DEFINITION: an arc or ring containing the colours of the spectrum in consecutive bands, formed in the sky by the refraction, reflection, and dispersion of light in rain or fog.

When light from a distant source, such as the sun, strikes a collection of water drops such as rain, spray, or fog a rainbow may appear. It appears as a multicoloured arc whose “ends” seem to touch the Earth. Rainbows are seen only when the observer is between the sun and the water drops, so rainbows appear in the part of the sky opposite the sun. The centre of the rainbow’s arc is located on an imaginary line extending from the light source through the observer’s eye to the area of the water drops.

Rainbows are most commonly seen when the sun’s rays strike raindrops falling from distant rain clouds. Generally, this is only in the early morning or late afternoon. When the sun is too far above the horizon no rainbow can be seen.

When the sun is lower in the sky, however, part of the arc becomes visible. In fact, if the sun is low enough and the observer is located in a place that is high enough, such as on a mountain, in an aeroplane or a spaceship, the observer may see a circular rainbow.

The most brilliant and most commonly seen rainbow is called the primary rainbow. The arcs of colour in a rainbow are caused by the refraction, or bending, and internal reflection of light rays that enter the raindrops. A ray of white sunlight is actually composed of all the colours of the spectrum. Inside the drop the ray of white light is separated into the colours that make it up and reflected back toward the observer. In the primary rainbow the colours are, from inside to outside, violet, blue, green, yellow, orange, and red. The red band makes an angle of about 42 degrees with the sun’s rays, and the other coloured bands make successively smaller angles. Sometimes another less intense rainbow may also be seen; this is called the secondary bow. The secondary bow, when visible, is seen outside the primary bow and with its colour sequence reversed. It is produced by light that has been reflected from two different points on the back of the drop before emerging into the air. Higher-order rainbows are very weak and so are rarely seen.

Occasionally, faintly coloured rings are seen just inside the primary bow. These are called spurious, or supernumerary, bows. When raindrops are extremely fine, an almost white bow, called a fog bow, is produced. A fog bow at night, sometimes called a lunar rainbow, is made by sunlight reflected from the moon and appears as a ring around the moon.

DEFINITION: 1 the sensation resulting from stimulation of the retina of the eye by light waves of certain lengths 2 the property of reflecting light of a particular wavelength: the distinct colours of the spectrum are red, orange, yellow, green, blue, indigo, and violet, each of these shading into the next; the primary colours of the spectrum are red, green, and blue, the light beams of which variously combined can produce any of the colours 3 any colouring matter; dye; pigment; paint: the primary colours of paints, pigments, etc. are red, yellow, and blue, which, when mixed in various ways, produce the secondary colours (green, orange, purple, etc.): black, white, and grey are often called colours (achromatic colours ), although black is caused by the complete absorption of light rays, white by the reflection of all the rays that produce colour, and grey by an imperfect absorption of all these rays 4 any colour other than black, white, or grey; chromatic colour: colour is distinguished by the qualities of hue (as red, brown, yellow, etc.), lightness (for pigmented surfaces) or brightness (for light itself), and saturation (the degree of intensity of a hue) 5 colour of the face; esp., a healthy rosiness or a blush 6 the colour of a person’s skin 7 skin pigmentation of a particular people or racial group, esp. when other than white 8 [pl. ] a coloured badge, ribbon, costume, etc. that identifies the wearer 9 [pl. ] a) a flag or banner of a country, regiment, etc. b) the armed forces of a country, symbolized by the flag [to serve with the colours ] 10 [pl. ] the side that a person is on; position or opinion [stick to your colours ] 11 outward appearance or semblance; plausibility 12 appearance of truth, likelihood, validity, or right; justification [the circumstances gave colour to his contention] 13 general nature; character [the colour of his mind] 14 vivid quality or character, as in a personality, literary work, etc. 15 Art the way of using colour, esp. to gain a total effect 16 Law an apparent or prima-facie right 17 Mining a trace of gold found in panning 18 Music a) timbre, as of a voice or instrument; tone colour b) elaborate ornamentation 19 Particle Physics a unique, hypothetical force or charge on each type of quark that controls how quarks combine to form hadrons: although called red, green, and blue, they are not related to visual colours 20 Photog., TV, etc. reproduction of images in chromatic colours rather than in black, white, and grey

One of the most striking features of the visible world is the abundance of colour The most extensive parts of the Earth and its atmosphere air, soil, and water are usually coloured The sky can be blue or black or grey and even reddish or purplish. Soils can be black or brown or grey and even red.

