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476. The rainbow. There is formed in nature a very beautiful spectrum with which everyone is familiar the rainbow.
Let a spherical bulb F (Fig. 445) 1 or 2 inches in diameter be filled with water and held in the path of a beam of sunlight which enters the room through a hole in a piece of cardboard AB. A miniature rainbow will be formed on the screen around the opening, the violet edge of the bow being toward the center of the circle and the red outside. A beam of light which enters the flask at C is there both refracted and dispersed; at D it is totally reflected; and at E it is again refracted and dispersed on passing out into the air. Since in both of the refractions the violet is bent more than the red, it is obvious that it must return nearer to the direction of the incident beam than the red rays. If the flask were a perfect sphere, the angle included between the incident ray OC and the emergent red ray ER would be 42°; and the angle between the incident ray and the emergent violet ray EV would be 40°.
FIG. 445. Artificial rainbow
The actual rainbow seen in the heavens is due to the refraction and reflection of light in the drops of water in the air which act exactly as did the flask in the preceding experiment. If the observer is standing at E with his back to the sun, the light which comes from the drops so as to make an angle of 42° (Fig. 446) with the line drawn from the observer to the sun must be red light; while the light which comes from drops which are at an angle of 40° from the eye must be violet light. In direct sunshine the prismatic color seen in a dewdrop changes to another color when the head is shifted sidewise. It is clear that those drops
whose direction from the eye makes any particular angle with the line drawn from the eye to the sun must lie on a circle whose center is on that line. Hence we see a circular arc of light which is violet on the inner edge and red on the outer edge. A second bow having the FIG. 446. Primary and secondary rainbows red on the inside and the violet on the outside is often seen outside of the one just described, and concentric with it. This bow arises from rays which have suffered two internal reflections and two refractions, in the manner shown in Fig. 446.
477. Continuous spectra. Let a Bunsen burner arranged to produce a white flame be placed behind a slit s (Fig. 447). Let the slit be viewed through a prism P. The spectrum will be a continuous band of color. If now the air is admitted at the base of the burner, and if a clean platinum wire is held in the flame directly in front of the slit, the whitehot platinum will also give a continuous spectrum.*
All incandescent solids and liquids are found to
give spectra of this type FIG. 447. Arrangement for viewing spectra
which contain all the
wave lengths from the extreme red to the extreme violet. The continuous spectrum of a luminous gas flame is due to
*By far the most satisfactory way of performing these experiments with spectra is to provide the class with cheap plate-glass prisms, like those used in Experiment 50 of the authors' Manual, rather than to attempt to project line spectra.
the incandescence of solid particles of carbon suspended in the flame. The presence of these solid particles is proved by the fact that soot is deposited on bodies held in a white flame.
478. Bright-line spectra. Let a bit of asbestos or a platinum wire be dipped into a solution of common salt (sodium chloride) and held in the flame, care being taken that the wire itself is held so low that the spectrum due to it cannot be seen. The continuous spectrum of the preceding paragraph will be replaced by a clearly defined yellow image of the slit which occupies the position of the yellow portion of the spectrum. This shows that the light from the sodium flame is not a compound of a number of wave lengths, but is rather of just the wave length which corresponds to this particular shade of yellow. The light is now coming from the incandescent so um vapor and not from an incandescent solid, as in the preceding experiments.
Let another platinum wire be dipped in a solution of lithium chloride and held in the flame. Two distinct images of the slit, s' and s" (Fig. 447), will be seen, one in red and one in yellow. Let calcium chloride be introduced into the flame. One distinct image of the slit will be seen in the green and another in the red. Strontium chloride will give a blue and a red image, etc. (The yellow sodium image will probably be present in each case, because sodium is present as an impurity in nearly all salts.)
These narrow images of the slit in the different colors are called the characteristic spectral lines of the substances. The experiments show that incandescent vapors and gases give rise to bright-line spectra, and not continuous spectra like those produced by incandescent solids and liquids (see on opposite page). The method of analyzing compound substances through a study of the lines in the spectra of their vapors is called spectrum analysis. It was first used by Bunsen in 1859.
479. The solar spectrum. Let a beam of sunlight pass first through a narrow slit S (Fig. 448), not more than millimeter in width, then through a prism P, and finally let it fall on a screen S', as shown in Fig. 448. Let the position of the prism be changed until a beam of white light is reflected from one of its faces to that portion of the screen which was previously occupied by the central portion of the spectrum.
Then let a lens L be placed between the prism and the slit, and moved back and forth until a perfectly sharp white image of the slit is formed on the screen. This adjustment is made in order to get the slit S and the screen S' in the positions of conjugate foci of the lens. Now let the prism be turned to its original position. The spectrum on the screen will then consist of a series of colored images of the slit arranged side by side. This is called a pure spectrum, to distinguish it from the spectrum shown in Fig. 440, in which no lens was used to bring the rays of each particular color to a particular point, and in which there was therefore much overlapping of the different colors. If the slit and screen are exactly at conjugate foci of the lens, and if the slit is sufficiently narrow, the spectrum will be seen to be crossed vertically by certain dark lines.
FIG. 448. Arrangement for obtaining a pure spectrum
These lines were first observed by the Englishman Wollaston in 1802, and were first studied carefully by the German Fraunhofer in 1814, who counted and mapped out as many as seven hundred of them. They are called, after him, the Fraunhofer lines. Their existence in the solar spectrum shows that certain wave lengths are absent from sunlight, or, if not entirely absent, are at least much weaker than their neighbors. When the experiment is performed as described above, it will usually not be possible to count more than five or six distinct lines.
480. Explanation of the Fraunhofer lines. Let the solar spectrum be projected as in § 479. Let a few small bits of metallic sodium be laid upon a loose wad of asbestos which has been saturated with alcohol. Let the asbestos so prepared be held to the left of the slit, or between the slit and the lens, and there ignited. A black band will at once appear in the yellow portion of the spectrum, in the place where the color is exactly that of the sodium flame itself; or, if the focus was sufficiently sharp so that a dark line could be seen in the yellow before the sodium
was introduced, this line will grow very much blacker when the sodium is burned. Evidently, then, this dark line in the yellow part of the solar spectrum is in some way due to sodium vapor through which the sunlight has somewhere passed.
The experiment at once suggests the explanation of the Fraunhofer lines. The white light which is emitted by the hot nucleus of the sun, and which contained all wave lengths, has had certain wave lengths weakened by absorption as it passed through the vapors and gases surrounding the sun and the earth. For it is found that every gas or vapor will absorb exactly those wave lengths which it is itself capable of emitting when incandescent. This is for precisely the same reason that a tuning fork will respond to, that is, absorb, only vibrations which have the same period as those which it is itself able to emit. Since, then, the dark line in the yellow portion of the sun's spectrum is in exactly the same place as the bright yellow line produced by incandescent sodium vapor, or the dark line which is produced whenever white light shines through sodium vapor, we infer that sodium vapor must be contained in the sun's atmosphere. By comparing in this way the positions of the lines in the spectra of different elements with the positions of various dark lines in the sun's spectrum, many of the elements which exist on the earth have been proved to exist also in the sun. For example, Kirchhoff showed that the four hundred sixty bright lines of iron which were known to him were all exactly matched by dark lines in the solar spectrum. Fig. 449 shows a copy of a and iron spectra
FIG. 449. Comparison of solar