A spectroscope is a device for splitting a beam of radiation into its component frequencies and delivering them to a screen or detector for detailed study.
Many hot objects emit a continuous spectrum of radiation, containing light of all wavelengths. A hot gas may instead produce an emission spectrum, consisting only of a few well-defined emission lines of specific frequencies, or colors. Passing a continuous beam of radiation through cool gas will produce absorption lines at precisely the same frequencies as are present in the gas's emission spectrum. Kirchhoff's laws describe the relationships among these different types of spectra. The emission and absorption lines produced by each element are unique—they provide a "fingerprint" of that element. The study of the spectral lines produced by different substances is called spectroscopy . Spectroscopic studies of the Fraunhofer lines in the solar spectrum yield detailed information on the Sun's composition.
Atoms are made up of negatively charged electrons orbiting a positively charged heavy nucleus consisting of positively charged protons and electrically neutral neutrons . In normal circumstances the number of orbiting electrons equals the number of protons in the nucleus, and the atom as a whole is electrically neutral. The number of protons in the nucleus determines the type of element that the atom represents. The Bohr model of the atom was an early attempt to explain how atoms can produce emission and absorption line spectra. An atom has a minimum-energy ground state , representing its "normal" condition. If an orbiting electron is given enough energy, it can escape from the atom, which is then said to be ionized . Between these two states, the electron can exist only in certain well-defined states each with a very specific energy. The electron's energy is said to be quantized . In the modern view the electron is envisaged as being spread out in a "cloud" around the nucleus but still with a sharply defined energy.
Electromagnetic radiation exhibits both wave and particle properties. Particles of radiation are called photons . The energy of a photon depends on its color—it is directly proportional to the photon's frequency. As electrons move between energy levels within an atom, the difference in the energy between the states is emitted or absorbed in the form of photons. Because the energy levels have definite energies, the photons also have definite energies, and hence colors, which are characteristic of the type of atom involved.
Molecules are groups of two or more atoms bound together by electromagnetic forces. Like atoms, molecules exist in energy states that obey rules similar to those governing the internal structure of atoms. Again like atoms, when molecules make transitions between energy states, they emit or absorb a characteristic spectrum of radiation that identifies them uniquely.
Astronomers apply the laws of spectroscopy in analyzing radiation from beyond Earth. Several physical mechanisms can broaden spectral lines. The most important is the Doppler effect, which occurs because stars are hot and their atoms are in motion, or because the object being studied is rotating or in turbulent motion.
1. Emission spectra are characterized by narrow, bright lines of different colors. HINT
2. Imagine an emission spectrum produced by a container of hydrogen gas. Changing the amount of hydrogen in the container will change the colors of the lines in the spectrum. HINT
3. In the previous question, changing the gas in the container from hydrogen to helium will change the colors of the lines occurring in the spectrum. HINT
4. An absorption spectrum appears as a continuous spectrum that is interrupted by a series of dark lines. HINT
5. The wavelengths of the emission lines produced by an element are different from the wavelengths of the absorption lines produced by the same element. HINT
6. Gustav Kirchhoff is credited with the discovery of blackbody radiation. HINT
7. The density of the hot gas producing an emission spectrum must be very high. HINT
8. The energy of a photon is inversely proportional to the wavelength of the radiation. HINT
9. The ground state of an atom is that in which the electron is in its lowest energy level (orbital). HINT
10. An electron can have any energy within an atom so long as it is above the ground state energy. HINT
11. An atom can remain in an excited state indefinitely. HINT
12. Emission and absorption lines correspond to the specific energy differences between orbitals in an atom. HINT
13. The number of electrons in an atom or ion determines the identity of the element it represents. HINT
14. More than one element or molecule can have the same emission or absorption spectrum. HINT
15. Spectral lines of hydrogen are relatively weak in the Sun because the Sun contains relatively little hydrogen. HINT
1. A _____ is a glass wedge that disperses light into a spectrum. HINT
2. Blackbody radiation is an example of a _____ spectrum. HINT
3. Fraunhofer discovered absorption lines in the _____. HINT
4. A continuous spectrum can be produced by a luminous solid, liquid, or _____ gas. HINT
5. An absorption spectrum is produced when a _____ gas lies in front of a source of continuous radiation. HINT
6. Light behaves both as a wave and as a _____. HINT
7. The experiment that demonstrated as the _____ caused Einstein to realize that light does not always behave like a wave. HINT
8. Protons carry a _____ charge; electrons carry a _____ charge. HINT
9. When one or more electrons are stripped from the atom, the atom is said to be _____. HINT
10. When an electron moves to a higher energy level in an atom it _____ a photon of a specific energy. HINT
11. When an electron moves to a lower energy level in an atom it _____ a photon of a specific energy. HINT
12. The "specific energy" of the photon referred to in the last two questions is exactly equal to the energy _____ between the energy levels involved. HINT
13. Changes in molecular vibration states produce spectral features in the _____ part of the electromagnetic spectrum. HINT
14. Changes in molecular rotational states produce spectral features in the _____ part of the electromagnetic spectrum. HINT