Bodies of water look blue or green. One of the important ways people obtain information about the world is by looking at the colours of things. When the green leaves of a plant turn brown, it may be a sign that the plant is sick. It can also be a sign of the season of year, since in the autumn the leaves of many trees turn brown.

The colour of a fruit can reveal whether it is ripe. A green banana is unripe, a yellow one is ripe, and a yellow banana with brown and black spots is overripe. A green tomato is unripe, but a red one is ripe. Colour can also indicate the flavour of foods. Brown rice has a different flavour from that of white rice.

What does it mean to say that a tomato is red? Is colour part of the tomato in the same way that shape is? A tomato examined in the dark is still perceived as round but not as being red. It has no colour at all. Moreover, if a bright blue light is shined only on the tomato, it does not look red but black. So colour, unlike shape, depends on light. In fact, it cannot exist apart from light. Yet in a sense the tomato can be described as red. Somehow, if the right kind of light shines on it, the tomato looks red. The colour of the tomato has something to do with the way light interacts with it.

But colour also has something to do with the persons and animals who see it. For the tomato to be red, viewers able to perceive colour are needed. Many kinds of animals cannot distinguish colours They see only in black, white, and greys A guinea pig looking at a tomato sees only a grey object. Colour exists the tomato is red because something happens in the eyes and the brains of certain persons and animals that enables them to perceive colour

It is possible to study colour from many points of view. Chemists and physicists, for example, have a special interest in colour Sometimes the molecular structure of chemicals or the physical arrangement of their atoms may reveal why they reflect only certain kinds of coloured light.

Physicists who study optics a branch of physics have developed theories of colour Biologists and psychologists use many interesting techniques to find out what enables people’s eyes and brains to perceive colour

Light from the noon-time sun looks white. But if a ray of white light is aimed at a prism, a broad band of different colours looking like a rainbow emerges. This colour array is called the visible spectrum.

In the 17th century Isaac Newton discovered that a second prism could not add more colour to light that had already passed through a prism. Red stayed red, green stayed green, and so on. But he observed that the second prism could spread the colours of the spectrum farther apart. A narrow red beam entering the second prism would emerge as a wider band of red. Newton also found that if he turned the second prism upside down so that the entire coloured band coming from the first prism entered it, white light would emerge. From these experiments he concluded that white light is a mixture of many different colours and that a prism is somehow able to bend it in such a way that the individual colours separate.

In the late 19th century the theory that light travels in the form of electromagnetic waves won acceptance. Waves are described by their speed, their wavelength, and their frequency. In a given medium, such as air or a vacuum, all light waves travel at the same speed, but they differ in wavelength and frequency. Wavelength and frequency are inversely proportional to each other the longer the wavelength, the lower the frequency. For the visible light spectrum, scientists commonly specify only the wavelength.

Each colour is associated with a range of wavelengths. The name green or red does not apply to just one colour A wide segment of the spectrum contains colours that are called green. These include blue-green, apple green, and chartreuse, as well as many intermediate greens. Another wide segment contains colours that are called red. Colours of nearly the same wavelength look exactly alike to the human eye.

The colours of the spectrum range, in order, from violet, through blue, green, yellow, and orange, to red. The wavelengths of violet are the shortest, ranging from 380 to about 450 nanometres (A nanometre is one billionth of a meter long.) Wavelengths of red are the longest, ranging from about 630 to 760 nanometres Wavelengths shorter than those of violet are called ultraviolet radiation; wavelengths longer than those of red are infra-red radiation. They produce no sensation of colour in humans. “Black” is the absence of colour

Additive Mixing with Coloured Light

Newton discovered that by mixing two differently coloured rays of light he could produce other colours When he projected light beams from different prisms onto a white background, he found that sometimes the new colour looked like one of the other colours of the spectrum.

