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Third Law of the Spectrum

المؤلف:  GEORGE A. HOADLEY

المصدر:  ESSENTIALS OF PHYSICS

الجزء والصفحة:  p-483

2025-12-21

662

Third Law of the Spectrum-Demonstration. - Examine sunlight by the spectroscope, and the colored spectrum will be seen to be crossed by a series of dark lines. Moisten a platinum wire with a solution of salt and hold it in a Bunsen flame in front of the slit. One of the dark lines, the D line, will be made darker. Shut out the sunlight and examine the sodium flame alone, and the D line will show in the same place as a bright yellow line.

We see in this demonstration that the passage of sunlight through sodium vapor - itself capable of giving a bright line- intensifies the dark line in the solar spectrum. The fact that this dark line is in the same place as the bright line does not mean that there is no sodium in the sun, but instead means that the sodium vapor in the sun's atmosphere absorbs the light of the same wave length as that given out by incandescent sodium, and makes this part of the spectrum look dark in comparison with the rest of it. Such spectra are called absorption spectra.

LAW III. The vapor of any substance will absorb the light. given out by that substance in a state of incandescence.

Absorption spectra can be shown by holding in front of the slit of a spectroscope a test tube containing a solution of potassium permanganate when the solar spectrum is being observed. It will then be seen that the spectrum is crossed by a number of dark bands. The vapors of barium and strontium will produce similar results.