The Wonders of Optics

by Fulgence Marion

C. Beginning of orange.

D. Middle of yellow.

E. Middle of green.

F. Beginning of blue.

G. Middle of indigo.

H. Middle of violet.

The designations of these lines have been retained to the present day, and they have been named after the Munich philosopher, being known as Fraunhofer’s lines. They are to be seen in all parts of the spectrum, and increase in number and fineness according as the width of the slit through which the light passes is diminished. It may be asked, how it happens that they increase in proportion to the narrowness of the aperture admitting the light? A little consideration will soon show the reason of this.

When a beam of light is passed through a hole of, let us say, the eighth of an inch in diameter and decomposed by a prism, the spectrum so produced is imperfect, inasmuch as an infinite number of spectra are thus superposed, and for this reason, that the rays of light entering on the right side of the aperture will give a spectrum falling in a different place to that formed by the rays entering on the left. In order, therefore, to diminish the confusion caused by the superposition of a number of spectra, the aperture ought to be reduced to a narrow slit. When the thin slice of light passing through the slit is decomposed by the prism, we find that not only is the purity of the colours greatly increased, but the lines in question make their appearance more or less in all parts of the coloured band.

These lines are very unequally distributed, some being crowded together in masses, while others are extremely faint, and are separated by large intervals. Their position is well marked and determined, no matter from what source we obtain our beam of sunlight. Whether the spectrum be produced from the sun itself, or from the reflected light proceeding from the moon or planets, they are still found in the same place; only that in the latter case they are not so numerous, on account of the light being much fainter. For many years the cause of these lines remained a complete mystery, and it was not until Bunsen and Kirchhoff undertook their investigation that a satisfactory explanation of their origin was arrived at. In order to explain this, we must consider briefly the properties of the spectra of flames, and other luminous bodies.

If, instead of the light of the sun, we examine prismatically the light given off by an incandescent body, such as a white-hot piece of platinum, we shall find that the lines seen in the solar spectrum are absent, and that we have a continuous band of coloured light quite uninterrupted by dark spaces or bands. The same absence of lines is seen in the spectra of the electric light and the flame of an ordinary candle, the light in each of these cases being produced by particles of carbon in a state of vivid incandescence. But if we examine the flame of incandescent gases, we shall find a spectrum of an entirely new kind. Thus if we examine an ordinary gaslight through a slit with a prism, we shall obtain a continuous spectrum, in consequence of the luminous portion of the flame consisting of solid carbon in a state of incandescence; but if we turn down the flame, so as to lessen the amount of carbon to be burned, we shall find the whole of that body is converted into feebly luminous gas, giving off a faint reddish blue light. If we now again examine it in the same manner, we shall find that the spectrum produced consists of black spaces, here and there crossed by a few faint coloured lines or bands. The reason of this is obvious: in the faint flame caused by the carbon and hydrogen in a state of luminous vapour, which only have a few of the colours of the spectrum, which, when passed through the prism, fall into their proper places. All substances with which we are acquainted are capable of being converted into luminous vapour by means of heat, and when thus burnt produce flames of more or less faint luminosity, generally characteristically coloured. A piece of soda inserted in the wick of a spirit-lamp gives a yellow tinge to the flame; a morsel of saltpetre (nitrate of potash) or nitrate of strontia will give a purple and crimson tint respectively. These hues are caused by the metals sodium, potassium, and strontium contained in these salts being converted into luminous vapour. On analyzing these coloured flames with a prism, as before, we should find in the case of the soda a single broad yellow line, situated just in the middle of the yellow portion of the spectrum, the rest of the space where the spectrum should be being perfectly dark. The reason of this is pretty simple. Sodium burns with a pure yellow flame, consequently when passed through a prism it cannot split into any other colours, but takes its place in the position belonging to yellow of that particular hue. Were it a little more orange or green in tint, it would take its place nearer to the red or violet end of the spectrum. The light from saltpetre, which contains potassium may next be examined. It will be found to tinge the flame with the spirit-lamp of a beautiful purple. We can almost guess what will happen when this flame is submitted to the action of the prism. We shall find that the purple light emitted will split into red and violet, which will immediately arrange themselves in their proper positions according to their hues. If in like manner we substitute nitrate of strontia for saltpetre, we shall get a splendid crimson flame which is decomposed by the prism into red, orange, or blue.