Red and yellow, for example, could be mixed to look like the orange of the spectrum. But colours could also be created in this way that did not look like any of the spectral colours Thus red and violet could form purples that did not match any colour in the spectrum. Newton also observed that as certain coloured lights were combined, a grey or white patch of light was produced. He found that he could often obtain white light by mixing the beams of three different colours

Almost all colours can be matched by three beams of differently coloured light. The greatest number of different colours can be produced when the three colours are chosen from the middle and the two ends of the spectrum. In other words, a combination of one of the reds, a green, and a blue or violet will produce the greatest range of colours For this reason red, green, and deep blue are called the primaries for additive colour mixing, or additive primaries. These three colours are used more than any other combination of colours to mix coloured light beams.

When only two of the additive primaries are mixed in a certain amount, the resulting colour is called the complementary colour, or complement, of the third additive primary. When red and green light beams are mixed, the resulting colour is yellow, the complementary colour of blue. A mixture of red and blue makes a purplish colour called magenta, the complement of green. And green and blue mixed together form cyan, the complement of red.

When the additive primaries are mixed in other amounts, intermediate colours are formed. This fact is the basis of the science of colorimetry, or colour measurement. Once the three primary colours are agreed upon, most other colours can be defined by the amounts of the three primary colours that, mixed together, match the new colour

Subtractive Colour Mixing

When light strikes an object, it may be transmitted, absorbed, or reflected. A windowpane, for example, transmits almost all the light that strikes it. Since it does not change the light, the pane looks colourless, or clear. A blackboard free of chalk dust, on the other hand, absorbs almost all the light that strikes it and therefore since blackness is the absence of light looks dull and black. A plaster wall both reflects and absorbs light. If the wall is white, it reflects almost all the light that falls on it.

Sometimes a substance absorbs some but not all the colours that reach it. For example, a red tomato absorbs all wavelengths but those of red, which, after bouncing from molecule to molecule within the top layers of the tomato, are redirected outward. When blue light (which does not contain red wavelengths) shines on a tomato, the blue wavelengths are absorbed. The tomato then looks black because no light is reflected from it.

Transparent red objects such as red cellophane, red plastic, or red glass absorb all wavelengths but red ones, which they partly transmit and partly reflect. Such transparent objects are called colour filters because when white light strikes them they filter out all colours except their own, which can pass through them easily.

Colour filters are the basis of subtractive colour mixing, just as coloured beams of light are the basis of additive mixing. Subtractive colour mixing is a complicated procedure because the different dye molecules in two different filters may produce the same colour sensation yet absorb different wavelengths of light. The description of subtractive colour mixing that follows assumes that ideal filters are used.

When a beam of white light strikes a yellow filter, the wavelengths that make up yellow can pass through the filter while all other wavelengths are absorbed. Since yellow is a mixture of green and red light, the wavelengths of those colours pass through, but the wavelengths of blue the complement of yellow are absorbed. Yellow is sometimes called minus-blue, since it can filter out blue light. Similarly, a magenta filter allows wavelengths of red and blue to pass but absorbs wavelengths of its complement, green. For this reason, magenta is sometimes called minus-green.

If a yellow filter (minus-blue) is placed on top of a magenta filter (minus-green) and a beam of white light is passed through them, the yellow filter absorbs blue, the magenta filter absorbs green, and only red light emerges.

A cyan filter (minus-red) absorbs its complement, red. If a yellow, a cyan, and a magenta filter are aligned in front of a beam of white light, all three of the additive primaries are absorbed, and no light emerges. This is called subtractive colour mixing because the filters absorb, or subtract, colour from a beam of light.

Pointillism (or divisionism), impressionistic painting process; the chief exponents were French artists Georges Seurat and Paul Signac.

Paint mixtures usually exhibit the complex behaviour of subtractive mixing. A mixture of yellow and cyan watercolours gives one of several greens, depending on what pigments make up the original cyan and yellow paints. If magenta is then added, black or grey results. However, pigments can be combined in additive mixtures by means of special techniques. A famous method is divisionism, sometimes called pointillism, which was used by some post-impressionist painters. They painted tiny dots of pure spectrum colours next to one another so that light reflected by one dot would combine with light reflected by a second dot in an additive mixture. One of the most famous paintings of this school is Georges Seurat’s ‘Sunday Afternoon on the Island of the Grande Jatte’.

