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Light and the Electromagnetic Spectrum © Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION © Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION © Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION © Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION © Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION © JonesLight & Bartlett and Learning, LLCthe © Jones & Bartlett Learning, LLC NOTElectromagnetic FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION4 Spectrum © Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC NOT FOR SALEJ AMESOR DISTRIBUTIONCLERK MAXWELL WAS BORN IN EDINBURGH, SCOTLANDNOT FOR IN 1831. SALE His ORgenius DISTRIBUTION was ap- The Milky Way seen parent early in his life, for at the age of 14 years, he published a paper in the at 10 wavelengths of Proceedings of the Royal Society of Edinburgh. One of his first major achievements the electromagnetic was the explanation for the rings of Saturn, in which he showed that they con- spectrum. Courtesy of Astrophysics Data Facility sist of small particles in orbit around the planet. In the 1860s, Maxwell began at the NASA Goddard a study of electricity© Jones and & magnetismBartlett Learning, and discovered LLC that it should be possible© Jones Space & Bartlett Flight Center. Learning, LLC to produce aNOT wave FORthat combines SALE OR electrical DISTRIBUTION and magnetic effects, a so-calledNOT FOR SALE OR DISTRIBUTION electromagnetic wave. His analysis of this hypothetical wave showed that its speed would be 300,000 kilometers/second. Because this is the speed of light, Maxwell concluded that he had discovered the nature of light: Light is an electromagnetic wave. © JonesIn 1888, & Bartlett 9 years afterLearning, Maxwell’s LLC death, radio waves were ©discovered Jones by& BartlettHeinrich Learning, LLC NOTHertz FOR and SALE were shown OR DISTRIBUTION to have properties similar to those of light.NOT This FOR verified SALE OR DISTRIBUTION Maxwell’s prediction. The importance of Maxwell’s work is indicated in the following quotation from the Nobel Prize winner Richard Feynman: From a long view of human history—seen from, say ten thousand years from now—there can be little doubt that the most significant event of the 19th century © Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION 95 © Jones & Bartlett Learning,4TH LLC. NOTPAGES FOR SALE OR DISTRIBUTION. 96 CHAPTER 4 Light and the Electromagnetic Spectrum u The Feynman Lectures on will be judged as Maxwell’s discovery of the laws of electrodynamics. The American Civil © JonesPhysics &. Vol. Bartlett 2 (Reading, MA: Learning, LLCWar will pale into provincial© Jones insignificance & Bartlett in comparison Learning, with this LLC important scientific NOT FORAddison-Wesley SALE Publishing OR DISTRIBUTIONevent of the same decade.NOT FOR SALE OR DISTRIBUTION Co., 1964). An important part of your study of astronomy is to learn about the objects in our uni- verse, but perhaps more important is to see how astronomy functions by learning how we know what we know about these objects. The only thing we obtain from them is the radia- tion© Jones they emit. & Bartlett This radiation, Learning, including LLC not only visible light but© many Jones other & typesBartlett of ra -Learning, LLC diation,NOT FOR carries SALE to us aOR tremendous DISTRIBUTION amount of information. To understandNOT FOR how SALE astronomers OR DISTRIBUTION analyze radiation to answer questions about celestial objects, it is necessary to learn some- thing about radiation itself. In this chapter, we examine the nature of light and show how we measure three major properties of stars: their temperatures, their compositions (that is, the chemical © Jones & Bartlett elements Learning, of which they LLC are made), and their speeds© Jones relative & toBartlett the Earth. Learning, Recall that LLC NOT FOR SALEwe have OR a DISTRIBUTION tool, in the form of Kepler’s third law,NOT which FOR allows SALE us to OR find DISTRIBUTION the total mass of a binary system. In addition, there are other tools, described elsewhere in this text which allow us to learn more about the stars (for example, how far away and how big they are). © Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC NOT FOR SALE OR DISTRIBUTION4-1 The Kelvin TemperatureNOT FOR SALE Scale OR DISTRIBUTION Light transmits energy. We know this because we can clearly feel the warmth of sunshine. When we think of the Sun as an energy source (even though very different from other energy© Jones sources, & Bartlett such as aLearning, fireplace or LLCa very hot iron bar), an obvious© Jones question & Bartlett to ask Learning, LLC Kelvin temperature scale isNOT how FOR“hot” itSALE is. What OR is DISTRIBUTIONits