitive, never! The eye has no problem with different colors of light. Its automatic focusing system is fast and efficient. It can see in three dimensions. It can detect millions of gradations of light and shades of color. It can create and record a new image every tenth of a second. The list goes on and on. What a masterpiece—the human eye!
How grateful we are for the ability to see—with or without the use of optical devices! The increased knowledge of things large and small, visible and invisible, has brought many tangible benefits. But above all, the marvelous gift of vision, coupled with what is learned through the science of optics, should help us see the wisdom and love of the one who provided these things, the Creator, Jehovah God.—Psalm 148; Proverbs 20:12.
[Pictures on page 23]
The spectacular Orion nebula, 1,300 light-years away
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NASA photo
Inset: One of the telescopes at Kit Peak National Observatory, Arizona, U.S.A.
[Pictures on page 24]
Top: The base of a single scale on a moth’s wing, magnified by electron microscope
Bottom left: At 40,000 times magnification, even more detail is seen, illustrating the intricate design present in the structure of all living things.
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Top and bottom left: Outdoor Pictures
Bottom right: Hooke’s early compound microscope from “Micrographia,” by Robert Hooke, 1665
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Historical Pictures Service