15. High temperatures, rotation, and magnetic fields all tend to _____ spectral lines. HINT
1. What is spectroscopy? Why is it so important to astronomers? HINT
2. Describe the basic components of a simple spectroscope. HINT
3. What is a continuous spectrum? An absorption spectrum? HINT
4. Why are gamma rays generally harmful to life forms, but radio waves generally harmless? HINT
5. In the particle description of light, what is color?
6. Give a brief description of a hydrogen atom. HINT
7. What is the normal condition for atoms? What is an excited atom? What are orbitals? HINT
8. Why do excited atoms absorb and reemit radiation at characteristic frequencies? HINT
9. How are absorption and emission lines produced in a stellar spectrum? What information might absorption lines in the spectrum of a star reveal about a cloud of cool gas lying between us and the star? HINT
10. According to Kirchhoff's laws, what are the necessary conditions for a continuous spectrum to be produced? HINT
11. Why is the H
absorption line of hydrogen in the Sun relatively weak, even though the Sun has abundant hydrogen? HINT
12. How do molecules produce spectral lines unrelated to the movement of electrons between energy levels? HINT
13. How does the intensity of a spectral line yield information about the source of the line? HINT
14. How does the Doppler effect cause broadening of a spectral line? HINT
15. List three properties of a star that can be determined from observations of its spectrum. HINT
1. What is the energy (in joules and electron volts) of a 450-nm blue photon? A 200-nm ultraviolet photon? HINT
2. What are the frequency and wavelength of a 2-eV red photon? Repeat your calculation for an 0.1-eV infrared photon and a 5000-eV (5 keV) 
ray. HINT
3. How many times more energy has an 0.0001-nm gamma ray than a 10-MHz radio photon? HINT
4. Calculate the wavelength and frequency of the radiation emitted by the electronic transition from the 10th to the 9th excited state of hydrogen. In what part of the electromagnetic spectrum does this radiation lie? Repeat the question for transitions from the 100th to the 99th excited state, and from the 1000th to the 999th excited state. HINT
5. How many different photons (that is, photons of different frequencies) can be emitted as a hydrogen atom in the third excited state falls back, directly or indirectly, to the ground state? What are their wavelengths? HINT
6. The H line of a certain star is received on Earth at a wavelength of 655 nm. What is the star's radial velocity with respect to Earth? HINT
7. In a demonstration of the photoelectric effect suppose that a minimum energy of 5 10 -19 J (3.1 eV) is required to dislodge an electron from the metal surface. What is the minimum frequency (or longest wavelength) of radiation for which the detector registers a response? HINT
8. At a temperature of 5800 K, hydrogen atoms in the solar atmosphere have typical random speeds of about 12 km/s. Use this information to estimate the thermal width (in nanometers) of the 656.3-nm solar H line. HINT
9. Turbulent motion in a radio-emitting gas cloud broadens a 1.2-GHz (1 gigahertz ? 109 Hz) radio line to a width of 0.5 MHz. Estimate the speed at which the gas within the cloud is moving. HINT
10. Estimate how fast, in revolutions per day, the Sun would have to rotate in order for rotational broadening to be comparable to thermal broadening in determining line widths (see problem #8). HINT
1. Find a spectrum of the Sun that also has a wavelength scale alongside. Figure 16.8 is a good example; however, you may want to enlarge it on a copying machine. Select various absorption lines and determine their wavelengths by interpolation. Now, try to identify the element that produced these lines. Use a reference of lines such as found in Moore's A Multiplet Table of Astrophysical Interest. Other references may be found in the astronomy, chemistry, or physics sections of your library. Work with the darkest lines first before trying the fainter lines.
2. Use a handheld spectroscope, available through Learning Technologies Inc. While in the shade, point the spectroscope at a white cloud or white piece of paper that is in direct sunlight. Look for the absorption lines in the Sun's spectrum. Note their wavelength from the scale inside the spectroscope. Compare your list with the Fraunhofer lines given in many physics, astronomy, or chemistry reference books.
