Wood: Seeing the Unseen - Looking at Things with invisible Light

Popular Science, heinäkuu 1916

By R. W. Wood
Professor of Experimental Physics, John Hopkins University

Professor Wood is one of the most distinguished of American physicists. He has recently attracted attention to himself by ingeniously photographing the common objects around us, as well as the planets, with light that our eyes can never see. Thus he has opened an entirely new world, the exploration of which teems with boundless possibilities. The following article from Professor Wood's pen explains as simply as possible how he conducted his investigation and what may be seen in the strange world that our imperfect eyes can never behold. - EDITOR.

If you could strike all the keys of a piano at once, from the deepest base note to the topmost treble, you would create a medley or cacophony in which it would be impossible to pick out one sound from another. White light is very much like that. it is a blending of many different kinds of light.

The analogy between light and sound is closer than may be supposed, if they are regarded merely as vibrations. The characteristic that distinguishes the lowest base note from the highest treble on a piano is pitch, and pitch depends on frequency of vibration. So it is with light. Low vibrations manifest themselves as red colors; high vibrations as violet hues. Just as there is a perfect musical octave comprised of notes each having a definite pitch or frequency of vibration, so there is a light scale, manifestng itself in color notes, each also having a definite pitch or frequency. But while the frequency of the vibrations that produce musical notes is measured at most by thousands per second, the vibrations that manifest themselves to our eyes as light must be measured by trillions per second.

A photograph taken with ultra-violet light reveals no shadows. White objects appear black, and everything seems veiled n a thin fog.

There are sounds so thin and shrill, so highly pitched that only sensitive ears can hear them. Beyond them are notes that no human ear can hear at all. With light it is the same. there are octaves of light which our eyes can never hope to see. Perhaps the best known of invisible rays are those used in wireless telegraphy; they are produced by vibrations of far lower frequency than those we see as sunlight.

Infra-red is as strange as the ultra-violet. The sky appears black, foliage a beautiful rich red, and there are long, heavy shadows.

When you strike the middle "C" on a piano you hear a single musical note. And so, when you look at the world about you through a pane of red glass, you see things in a single light-note, as it were. Change the color of the glass and the world appears different. The same trees, the same flowers, the same houses are there, but with one color details are obscured and with another intensified.

It is perfectly possible to view the world with invisible rays and to learn things about which we never dreamed of in our philosophy - only we must use an eye, which, unlike our own eyes, will see the unknown world for us and make a picture of it which we can perceive. The ordinary photographic camera is such an eye. The sensitized plate is extraordinarily responsive to those very high-pitched vibrations that do not affect the eye. All that remains is to strike the single note in a given octave of light, with which the world is to be viewed in order to see things as they are but as we never see them.

In order to see the world with invisible ultra-violet rays someting better than glass must be employed; for glass is almost as opaque to them as a plate of sheet-iron. Quartz must be used, since quartz is transparent to them. Hence a quartz lens must be fashioned for the camera. To exclude all but violet rays from the lens a filter must be employed - a kind of sieve through which only the ultra-violet rays will pass, just as only red rays will pass through red glass. Some fifteen years ago i discovered that an aniline dye, called nitroso-dimethyl aniline, would exclude all but the ultra-violet rays, the effect of which i wished to study. Thin films of silver are also serviceable, as well as the vapor of bromine, contained in a rectangular transparent cell.

White ink made from Chinese white and written on white paper is practically invisible to our eyes. Photograph it with ultra-violet rays by means of the devices mentioned and it appears on the photograph as it had been written with the blackest ink. Landscapes photographed by ultra-violet rays reveal no shadows. This means that the molecules of air or the particles of dust in the atmosphere completely scatter the rays, from which it follows that the greater part of the ultra-violet light that reaches the surface of the earth comes from the sky and not directly from the sun. if we saw only with ultra-violet light the world would appear as it does when a thin mist hovers over everything. We should, indeed, see the sun, but it would be very dull, and there would be no shadows, just as there are none on a foggy day. Garden flowers which are white inthe sun, phlox for example, become almost black. Who knows ut this ability of white flowers to absorb ultra-violet rays may play some economic part in the growth in the plant? i made some experiments to answer that question, but without success. But who knows what the result would be after several generations of plants had been grown without the influence of ultra-violet light?

A check which was "raised" from twenty-four to twenty-four hundred dollars. The upper photograph, made with ultra-violet rays, shows the erasure plainly; the lower photograph, made by ordinary light, reveals nothing suspicious.

It must not be supposed that there is but one ultra-violet light. There are indeed as many colors that we cannot see in the ultra-violet region as there are in the rainbow. Unfortunately the camera and the sensitized plate do not give us true colors, as every kodak user knows; but they do indicate color differences in black and white. The photographs which i have made afford convincing evidence that there are a myriad hues in ultra-violet octaves. Thus all white flowers fo not appear equally dark on ultra-violet photographs. White geraniums photograph much lighter than common white phlox.

