KIRCHHOFF'S LAWS: THE NATURE OF LIGHT SOURCES
When we analyze light from various astronomical sources, we do not always find a continuous rainbowcolored sequence of wavelengths. Spectra can be classified and interpreted according to laws formulated by the German chemist Gustav Kirchhoff more than a century ago. The three basic types of spectracontinuous, emission, and absorption-and the physical conditions under which they are formed are given by Kirchhoff's laws.:
KIRCHHOFF'S FIRST LAW-CONTINUOUS SPECTRUM: The spectrum of a radiating solid, liquid, or highly pressurized gas is an uninterrupted sequence of wavelengths known as a continuous spectrum.
KIRCHHOFF'S SECOND LAW-EMISSION, OR BRIGHT-LINE, SPECTRUM: The spectrum of a radiating rarefied gas is a set of isolated or discrete wavelengths whose appearance is a series of bright-colored lines that form a pattern characteristic of the chemical composition of the gas.
KIRCHHOFF'S THIRD LAW-ABSORPTION, OR DARK-LINE, SPECTRUM: Light from a radiating source producing a continuous spectrum will, if it passes through a cooler gas, have certain specific wavelengths characteristic of the cooler gas removed from the spectrum. The spectrum appears continuous except where it is crossed by dark lines, which indicates that these wavelengths have been removed.
There are many common examples of light sources whose spectrum is one of the three basic types. As one example, the spectrum of the glowing filament of an electric light bulb is a continuous spectrum. The spectrum of a neon sign is an example of an emission spectrum. The spectrum of a gas composed of molecules (which consist of two or more atoms) is actually many sets of very closely spaced spectral lines known as emission bands. And, as a final example, the spectrum of the sun and most stars is an absorption spectrum.
We will see additional examples of each type of spectrum at many points in the remaining chapters. The important point to remember is that the type of spectrum for a light source tells us something about the conditions in and around that source.
When we analyze light from various astronomical sources, we do not always find a continuous rainbowcolored sequence of wavelengths. Spectra can be classified and interpreted according to laws formulated by the German chemist Gustav Kirchhoff more than a century ago. The three basic types of spectracontinuous, emission, and absorption-and the physical conditions under which they are formed are given by Kirchhoff's laws.:
KIRCHHOFF'S FIRST LAW-CONTINUOUS SPECTRUM: The spectrum of a radiating solid, liquid, or highly pressurized gas is an uninterrupted sequence of wavelengths known as a continuous spectrum.
KIRCHHOFF'S SECOND LAW-EMISSION, OR BRIGHT-LINE, SPECTRUM: The spectrum of a radiating rarefied gas is a set of isolated or discrete wavelengths whose appearance is a series of bright-colored lines that form a pattern characteristic of the chemical composition of the gas.
KIRCHHOFF'S THIRD LAW-ABSORPTION, OR DARK-LINE, SPECTRUM: Light from a radiating source producing a continuous spectrum will, if it passes through a cooler gas, have certain specific wavelengths characteristic of the cooler gas removed from the spectrum. The spectrum appears continuous except where it is crossed by dark lines, which indicates that these wavelengths have been removed.
There are many common examples of light sources whose spectrum is one of the three basic types. As one example, the spectrum of the glowing filament of an electric light bulb is a continuous spectrum. The spectrum of a neon sign is an example of an emission spectrum. The spectrum of a gas composed of molecules (which consist of two or more atoms) is actually many sets of very closely spaced spectral lines known as emission bands. And, as a final example, the spectrum of the sun and most stars is an absorption spectrum.
We will see additional examples of each type of spectrum at many points in the remaining chapters. The important point to remember is that the type of spectrum for a light source tells us something about the conditions in and around that source.
