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Unformatted text preview: 1 Wednesday, September 21: Introduction This course deals with continuum radiation processes of astrophysical inter- est. (Processes that produce spectral lines, rather than continuum radiation, are dealt with in Astronomy 823: Theoretical Spectroscopy.) The nature of electromagnetic radiation, otherwise known as light, 1 was long a subject of debate. Isaac Newton, in the 17th century, believed that light consisted of a stream of particles. In the early 19th century, however, Thomas Young demonstrated that light showed the properties of diffraction and interference, and thus had to consist of waves. In the early 20th century, though, Albert Einstein showed that the photoelectric effect can only be accounted for if light consists of a stream of particles, called photons. We can take advan- tage of the wave/particle duality of light, and treat it either as a stream of particles or as a propagating wave (whichever makes the solution to a given problem easier!) When we think of light as a wave, its wavelength is = c/ , (1) where is the frequency, and c = 3 . 10 10 cm s- 1 when the light is propa- gating through a vacuum. When we think of light as particles, the energy of an individual photon is E = h , (2) where Plancks constant is h = 6 . 6 10- 27 erg s in cgs (centimeter, gram, second) units. Note that Plancks constant is a small number, and thus each photon carries only a small amount of energy. Electromagnetic radiation consists of a spectrum of light, ranging from short wavelength to long (that is, from high frequency to low, or from high photon energy to low). By convention, the spectrum is divided into rays, X rays, ultraviolet, visible, infrared, microwave, and radio waves. These divisions, although useful in practice, are the results of the quirks of history. Light can be produced in many ways. Consider lighting a candle. The candle flame produces a continuous spectrum, emitted primarily by the soot particles within the flame. The temperature of a typical candle flame is about 700 Kelvin, but its color is much yellower than a blackbody at that 1 I will use electromagnetic radiation and light interchangeably; when I want to speak specifically about those wavelengths that our eyes can detect, Ill use the phrase visible light. 1 temperature. 2 This color anomaly results from the fact that the soot particles are generally less than a micron across, and are very inefficient at making light with a wavelength of more than one micron. The light energy is thus carried away by light with wavelengths of less than a micron; much of the light lies in the visible range of the spectrum (400 nm &lt; &lt; 750 nm). The moral of the candles story is that thermal radiation is not necessarily blackbody radiation....
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This note was uploaded on 07/17/2008 for the course ASTRO 822 taught by Professor Ryden during the Fall '05 term at Ohio State.
- Fall '05