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- From: Noyes, William A. A Textbook of Chemistry. Henry
Holt and Company, New York (1919) pp. 425-427.
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- Copyright © 1998 Richard A. Paselk
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- THE SPECTROSCOPE
- Frontspiece: PLATE
OF SPECTRA
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- The simplest form of spectroscope is shown
in Fig. 97. The prism A is of glass having a.high dispersive
power. A narrow slit at B is illuminated by the flame or light
which is to be examined. Between the slit and the prism is placed
a.lens at C, which renders the rays of light from the slit parallel
before they
- reach the prism. The prism is set in such
a manner that the angle of incidence on the first face is the
same as the angle of emergence from the second face, as this
gives the purest spectrum. The light is examined by means of
the telescope D. A scale placed at E, illuminated by a light
placed before it and whose image is reflected from the surface
of the prism, serves to locate the position of the lines.
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- Such a flame as that of acetylene gives a
continuous spec- trum, indicating that molecules of solid carbon
in the white flame are vibrating in all possible periods required
to give white light. If a Bunsen flame is placed before the slit
and some com- pound of sodium, as sodium chloride, is introduced,
the flame assumes a brilliant yellow color, and with a single
prism spec- troscope the spectrum consists of a single, bright
yellow line.
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- A spectroscope having several prisms, or
a spectroscope using a metallic mirror (" grating ")
ruled with many thousands of equidistant lines and which gives
a diffraction spectrum, will separate the line into two lines
situated close together. The wave lengths of the lines are 0.5896
and 0.5590 microns, the micron being the thousandth part of a
millimeter. The physical significance of these lines seems to
be that under the conditions of the flame either the sodium atoms
as a whole or, more probably, portions of the sodium atoms or
electrons within or around them vibrate at a definite rate, which
is independent of the temperature. This rate is almost inconceivably
rapid. The velocity of light is about 300,000 kilometers per
second. This is equal to 3 X 10 14 microns, and since
the wavelength of the sodium light is only 0.59 micron, the number
of vibrations per second must be approximately
per
second.
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- There are two dark lines in the solar spectrum
which coincide exactly with the bright yellow lines of the sodium
spectrum. This is explained by supposing the interior of the
sun to be an incandescent mass which gives out light vibrations
of all wave lengths corresponding to the visible spectrum. The
photosphere of the sun, on the other hand, consists of a gaseous
envelope or atmosphere containing many different elements, among
these sodium. The sodium atoms, if they have the power of producing
light waves in the ether by their vibrations, must also be able
to absorb waves of the same length from the ether, exactly as
a tuning fork is set in vibration by sound waves of its own pitch,
while waves of a different pitch do not affect it. The sodium
atoms in the photosphere, therefore, absorb the waves of their
own particular rate; and while they give the energy absorbed
back again to the ether, they dissipate the energy by spreading
it in all directions instead of allowing it to pass on toward
the observer. The result is that the portion of the spectrum
corresponding to the sodium vibrations will be rela- tively dark.
By means of this principle it has been possible to show that
more than thirty elements found on the earth are found also in
the sun. One of these elements (helium), indeed, was discovered
in the sun before it was found on the earth. For the purpose
of comparing spectra it is convenient to place a right-angled
prism before the slit of the spectroscope in such a manner as
to cover one half of it. This may be made to reflect the light
from a second flame into the slit in such a way that the spectrum
from one flame will occupy the upper half of the field of vision
while the spectrum from the other flame will occupy the lower
half. In this manner the coincidence of lines in the two spectra
may be readily observed.
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- In another form, known as the direct vision
spectroscope, a series of prisms of different kinds of glass
are so combined that one kind of glass counter-balances the mean
refractive index of the other, while the dispersive effects are
not counterbalanced. The effect is exactly the reverse of that
in an achromatic lens. Such spectroscopes are especially suitable
for the detection of the alkali and alkali-earth metals in qualitative
analysis. By means of the spectroscope it is possible to detect
1/3000000 milligram of sodium. Only the methods used in studying
radioactive sub- stances are more sensitive than this.
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- To obtain the spectra of iron, copper and
other metals, which are volatile only at high temperatures, electric
sparks from a Rumkhorf coil are passed between terminals of the
metal, or, in some cases, between platinum wires, one of which
is in a small cup containing a solution of a salt of the metal.
The spectra of gases are observed in Plucker tubes, which have
a narrow portion through which the electric discharge is passed.
- © R. Paselk
- Last modified 22 July 2000