Colours produced by the subtraction of wavelengths, or filtering, often occur in nature. The reds and oranges of a sunset are caused by the filtering action of the sky. The sky scatters light of short wavelengths, such as blue. At midday, when the sun is overhead, the scattered blue light does not have to travel through very much air to reach a viewer. The sky looks blue because a great deal of blue light is reflected from it. But at sunset the light must travel through much more air on its way to Earth. The blue is soon scattered, and only the colours of longer wavelengths combined to appear orange and red can be seen.

From earliest times the rainbow had delighted and puzzled observers. Men invented myths to explain the beautiful arc of multicoloured light that appeared after the rain. But a scientific answer to the puzzle of the rainbow did not come until 1666. In that year Sir Isaac Newton began investigating the problem of eliminating the colour fringes in telescope lenses. (Scientists now call these colour fringes chromatic aberration.) He decided that the trouble might lie in the character of light itself. So he began to study how light formed colours

He admitted a small beam of sunlight into a darkened room and passed it through a prism. The beam produced a band of colours just like the rainbow, ranging from red through yellow, green, and blue to violet. He then passed each of these colours through other prisms and found that they did not change. But when he passed the whole band of coloured lights through a prism in reverse position, the coloured band became white sunlight again.

From this he reasoned that white light is really a mixture of coloured lights, and that each colour is bent by a different amount when it passes through the prism. This difference in bending enables each colour to stand out separately and be visible. The band of coloured lights thus formed is called a spectrum. The rainbow is actually a spectrum, formed by sunlight passing through raindrops.

Separating light into its colours is called dispersion. It is accomplished by refraction (bending) of light in the prism. Each of the colours has its own wavelength. The wavelength determines how much each colour will bend. Red bends the least, violet the most. If the light beam strikes the prism at a certain angle, the amount of bending for each colour is always the same. Each colour then falls in exactly the same place on a screen, so its position is enough to identify it.

Scientists use the dispersive action of the prism in the spectroscope. The spectroscope reveals that the spectral pattern of light is different for various classes of light sources. Light from the sun, from certain lamp filaments, and from molten metals each produces a spectrum which has all colours in an unbroken array. Such a pattern is called a continuous spectrum. Incandescent gases give off only certain colours, in fine lines. Their spectra are called bright-line spectra. Both bright-line and continuous spectra are emission spectra, produced by emitted light.

Fraunhofer, Joseph von (1787-1826), German optician and physicist; worked to improve optical instruments; invented a heliometer and a micrometer.

In the early 1800s Joseph von Fraunhofer observed that the continuous spectrum was crossed by many dark lines. He charted more than 700 of them, but he was unable to explain their meaning. Because of his discovery, however, they are called Fraunhofer lines.

Kirchhoff, Gustav Robert (1824-87), German physicist, born in Konigsberg, East Prussia (now Kaliningrad, Russia); developed spectrum analysis and discovered caesium and rubidium (with Bunsen); explained the Fraunhofer lines; professor of physics at Heidelberg 1854-74, at Berlin 1874-87.

The meaning of the Fraunhofer lines was discovered about 50 years later by Gustav Kirchhoff and Robert Bunsen. With a spectroscope they studied the spectra of certain substances which were vaporized in the non luminous Bunsen burner flame. Each vapour showed a characteristic bright-line spectrum. But when emitted light was passed through a cooler vapour of the same substance, the bright lines were replaced by dark ones in the same position.

This replacement of bright by dark lines meant that the second vapour had absorbed the characteristic light of the first. Later experiments showed that the cooler vapour absorbs those light waves which it would normally emit at a higher temperature. In 1859 Kirchhoff published his findings in his laws of radiation and absorption. The spectral pattern thus formed is called a dark-line, or absorption, spectrum.