In the opening paragraphs of this article light and sound were compared. it was stated that just as there are inauduble sounds there are invisible lights. There is a difference, however, between the sound rays and light rays. As you go below the scale of musical notes, as you lower the number of vibrations, you hear not musical notes but distincts beats or blows. That happens when there are less than sixteen vibrations in a second. But - you hear. As you go down the light scale beyond red, the vibrations decrease in number by millions in a second. But - you do not see. in other words there is but one small octave of visible light. Above and below that octave we see nothing with our eyes.

Photograph taken with infra-red light. Note the black sky, the white trees silhouetted against it, and the deep shadows.

It is obvious that the world is fully as well worth studying in light below red (indra-red) as in light above violet. When we reach the infra-red rays we are dealing with heat rays. A glass lens will answer our purpose in this case, but we must use a screen or color filter which absorbs all of the visible and ultra-violet light, while transmitting the infra-red.

As the camera reveals it, the infra-red world is as startling as the ultra-violet world. The sky appears in photographs as black as midnight; foliage snow white. The shadows are intensely black, simply because most of the light comes directrly from the sun and not from the sky.

Applied to purely scientific investigation this utilization of infra-red and ultra-violet rays has vast possibilities. i have made photographic studies of the heavenly bodies with invisible rays, and the results obtained prove convincingly that many new facts can be reached in this way.

The Moon is a dead, arid, airless body which has long ceased to interest most astronomers. Every one of its many thousand extinct craters has been plotted; its great mountain ranges have all been named; and its so-called "seas" and basins have been mapped. it seemed impossible years ago to add anything substantial to our knowledge of the Moon. i made some experiments at my summer home on Long island with a horizontal reflecting telescope of fifty-six-foot focus and fourteen-inch aperture to ascertain what might be revealed if the Moon were photographed with ultra-violet light. While there is very little difference between ordinary photographs of the lunar surface and those made with ultra-violet radiation alone, there is enough that is significant. The brightest of all extinct lunar craters is called Aristarchus. Photographed with ultra-violet rays, Aristarchus shows a dark patch which is not to be seen on a photograph made with visible light. i made an enlargement of the region in which this crater appears, and it is evident that there is in its neighborhood a large deposit of some material which can be revealed only by ultra-violet rays. These photopgraphs of the Moon prove that by systemically studying the lunar surface with invisible rays, we may some day discover what the Moon is made of almost with as much certainty as if we could analyze a piece of it in an earthly laboratory.

In the late autumn of last year, through the courtesy of Professor Hale, the great sixty-inch reflecting telescope of the Mount Wilson Observatory in California was placed at my disposal for four nights. The instrument is the largest of its kind in the world. Photographs of Saturn and Jupiter were made by means of infra-red, yellow, violet and ultra-violet light.

Infra-red Yellow Violet Ultraviolet
Saturn made by Professor Wood with various rays, showing how much more is revealed by some rays than others

Both Saturn and Jupiter are striped with belts which have been the subject of much discussion among astronomers. Study the accompanying photographs and you will see how different is the aspect of the planets when photographed with different rays, whether visible yellow or invisible infra-red or ultra-violet. The belts on the ball of each planet, which can be seen with the eye in a telescope and which are very distinct on photographs made with visible yellow rays, vanish almost completely when photographed with infra-red rays. When ultra-violet light s used a remarkable transformation of the planet occurs. A broad dark equatorial belt surrounds each planet, and a large dark polar cap appears. This equatorial portion is the brightest part of each planet when photographed with visible yellow light. When ultra-violet is employed the bright belts vanish. The equatorial dark belts are still recorded, but they are slightly narrower than when photographed in violet light. Moreover the dark polar cap has decreased in size.

Variations in the intensity of the inner and outer ring of Saturn are also shown in the different photographs. The surface features of both Saturn and Jupiter have been repeatedly photographed, but not with the result of adding much to our knowledge. At last we have a method which may enable the astronomer to interpret the puzzling belts intelligently. it is much too early to venture an opinion. Much work remains to be done with the spectroscope. Perpahps it may turn out that the bands of Saturn may be due to some substance which has not been made in any earthly laboratory or to some substance, which has never studied in layers thick enough to bring out the characteristic appearance. it is also possile, though hardly probable, that the belt is due to a fine mist or dust which absorbs violet light; but it seems unlikely that such a mist would appear dark for the simple reason that it would reflect equally as much light as it absorbed. As a venture we might attribute the belt to chlorine gas, which absorbs violet and ultra-violet light powerfully and is transparent to yellow light. When we recall the enormous quantity of chlorine locked up in the salt of the ocean it is perhaps possible that large quantities may exist free in the atmosphere of young planets like Jupiter and Saturn.

It seems highly propable that extremely valuable results may be obtained if these methods are applied to the planet Mars. Unfortunately, at this time Mars is too far away, and the photographs which i made show nothing of interest.

Infra-red Yellow Violet Ultraviolet
Photographs of Jupiter made by Professor Wood with different rays

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