temperature? Even more important,NOT what FOR do we SALE mean ORby DISTRIBUTION A temperature scale with the term “temperature,” and how do we measure it? It should not be surprising that we its zero point at the lowest started measuring temperatures long before we understood what temperature is. Check possible temperature (“absolute the accompanying Tools of Astronomy box where we describe the temperature scales zero”) and a degree that is the currently in use. same size (same© temperatureJones & BartlettThe commonlyLearning, used LLC temperature scale in science© Jones is the Kelvin & Bartlett scale; its Learning, zero point corLLC- difference) asNOT the Celsius FOR degree. SALEresponds OR toDISTRIBUTION the lowest temperature possible (absoluteNOT zero), FOR about SALE 2273°C. OR DISTRIBUTION The intervals on the Kelvin scale are the same size as on the Celsius scale and, thus, in Kelvin temperature, the TK 5 TC 1 273. freezing point of water is 273 K and the boiling point is 373 K. (No degree symbol is included; the latter temperature is stated as “373 kelvin.”) FIGURE B4-1 includes the Kelvin temperature scale on the right. © Jones & Bartlett Learning,We LLC now recognize that temperature© Jones is a fundamental& Bartlett Learning,quantity, as areLLC mass and time. As NOT FOR SALE OR DISTRIBUTIONsuch, it cannot be expressed in termsNOT of FORother quantities;SALE OR however, DISTRIBUTION it is a good approximation for us to say that given the temperature of an object, such as a pot of water on a stove, we can calculate the average speed of each of its constituent particles; that is, temperature is a mea- sure of their average kinetic energy. As the temperature of an object increases, each of its con- kinetic energy An object’s stituent particles moves faster, whereas as the temperature decreases, the particle speed also energy due to motion. For an decreases (although not in a linear fashion). At absolute zero, we have a state of minimum object of mass m and speed , © Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC atomic motion. its kinetic energy is equal to NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION (1/2)m 2. EXAMPLE A star’s surface temperature is 6000 K. What is its temperature in degrees Celsius and Fahrenheit? © Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION SOLUTION In degrees Celsius, the star’s temperature is 6000 2 273 5 5727°C. In degrees Fahrenheit, it is 9 5727 1 32 5 10,341°F. 5 © Jones & Bartlett Learning,For a largeLLC enough Kelvin temperature,© Joneswe can approximate & Bartlett its CelsiusLearning, equivalent LLC by the same number, NOT FOR SALE OR DISTRIBUTIONwhile its Fahrenheit equivalent is approximatelyNOT FOR twice SALE as large OR in DISTRIBUTIONvalue. © Jones & Bartlett Learning,4TH LLC. NOTPAGES FOR SALE OR DISTRIBUTION. 4-2 The Wave Nature of Light 97 © Jones &Tools Bartlett Learning, of Astronomy LLC © Jones & Bartlett Learning, LLC NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION Temperature Scales Around 1592, Galileo invented the first device indicating temperatures up to the boiling point of water, but let’s think the “degree of hotness”© of Jonesan object. & The Bartlett invention Learning, of LLCfor a minute about what these© Jones temperature & Bartlett scales can andLearning, LLC the thermometer, in 1631, is credited to J. Rey, a French cannot tell us. What does it mean to say that object A is physician. The scale in NOTcommon FOR use inSALE the United OR StatesDISTRIBUTION is two (or a thousand) times hotterNOT than FOR object SALE B, which OR is DISTRIBUTION at the Fahrenheit scale, but most of us are at least somewhat fa- 0°F? What is then the temperature of object A? The obvious miliar with the Celsius temperature scale. The freezing point answer is 0°F, but we know there is something wrong with of water is defined as 0°C or 32°F, whereas the boiling point this. (We could ask a similar question using 0°C.) Neither of water© Jonesis defined & as Bartlett 100°C or 212°F.Learning, Because LLC 180 divisions scale—nor,© forJones that matter, & Bartlett any other Learning, scale that defines LLC its zero on the Fahrenheit scale correspond to 100 divisions on the mark arbitrarily—has anything to do with what temperature CelsiusNOT scale, FOR a Fahrenheit SALE degree OR DISTRIBUTION is smaller than a Celsius “is.” Clearly,NOT we FORwould likeSALE to have OR an DISTRIBUTION “absolute” tempera- degree by a factor of 180/100 5 9/5. Thus, the two scales are ture scale, one on which the zero mark is associated with related by the lowest temperature possible (absolute zero). Such a scale exists (Figure B4-1). It is called the Kelvin scale and is the one 9 5 T 5 32 1 T or T 5 (T 2 32).
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