Dark Lines Superimposed on a Continuous Spectrum

Light and Matter 2

Spectroscopy

Every source of EM radiation produces a unique distribution of frequencies (or wavelengths). This distribution is known as the spectrum of the source. The study of spectra is spectroscopy. In order to investigate the spectrum it must be spread out into its component wavelengths. This dispersion can be accomplished by using a prism, or more effectively by using a diffraction grating.

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Types of Spectra

There are three distinct types of spectra, each with its own characteristics, and each produced under a different set of circumstances.

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Continuous Spectra

When a solid, liquid, or dense gas is heated it emits a continuous range of frequencies – a continuous spectrum. For example, when the filament of a light bulb is heated by passing an electric current through it a continuous spectrum of light is emitted. This can be verified by passing the light through a prism or diffraction grating.

Emission Spectra

When a dilute gas (or a vaporized solid or liquid) is heated to a high temperature it emits a spectrum consisting of discrete frequencies rather than continuous band. While the intensity and width of the emission lines can be influenced by altering the conditions under which the radiation is produced, the pattern of the lines is unique to the chemical make-up of the material producing the radiation.

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Absorption Spectra

When a continuous spectrum of light is passed through a dilute, relatively cool gas or vapor the resulting spectrum consists of dark lines superimposed upon the incident continuous spectrum. This type of spectrum, first discovered by Wollaston and studied extensively by Fraunhofer, is the most important for the study of individual stars.

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The emission lines and absorption lines for a given material are found to coincide precisely.

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Spectrographs & Spectroscopes

Devices which are designed to disperse the light forming the spectrum of a source are known as spectroscopes (simple version) or spectrographs (sophisticated version). In modern spectrographs the light gathered by a telescope is passed through the dispersing device and then focussed on a recording device, usually an electronic device such as a CCD.

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Kirchhoff's Rules

1.) Solids, liquids, and dense gases emit a continuous spectrum.

2.) A dilute, hot gas or vapor emits an emission spectrum consisting of discrete lines at locations characteristic of the emitting material.

3.) A dilute, cool gas or vapor passes an absorption spectrum consisting of discrete dark lines superimposed on a continuous spectrum at locations characteristic of the absorbing material.

Atomic Structure and Production of Spectra

Atomic Models

The structure of matter has been a point of speculation for at least the last 2500 years.

Early Models

The earliest theories concerning the composition of matter were substance based. For example Aristotelian physical theory postulated that everything in the terrestrial realm was composed of earth, water, air, and fire. Other similar substance oriented theories were also devised.

In the 18th and 19th centuries it became clear that there exists a set of basic elements, with the fundamental units of each element being atoms. As the different species of atoms were discovered and classified it became clear that they fit a pattern as exemplified by the periodic table constructed by Mendeleev.

Near the end of the 19th century atoms were still regarded as indivisible units. Then several discoveries revealed that atoms were not fundamental, but themselves had structure. These discoveries included radioactivity and the discovery of the electron and its properties.

The first model to incorporate an internal structure for the atom was the "plum pudding" model which postulated that the bulk of the atom consisted of positively charged material with the tiny negatively charged electrons embedded throughout it like the raisins in a pudding. This model was discredited by Rutherford's alpha particle scattering experiment, which indicated that most of the mass of the atom was concentrated in a very small volume compared with the overall volume of the atom.

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Bohr Model

Based on Rutherford's results, Bohr constructed a model of the atom in which a very small, massive, positively charged nucleus is surrounded by the very low mass, negatively charged electrons.

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Quantum (Schrödinger) Model

In the quantum picture of the atom, the electrons can be thought of as orbiting the nucleus in certain orbits on the average. However, at any specific time only the probability of the electron being in a certain location can be predicted. Thus the electron orbits are usually pictured as being smeared out clouds, rather than specific planet-like orbits. This unusual behavior is due to the dual nature which the electrons (and other particles) exhibit.

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Transitions and Spectra

In the Bohr model and the Schrödinger model only certain electron orbits are allowed, and when an electron makes a transition between allowed orbits it either absorbs or emits radiation. The energy of the radiation is equal to the difference in energies of the two orbits involved.

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