Spectral Lines - History, Spectrographs, Doppler Shift
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A spectral line is light of a single frequency, or wavelength, which is emitted by an atom when an electron changes its energy level. Because the energy levels of the electron vary from element to element, scientists can determine the chemical composition of an object from a distance by examining its spectrum. In addition, the shift of a spectral line from its predicted position can show the speed at which an astronomical object is moving away from Earth. The measurement of spectral lines is the basis of much of modern astronomy.
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Isaac Newton was the first to discover that light from the sun was composed of multiple frequencies. In 1666, by using a prism to break sunlight into its component colors, and then recombining them with a second prism, he showed that the light coming from the sun consisted of a continuous array of colors. Until then, some believed that the colors shown by a prism were generated by the prism itself…
Astronomers use a device called a spectrograph to disperse light into its constituent wavelengths in the same way that Newton's prism divided sunlight into its component colors. Spectrographs may have a prism or a diffraction grating (an optical element consisting of a ruled surface which disperses light due to diffraction) as their dispersive element. The resultant spectrum may be recorded…
Another way that spectral lines are used in astronomy is to determine the velocity of an object. An object which is moving away from Earth will have its spectral lines shifted to longer wavelengths due to the Doppler shift acting on the emitted photons. Similarly, objects moving towards Earth will be shifted to shorter wavelengths. By measuring the shift of a spectrum, the velocity with which the …
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