SPECTROSCOPY
Just as sound waves of various wavelengths are transported simultaneously through the same region of air, electromagnetic waves of different wavelengths can move through the same point in space and superimpose to form composite waves, or white light. Stars, for example, are white-light sources though the color of the composite light from the stars may be white, red, yellow, or blue. As stated earlier, white, or composite, light can then be dispersed, or separated, into its component colors, or wavelengths,
MEASURING THE DOPPLER EFFECT
to form a spectrum. The study of spectra is called spectroscopy. Let us briefly explain how we can accomplish the dispersion of composite light by using a triangular piece of glass called a pris.
In the refraction of a ray of light the angle through which light is refracted depends on wavelength; the angle of refraction is greater for shorter wavelengths than it is for longer ones. Consider white light passing through the slit of a narrow diaphragm and then through a glass prism. The light disperses into its component wavelengths, with shortwavelength light refracted through larger angles than long-wavelength light is. Thus waves of different wavelengths disperse in different directions. The result is a rainbow-colored sequence of images of the slit, containing an image for each wavelength present in the white light.
Another means of dispersing white light to produce a spectrum is the diffraction grating. Unlike the ordinary glass prism, which is transparent only to visible and infrared radiation, the grating is useful over a broad spectrum, from X-ray to infrared wavelengths. In its simplest form the diffraction grating is a plate containing a very large number of very narrow parallel slits, uniformly spaced at distances that are only a few times the wavelength of light. (By "large number" we mean many thousands of slits per centimeter.) The spectrum is viewed in the direction of the light source, as in Figure 4.9. Since the amount of the bending, or diffraction, of electromagnetic waves at each slit depends on its wavelength, composite light is dispersed into its component colors. (For more details on these dispersing devices as they are used in an instrument called a spectroscope.
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.
IDENTIFYING THE ELEMENTS FROM EMISSION OR ABSORPTION SPECTRA
An astronomical light source, such as a star or a gaseous nebula, contains a mixture of chemical species, each either emitting or absorbing its own set of wavelengths of electromagnetic radiation. With the aid of laboratory spectral analysis of the different chemical elements, astronomers can identify individual elements in the light source from the measured wavelengths of its spectral lines, regardless of whether they are emission 'or absorption lines.
Identification is done in the following way: Light from a celestial body is collected by a telescope and then passed through a spectrograph in order to disperse the white light from the light source and form its spectrum. The photographic plate on which the spectrum is recorded is called a spectrogram. As a standard against which unknown wavelengths in the astronomical spectrum can be measured, an emission spectrum of a known gas, such as neon or vaporized iron or titanium, is placed above and below the astronomical spectrum. (The mechanism for placing the laboratory spectrum on the astronomical spectrogram is a part of the telescope and spectrograph.) With these comparison lines of known wavelength the astronomer can determine the unknown wavelengths of the astronomical object's spectral lines. The absorption spectrum is gray with black absorption lines and the comparison spectrum of neon shows white emission lines on a black background.
Kirchhoff's laws of spectrum analysis tell us about the general physical conditions of the light source. And if the spectrum of the light source contains absorption or emission lines, we can measure their wavelengths and identify the chemical elements that are present.
Can more detailed information about the light source be found? Suppose we want to know the temperature of the light source. Can this be done? Yes it can, for special types of light sources known as ideal radiators, or blackbodies.
All objects radiate and absorb some form of electromagnetic radiation; the wavelength region and the amount of energy depend generally on the body's temperature and physical state. From laboratory experiments and from theory physicists in the nineteenth century analyzed how various bodies emit and absorb radiation as a function of temperature and wavelength. From this work they developed the concept of an idealized radiator called a blackbody.
BLACKBODY CONCEPT:
A blackbody is an imaginary body that, when cool, absorbs all the radiant energy falling on its surface so that it is black in color; when hot, the blackbody emits energy with 100 percent efficiency. (Real matter is generally less than 100 percent efficient when it radiates.)
