Spectrum

Spectrum. Light, coming from any source, can by suitable means be split up into Its component parts. This was first discovered by Newton, who allowed a fine beam of sunlight to enter a dark room through a small hole in the shutter. If allowed to fall unmolested upon a screen, an image of the sun was formed there, but when a prism was placed in its path, instead of a round image of the sun, there appeared a brilliantly coloured band upon the screen. In the accompanying figure H A is the beam of sunhght entering at H, and tending to form the sun's image at A¹. Ihe prism, however. affects the beam at A, and V R is the long band of colour formed on the screen. This coloured band was called bv Newton a spectrum, and the colours vary gradually from violet (V) through indigo, blue, green, yellow, and orange to red (R). It is noticed that the violet ray is bent most away from the original direction, H A A¹, of the beam. Hence the waves of violet light are said to be the most refrangible aud those of red light the least refrangible of the visible spectrum. However, our eyes are by no means able to detect the whole of solar radiation. Beyond the violet end of the spectrum there are waves capable of promoting active chemical decomposition, and this is specially the case with regard to silver salts. The presence of these ultra-violet waves can be shown in another way. Their rate of vibration is extremely rapid, but, if they fall upon some substances such as fluorescein or quinine sulphate, they produce slower vibrations in these bodies, and hence the eye is able to detect them. Bodies which possess this power are said to be fluorescent, and Professor Dewar has quite recently shown that many bodies, which are not fluorescent at ordinary temperatures, become brilliantly so when extremely cold, at about 180° C. If, therefore, we let the spectrum fall, not upon a white screen, but, upon one painted with quinine sulphate, we shall see the screen rendered luminous where it was originally dark beyond the violet end just as there are vibrations of too frequent periods to be detected by the eye, so also are there waves whose vibrations are too slow, and these occur beyond the red end of the spectrum. We constantly experience the fact that heat and light are in the habit of accompanying each other, and, if we examine different parts of the spectrum with a sensitive thermometer, we find the temperature low at the violet end, but rising in the red part, and continuing to rise rapidly in the invisible region beyond the red, until a maximum temperature is reached, after which it rapidly falls. The accompanying figure (Fig. 2) exhibits this alteration in temperature. A B is the length of the visible spectrum, and the height of the curve above this line-represents the heat at each point. Thus, B C is proportional to the temperature at B, the end of red spectrum, and D E is the maximum temperature at some point, D, in the ultra-red part. Every substance does not behave in the same manner to different radiations; by passing the light through a cell containing a solution of alum we can stop all the heat and let only the light through; by using a solution of iodine in carbon bisulphide we can get rid of the light and leave the radiant heat invisible to our eyes, but capable of an the heating effects possessed by the original beam. Instead of using sunlight for obtaining a spectrum, it is generally more convenient to use artificial light. If, however, we examine the light of all incandescent vapour, we find that we do not obtain a complete spectrum. A strong electric current is capable of heating silver to such an extent that it boils; its vapour is then seen to be green in colour; and, if this light be Sent through a prism, its spectrum simply consists of two green bands. Zinc, treated in a similar way, gives bands in the red and blue parts of the spectrum, but only darkness exists where the other colours might be. What is true of silver or zinc applies to every other metal; the heated vapour of each gives rise to its own particular bands and no others, and the bands are never the same for any two metals. Further, these bands are given when the metal is present in any form whatever. Sodium concealed in common salt, or copper hidden in brass, give their definite and unmistakable bands. An optical examination of the incandescent vapour of a substance must therefore prove the presence of any metal which it contains. This method of examination was first used by Bunsen and Kirchhoff, and is known as Spectrum Analysis. The metals Caesium and Rubidium were discovered in this way, for the substance containing them was found to give bands which did not agree with those obtainable from any known metal. Examination of substances in this way is usually performed by means of an instrument known as a spectroscope. In this instrument the only light which can reach the observer comes throngh a very fine slit at the end of a tube. The fineness of the slit is necessary to obtain a pure spectrum - i.e. one in which there is no overlapping of the different colours. In this tube is placed a convex lens, called a collimator, at a distance of its focal length from the slit. Light from the slit is therefore rendered parallel by the lens, and falls upon a prism suitably adjusted in position. For actually observing the effects a telescope is used. Now, so long as we are dealing with the light of an incandescent solid, we shall observe a continuous and pure spectrum. With an incandescent vapour containing no solid particles we have independent bright bands. But it was for long observed that a pure spectrum of the sun exhibited a number of dark lines interrupting the range of colours. A few of these were first noted by Wollaston, but many more were found by Fraunhofer, who characterised their positions; they are hence known as Fraunhofer's lines (Fig. 3), and called by the letters which he gave to them. The explanation of these lines is due to Kirchhoff. It had been noted by Fraunhofer that the dark lines D of the solar spectrum coincided exactly in position with the bright lines given by the yellow incandescent vapour obtained by burning alcohol containing salt. Kirchhoff obtained a weak solar spectrum with its characteristic D lines; on making the light pass through the salt flame, however, he got two bright lines instead - the bright lines of sodium; he then increased the intensity of his solar spectrum, still passing the light through the salt flame, and, as he did so, the bright D lines gradually faded away until at last they appeared much darker than when given by the solar light alone. Kirchhoff now obtained a pure spectrum by means of a limelight, passed the light through the salt flame, and got his spectrum interrupted by two dark lines, corresponding exactly with the D lines of the solar spectrum. It was, therefore, the case that the salt flame picked out from the complete spectrum just those waves which were the ones it could itself actually emit. It absorbed the rays which it would radiate. In the case of the experiment with the feeble solar light, when the D lines appeared rather bright the radiation by the salt flame exceeded its absorption of the same light. But as the solar light increased, the absorption rose until it exceeded the radiation, and thus by contact with the increased brilliancy of the other part of the spectrnm those bands appeared dark. Many other flames were then employed to artificially produce different Fraunhofer lines, and the mystery of their existence was cleared up. This discovery immediately opened up a wide fleld in astronomical investigation. The presence of the D lines proves that sodium exists in the vapour surrounding the sun, while the other lines also point to the presence of definite substances, such as iron, copper, nickel, etc., in a state of vapour. The same process of examination applied to the light from different stars has given us enormously increased knowledge of their composition.