3.8 Spectroscopy and Kirchhoff's Laws

Pre-Lecture Reading 3.8

Video Lecture

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Spectroscopy and Kirchhoff's Laws (17:55)

Supplementary Notes

Spectroscopy is the science of dispersing light into its component colors and measuring—and interpreting—the intensity of the light at each color (i.e., at each frequency/wavelength).

Types of Spectra

Figure 1

Kirchhoff's Laws

Kirchhoff's First Law A continuous spectrum is produced by a:

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Luminous solid
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Luminous liquid
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Hot, opaque gas

Kirchhoff's Second Law An emission line spectrum is produced by a:

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Hot or heated, transparent gas

Kirchhoff's Third Law An absorption line spectrum is produced by a:

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Backlit, cool, transparent gas

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Absorption of light by a cool, transparent gas heats that component of the gas, which later reemits the light in all directions.
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Absorption and emission lines occur at the same frequencies/wavelengths, but not necessarily in the same proportions.

Astronomical Application

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Absorption and emission line spectra can be used to determine both the chemical composition and the temperature of a transparent gas.

Exercise

Experiment with UNL's Three Views Spectrum Demonstrator.

Assignment 3

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Do Question 6.