For our purposes the most important feature of the blackbody is the way in which emitted radiant energy is spread in wavelength, or the spectral energy distribution. Scientists have found that the distribution of energy depends only on the blackbody'S temperature and not on its chemical composition. Note how the amount of radiant energy emitted by a blackbody varies with wavelength in a very recognizable way, even for different temperatures. The emission of radiant energy (or the brightness at each wavelength) covers a continuous range of wavelengths so that the spectrum of a blackbody is a continuous spectrum; that is, there are no color bands missing from its spectrum. At room temperature, lampblack (a finely powdered black soot) is very close to being a blackbody because it absorbs almost all the radiation incident upon it and reflects very little. Fortunately, the radiation emitted by stars tends to be much like that emitted by a blackbody.
In 1900 the German physicist Max Planck (1858-1947) derived a mathematical expression, now called Planck's law, that describes the distribution of brightness in the spectrum of a blackbody. There are two other distinguishing characteristics of the spectrum of blackbody radiation: First, the energy emitted by the blackbody is greater at every wavelength as the temperature increases. Thus the total amount of radiant energy emitted increases with increasing temperature, which is known as the Stefan-Boltzmann law. Second, the greatest amount of radiation is fou nd toward shorter wavelengths (blue end of the spectrum) as the temperature increases. This is known as Wien's displacement law.
The significance of the blackbody-radiation lawsPlanck's law, the Stefan-Boltzmann law, and Wien's law-is that bodies that emit electromagnetic radiation because they are hot, such as stars, do so much like a blackbody. Thus the blackbody-radiation laws are powerful diagnostic tools for measuring the temperature of these thermal sources of radiation. For the study of bodies that emit radiation not because they are hot (called nonthermal sources of radiation) but because of some selective physical processes, the blackbody-radiation laws are of no use. Fortunately, most of the celestial bodies-all the stars-are thermal sources of radiation and emit much like a blackbody. Some everyday examples of thermal sources of radiation are an incandescent light bulb, the burner on an electric stove, and the flame of a cutting torch. Examples of nonthermal sources are a fluorescent light, lightning, and a television screen.
Just as sound waves of various wavelengths are transported simultaneously through the same region of air, electromagnetic waves of different wavelengths can move through the same point in space and superimpose to form composite waves, or white light. Stars, for example, are white-light sources though the color of the composite light from the stars may be white, red, yellow, or blue. As stated earlier, white, or composite, light can then be dispersed, or separated, into its component colors, or wavelengths,
MEASURING THE DOPPLER EFFECT
to form a spectrum. The study of spectra is called spectroscopy. Let us briefly explain how we can accomplish the dispersion of composite light by using a triangular piece of glass called a pris.
In the refraction of a ray of light the angle through which light is refracted depends on wavelength; the angle of refraction is greater for shorter wavelengths than it is for longer ones. Consider white light passing through the slit of a narrow diaphragm and then through a glass prism. The light disperses into its component wavelengths, with shortwavelength light refracted through larger angles than long-wavelength light is. Thus waves of different wavelengths disperse in different directions. The result is a rainbow-colored sequence of images of the slit, containing an image for each wavelength present in the white light.
Another means of dispersing white light to produce a spectrum is the diffraction grating. Unlike the ordinary glass prism, which is transparent only to visible and infrared radiation, the grating is useful over a broad spectrum, from X-ray to infrared wavelengths. In its simplest form the diffraction grating is a plate containing a very large number of very narrow parallel slits, uniformly spaced at distances that are only a few times the wavelength of light. (By "large number" we mean many thousands of slits per centimeter.) The spectrum is viewed in the direction of the light source, as in Figure 4.9. Since the amount of the bending, or diffraction, of electromagnetic waves at each slit depends on its wavelength, composite light is dispersed into its component colors. (For more details on these dispersing devices as they are used in an instrument called a spectroscope.
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.
IDENTIFYING THE ELEMENTS FROM EMISSION OR ABSORPTION SPECTRA
An astronomical light source, such as a star or a gaseous nebula, contains a mixture of chemical species, each either emitting or absorbing its own set of wavelengths of electromagnetic radiation. With the aid of laboratory spectral analysis of the different chemical elements, astronomers can identify individual elements in the light source from the measured wavelengths of its spectral lines, regardless of whether they are emission 'or absorption lines.