Kirchhoff and Bunsen also noticed that characteristic arrays of lines are given off by the different chemical elements. For example, incandescent sodium always gives certain yellow lines near the middle of the spectrum, and no other element gives these lines. Thus when these lines appear, sodium must be present in the incandescent substance. If the lines are bright the light has come directly from the incandescent sodium. If they are dark the light has passed, somewhere along its path, through an absorbing vapour containing some gaseous sodium. Only minute quantities of an element are needed to make its lines appear. This makes it possible to identify the elements in unknown substances.

These discoveries not only explained the Fraunhofer lines in the spectrum of sunlight but made it possible to determine what chemical elements the sun contains. The absorption necessary to produce the dark lines was considered as taking place in the outer layers of incandescent gas surrounding the sun. For “analysis” of the sun, the dark lines could be compared with the bright-line spectra of different elements produced in the laboratory. Whenever they corresponded, scientists could be sure that the element existed in the sun. Stars likewise could be “analysed” as to chemical contents by this method.

Janssen, Pierre-Jules-Cesar (1824-1907), French astronomer, born in Paris; discoverer of helium in sun; founded and directed observatory on Mont Blanc 1893.

Lockyer, Joseph Norman (1836-1920), British astronomer and physicist, born in Rugby, England; pioneer in application of spectroscope to sun and stars; explained sunspots; between 1870 and 1905 conducted eight British expeditions for observing total solar eclipses (‘The Sun’s Place in Nature’; ‘Recent and Coming Eclipses’; ‘The Chemistry of the Sun’; ‘Inorganic Evolution’).

Ramsay, William (1852-1916), British chemist, born in Glasgow, Scotland; professor Bristol University 1880-87, University of London 1887-1913; discoverer of helium, neon, krypton, xenon; co-discoverer of argon; research in radioactivity led to new theory of transmutation of elements; knighted 1902; received 1904 Nobel prize.

Cleveite, mineral, oxide of uranium and lead; named for Per Theodor Cleve, a Swedish chemist; produces helium when heated with acid.

Scientists have obtained spectra corresponding to the different elements and have measured and charted every line. When they wish to learn the composition of a star, they photograph its spectrum and then check the lines against these charts for the elements. A notable triumph of the method was the discovery of helium. In 1868 P.J.C. Janssen (1824-1907), a French astronomer, and the English astronomer, Sir Norman Lockyer (1836-1920), independently discovered lines in the solar spectrum which could not be identified with the charted lines of any known element. Lockyer interpreted this to mean that an element unknown to us existed in the sun. He named it helium, after helios, Greek for “sun.” Then in 1895 Sir William Ramsay (1852-1916) found that the Norwegian mineral cleveite, when heated, gave off minute quantities of a light gas which he identified as helium by means of its spectrum.

Measuring Light Waves

Millimicron (mm), unit of wave length equal to one millionth of a millimetre or one thousandth of a micron; sometimes used in the measurement of light waves.

Angstrom unit (A, or A.U.), ten-millionth of a millimetre, used to describe length of light waves; named to honour Anders Jonas Angstrom.

Nanometre (nm), measuring unit in spectroscopy, equalling one billionth of a meter.

The units once commonly used to measure wavelengths were the millimicron, denoted by the symbol mm and equalling one millionth of a millimetre; and the Angstrom unit (A or A.U.), one ten millionth of a millimetre Wavelengths are still measured in various units, but the unit most commonly used in spectroscopic work today is the nanometre (nm), which is equal in length to the unit it replaced, the millimicron. This is one of the special units which science has accepted as a means of avoiding the excessively long decimal fractions which would be needed to express wavelengths as short as those of light, if measured in inches or centimetres For example, violet light has a wavelength of 410 nanometres. The following table gives the wavelengths which fall approximately in the centre of each of the coloured regions in visible light:

Violet 410 nanometres

Blue 470

Green 520

Yellow 570

Orange 620

Red 710

Since the colour of light is determined by its wavelength, this means that the shorter the wavelength the more the light is bent by passage through a given prism. Thus the wavelength (and the frequency) of the vibration causing the wave is judged from the amount of bending given by the prism. This is determined by the position of the spectral line on the screen or photographic plate.

