Obscure Trivia 0002
Taken from page's 27-29 in The Cosmos From Space by David H. Clark:
It is well known that if a beam of white light passes through a slit and a prism, it is split into a rainbow of colors. The prism bends the different components of white light (red being least, violet most) into a merging row of colors called a spectrum. A practical spectrometer (a device used to produce a spectrum) normally uses a grating rather than a prism to disperse the light. Since white light produces a continuous range of colors, it is said to have a continuous spectrum. By contrast, the spectrum of some particular light source may show just selected features of different colors against a dark background. Such a spectrum is called an emission-line spectrum, each spectral line being an image the slit shaped instrument aperture in a single color. An emission-line spectrum is a unique characteristic of the radiating material, a fingerprint or signature that allows it's unambiguous indentification. If a beam of white light displaying a continuous spectrum shines on a low-density gas that does not itself radiate, certain colors of the incident light are absorbed so that the emergent light shows dark lines in its spectrum against a continuous background. Such a spectrum is referred to as an absorbtion-line spectrum. The colors of light absorbed are the same as those that would be emitted by the gas if it were made to radiate, so that an absorbtion-line spectrum of gas reveals the same information as would it's emission-line spectrum. Since the spectrum of a particular light source provides important information on the composition, density, and tempature of the light source, spectroscopy, the study of spectra of different sources of electromagnetic radiation, has proved to be an extremely powerful technique in astronomicial observations from space.
Spectroscopy is also important in determining the velocity of a light source, through a phenomenon known as the Doppler effect. The most familiar manifestation of this phenomenon is the change of pitch noticed by a stationary observer when a source of sound approaches and passes. Thus, for example, when a police car approaches with its siren on, the sound waves are "bunched up" ahead of it (in the sence that the distance between adjacent sound wave crests is shortened, that is, the wave length is decreased and the sound is of higher pitch than when the siren is stationary). When the source of sound is receding, the waves are "stretched out" (in the sence that the distance between adjacent wave crests is lengthened), thereby increasing the wave length and making the pitch lower. As it is with sound waves, so it is with light waves. If a source of light (for example, a star or galaxy) is approaching an observer, the wave length (defining the "color") of any characteristic emission is decreased; the light is said to be blue-shifted ( emission features shift toward the blue end of the spectrum) When the source is receding, the light is red-shifted. The greater the speed of approach or recession, the greater the degree of blue-shift or red-shift. Thus the measurement of wave length shift (compared with a laboratory source) provides the velocity of the source along the line of sight.
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