Identification is done in the following way: Light from a celestial body is collected by a telescope and then passed through a spectrograph in order to disperse the white light from the light source and form its spectrum. The photographic plate on which the spectrum is recorded is called a spectrogram. As a standard against which unknown wavelengths in the astronomical spectrum can be measured, an emission spectrum of a known gas, such as neon or vaporized iron or titanium, is placed above and below the astronomical spectrum. (The mechanism for placing the laboratory spectrum on the astronomical spectrogram is a part of the telescope and spectrograph.) With these comparison lines of known wavelength the astronomer can determine the unknown wavelengths of the astronomical object's spectral lines. The absorption spectrum is gray with black absorption lines and the comparison spectrum of neon shows white emission lines on a black background.
Kirchhoff's laws of spectrum analysis tell us about the general physical conditions of the light source. And if the spectrum of the light source contains absorption or emission lines, we can measure their wavelengths and identify the chemical elements that are present.
Can more detailed information about the light source be found? Suppose we want to know the temperature of the light source. Can this be done? Yes it can, for special types of light sources known as ideal radiators, or blackbodies.
All objects radiate and absorb some form of electromagnetic radiation; the wavelength region and the amount of energy depend generally on the body's temperature and physical state. From laboratory experiments and from theory physicists in the nineteenth century analyzed how various bodies emit and absorb radiation as a function of temperature and wavelength. From this work they developed the concept of an idealized radiator called a blackbody.
BLACKBODY CONCEPT:
A blackbody is an imaginary body that, when cool, absorbs all the radiant energy falling on its surface so that it is black in color; when hot, the blackbody emits energy with 100 percent efficiency. (Real matter is generally less than 100 percent efficient when it radiates.)
For our purposes the most important feature of the blackbody is the way in which emitted radiant energy is spread in wavelength, or the spectral energy distribution. Scientists have found that the distribution of energy depends only on the blackbody'S temperature and not on its chemical composition. Note how the amount of radiant energy emitted by a blackbody varies with wavelength in a very recognizable way, even for different temperatures. The emission of radiant energy (or the brightness at each wavelength) covers a continuous range of wavelengths so that the spectrum of a blackbody is a continuous spectrum; that is, there are no color bands missing from its spectrum. At room temperature, lampblack (a finely powdered black soot) is very close to being a blackbody because it absorbs almost all the radiation incident upon it and reflects very little. Fortunately, the radiation emitted by stars tends to be much like that emitted by a blackbody.
In 1900 the German physicist Max Planck (1858-1947) derived a mathematical expression, now called Planck's law, that describes the distribution of brightness in the spectrum of a blackbody. There are two other distinguishing characteristics of the spectrum of blackbody radiation: First, the energy emitted by the blackbody is greater at every wavelength as the temperature increases. Thus the total amount of radiant energy emitted increases with increasing temperature, which is known as the Stefan-Boltzmann law. Second, the greatest amount of radiation is fou nd toward shorter wavelengths (blue end of the spectrum) as the temperature increases. This is known as Wien's displacement law.
The significance of the blackbody-radiation lawsPlanck's law, the Stefan-Boltzmann law, and Wien's law-is that bodies that emit electromagnetic radiation because they are hot, such as stars, do so much like a blackbody. Thus the blackbody-radiation laws are powerful diagnostic tools for measuring the temperature of these thermal sources of radiation. For the study of bodies that emit radiation not because they are hot (called nonthermal sources of radiation) but because of some selective physical processes, the blackbody-radiation laws are of no use. Fortunately, most of the celestial bodies-all the stars-are thermal sources of radiation and emit much like a blackbody. Some everyday examples of thermal sources of radiation are an incandescent light bulb, the burner on an electric stove, and the flame of a cutting torch. Examples of nonthermal sources are a fluorescent light, lightning, and a television screen.
