What the Unaided Eye Cannot See

TINY dust particles float unseen in the air. But then a shaft of sunlight streams through the window, and what had been invisible can suddenly be seen. The penetrating beam of light unmasks the particles to human vision.

Think further about visible light, which appears white or colorless to the unaided eye. What happens if sunlight shines at just the proper angle through droplets of water? The water serves as a prism, and we see a rainbow of beautiful colors!

Actually, objects around us reflect various wavelengths of light that our eyes see as color. Green grass, for example, does not of itself produce green light, but, rather, it absorbs all the wavelengths of visible light except green. The grass reflects the green wavelength back to our eyes. Thus, to our eyes the grass appears as green.

Aided by Man-Made Instruments

In recent years many things invisible to our unaided eyes have become visible by means of modern inventions. We can look through an ordinary microscope at a seemingly lifeless drop of water and discover that it is filled with all sorts of moving creatures. And a strand of hair, which to normal vision appears smooth and even, is seen to be rough and jagged. Very powerful microscopes can magnify objects one million times, the equivalent of enlarging a postage stamp to the size of a small country!

Now, with the use of even more powerful microscopes, researchers are provided with atom-scale contour images of surfaces. This gives them a view into what until quite recently was beyond the scope of human vision.

On the other hand, we may look up into the sky at night and see stars. How many? With the unaided eye, only a few thousand at most. But with the invention of the telescope nearly 400 years ago, people began to see many more. Then, in the 1920’s, a powerful telescope at the Mount Wilson Observatory revealed that there are galaxies beyond our own and that they too are filled with countless stars. Today, using sophisticated man-made means of probing the universe, scientists estimate that tens of billions of galaxies exist, many of which consist of hundreds of billions of stars!

It is truly amazing that telescopes have revealed that billions of stars, appearing as the Milky Way because of seemingly being so close together, are separated by incomprehensibly great distances. Similarly, powerful microscopes have helped the unaided eye to see that objects that appear to be solid are, in fact, composed of atoms that are primarily made up of empty space.

 The Infinitesimally Small

The tiniest speck that can be seen under an ordinary microscope is composed of more than ten billion atoms! Yet, in 1897 it was discovered that the atom has tiny orbiting particles called electrons. In time, the nucleus of the atom, around which the electrons orbit, was found to consist of larger particles—neutrons and protons. The 88 different kinds of atoms, or elements, that occur naturally on earth are basically the same size, but they vary in weight because each has a progressively larger number of these three basic particles.

The electrons—in the case of the hydrogen atom, one single electron—whirl through space around the atom’s nucleus billions of times every millionth of a second, thus providing shape to the atom and causing it to behave as if it were solid. It would take nearly 1,840 electrons to equal the mass of a proton or a neutron. Both the proton and the neutron are about 100,000 times smaller than the entire atom itself!

To get some idea of how empty an atom is, try to visualize the nucleus of a hydrogen atom in relation to the atom’s orbiting electron. If that nucleus, consisting of a single proton, were the size of a tennis ball, its orbiting electron would be about two miles [3 km] away!

A report on the centennial celebrations of the discovery of the electron commented: “Few think twice about celebrating something no one has seen, which has no discernible size and yet has a measurable weight, an electric charge—and spins like a top. . . . Today no one questions the idea that things we can never see do exist.”

Even Tinier Objects

Atom-smashing machines, capable of hurling particles of matter at one another, now offer scientists a glimpse inside the nucleus of the atom. As a result, many particles with strange-sounding names are written about—positrons, photons, mesons, quarks, and gluons, to name but a few. All are invisible, even to the most powerful microscopes. But with such equipment as cloud and bubble chambers and scintillation counters, traces of their existence are observed.

Researchers now see what was once invisible. As they do, they are grasping the significance of what they believe are the four fundamental forces—gravity, electromagnetic force, and two subnuclear forces designated the “weak force” and the “strong force.” Some scientists pursue a quest for what is termed a “theory of everything,” which they hope will provide one comprehensible explanation of the universe, from the macroscopic to the microscopic.

What lessons can be learned from seeing what the unaided eye cannot? And based on what they have learned, what conclusions have many reached? The following articles provide answers.

[Pictures on page 3]

Images of nickel atoms (top) and platinum atoms

[Credit Line]

Courtesy IBM Corporation, Research Division, Almaden Research Center