SPECTRUM AND SPECTROSCOPE (Part 1 of 3)

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.