Prism and Diffraction-Grating Spectroscopes

Modern spectroscopes used in very technical fields vary considerably in function and design and are often quite specialized for the specific substances they analyse However, the fundamental teaching instruments generally used in today’s classrooms are the relatively simple prism spectroscopes. These consist of a collimator (tube for admitting light), a glass prism, and a telescope. The collimator has a slit at one end to admit light and a lens on the other to concentrate it. The lens directs the light on the prism, which disperses the ray into its component colours Sometimes a train of prisms is used to increase the dispersion.

After the colours leave the prism they are focused on the object glass of the telescope. Each wavelength appears as a separate image of the collimator slit. When the telescope is replaced by a camera to photograph the lines, the device is called a spectrograph.

Rowland, Henry Augustus (1848-1901), U.S. physicist, born in Honesdale, Pa.; professor Johns Hopkins University 25 years; determined ohm and the mechanical equivalent of heat; discovered magnetic effect of electric convection.

A more powerful type of spectroscope uses a diffraction grating, invented by Fraunhofer in 1821. He made it by twisting a fine wire about two tiny screws. With it he measured the wavelengths of light with surprising precision. The modern precision grating consists of a plate of speculum metal or glass upon which fine lines, equidistant and parallel, have been ruled. Among the finest of these are the gratings made by H. A. Rowland. He invented a machine to rule the entire grating automatically, etching from 14,000 to 20,000 lines per 1 inch (2.5 centimetres). By means of such a grating, made on a concave surface, Rowland secured a spectrum band of sunlight more than 20 feet (6 meters) long. The grating uses a special application of the interference phenomenon of light.

Diffraction-grating spectroscopes can measure the wavelength of light with a precision of .000,000,000,001 centimetre (10^-12 centimetre). It is used as the dispersing medium in analysing visible light and ultraviolet rays. A photographic plate is usually used as the detecting device in the analysis.

Motion, Temperature, Magnetism

Doppler effect, law in physics discovered by Christian Doppler (1803-53); applied to sound, light, and radar from moving sources.

The spectroscope can also tell the astronomer whether a star is moving toward or away from the Earth by means of a phenomenon known as the Doppler effect. Everyone has noticed how the whistle of an approaching locomotive rises to a shrill note as it approaches then drops to a lower and lower tone as the train rushes away. The reason for this is that when the train approaches, its whistle is nearer to us each time a sound wave is emitted. The successive waves reach us a little more quickly and therefore have a higher pitch. When the train is receding, the waves are dragged out, and thus the pitch of the whistle is lowered.

Similarly, when a star moves toward the Earth, each light wave is shortened a little. Consequently, the lines shift their position toward the violet end of the star’s spectrum. When the star is moving away from the Earth, the wavelengths are lengthened somewhat, and the lines in the spectrum shift a little toward the red end. The amount of shift reveals the speed of the star’s motion; but since light in a vacuum travels at the tremendous speed of 186,282 miles per second (299,743 kilometres per second), the star must be travelling at a very great speed to create a noticeable effect.

Temperature, degree of hotness or coldness measured on a definite scale.

Temperature and pressure have certain effects on spectra. These effects can be detected and used to determine the approximate temperature of stars and the pressure of gases on distant bodies.

Zeeman, Pieter (1865-1943), Dutch physicist; professor physics and director Physical Institute, University of Amsterdam, 1900-35; discovered the Zeeman effect of magnetism on light; Nobel prize 1902.

Another marvellous revelation of the spectroscope is the connection between magnetism and light. In 1896 the Dutch physicist Pieter Zeeman (1865-1943) discovered that when light passed through the field of a strong electromagnet, the lines in the resulting spectrum were split into two or more lines. This influence of magnetism on light, which was named the Zeeman effect after its discoverer, has proved to be valuable in the detection and measurement of magnetism in the sun.

The Electromagnetic Spectrum

The coloured lights in the rainbow make up but a small portion of that huge spectrum of energy called electromagnetic radiation. The other groups include radio waves, microwaves, infra red light (heat), ultraviolet rays, X rays, and gamma rays. Despite the different effects they produce, each of these forms of energy travels through space as an electromagnetic disturbance. They are sometimes called forms of radiant energy.