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Chapter 4

Chapter Four Review: Spectroscopy

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University of Toronto Scarborough
Brian Wilson

rd Nov. 23 , 2010 ASTA01H Intro to Astronomy and Astrology Part I Chapter Four: Spectroscopy Spectral Lines A vital step, in how astronomers can analyze electromagnetic radiation received from space to obtain information about distant objects, is the formation of a spectrum – a splitting of the incoming radiation into its component wavelengths o But in reality no cosmic object emits a perfect blackbody spectrum A blackbody is an object that absorbs all electromagnetic energy falling on it. Blackbodies absorb and incandescently re-emit radiation in a characteristic, continuous spectrum. Because no light (visible electromagnetic radiation) is reflected or transmitted, the object appears black when it is cold Radiation can be analyzed with an instrument known as a spectroscope o In its most basic form, a spectroscope consists of an opaque barrier with a slit in it (to define a beam of light), a prism (to split the beam into its component colours), and an eyepiece or screen (to allow the user to view the resulting spectrum) Emission Lines Continuous spectra: emits radiation of all wavelengths (mostly in the visible range) o Viewed through a spectroscope, the spectrum of the light from the bulb would show the familiar rainbow of colours, from red to violet, without interruption Not all spectra are continuous however o For instance, if we took a glass jar containing pure hydrogen and passed an electric current through it, the gas would begin to glow – that is, it would emit radiation If we were to examine that radiation with a spectroscope, we would find that its spectrum consists of only a few bright lines on an www.notesolution.com otherwise dark background, quite unlike the continuous spectrum described for the light bulb • In this experiment, the light produced by the hydrogen does not consist of all possible colours, but instead includes only a few narrow, well-defined emission lines – thin “slices” of the continuous spectrum • The black background represents all the wavelengths not emitted by hydrogen After some experimentation, scientists found that, although they could alter the intensity of the lines – for example, by changing the amount of hydrogen in the jar or the strength of the electric charge – they could not alter the colours of the line (in other words, their frequency or wavelength) o After preforming further experiments on various other elements, sometimes the patterns of lines were fairly simple, and sometimes it was complex, but it was alwaysunique to the element being tested Thus, researchers could detect the presence of a particular atom or molecule solely through the study of the light it emitted The particular pattern of light emitted by a gas of a given chemical composition is known as theemission spectrum of the gas Absorption Lines When sunlight is split by a prism, at first glance it appears to produce a continuous spectrum o However, looking more closely with a spectroscope shows that the solar spectrum is interrupted vertically by a large number of narrow dark lines o We now know that these lines represent wavelengths of light that have been removed (absorbed) by gases present either in the outer layers of the Sun or in Earth’s atmosphere o These gaps in the spectrum are calla ebdsorption lines Kirchhoff’s Laws The analysis of the ways in which matter emits and absorbs radiation is called spectroscopy www.notesolution.com One early spectroscopist, Gustav Kirchhoff, summarized the observed relationships among the three types of spectra – continuous, emission lines, and absorption lines – he formulated three spectroscopic rules, now knowK nirshhoff’s Laws, governing the formation of spectra: o A luminous solid or liquid, or sufficiently dense gas, emits light of all wavelengths and so produces a continuous spectrum of radiation o A low-density, hot gas emits light whose spectrum consists of a series of bright emission lines that are characteristic of the chemical composition of the gas o A cool, thin gas absorbs certain wavelengths from a continuous spectrum, leaving dark absorption lines in their place, superimposed on the continuous spectrum. Once again, these lines are characteristic of the composition of the intervening gas – they occur at precisely the same wavelength as the emission lines produced by that gas at higher temperatures Atoms and Radiation When light interacts with matter on very small scales, it does not in a continuous way, but in a discontinuous, “stepwise” manner The challenge was then to find an explanation for this unexpected behaviour The eventual solution revolutionized our view of nature and now forms the foundation for all of physics and astronomy Atomic Structure To explain the formation of emission and absorption lines, we must understand not just the nature of light, but also the structureatf ms – the microscopic building blocks from which all mater is constructed Example: A hydrogen atom consists of an electron with a negative charge orbiting a proton carrying a positive charge o The proton forms the centra nlucleus of the atom o The hydrogen atom as a whole is electrically neutral as the proton and electron have equal and opposing charges www.notesolution.com How does this picture of the hydrogen atom relate to the characteristic emission and absorption lines associated with hydrogen gas? o If an atom absorbs some energy in the form of radiation, that energy must cause some internal change o Similarly, if the atom emits energy, the energy must come from somewhere within the atom o It is reasonable (and correct) to suppose that the energy absorbed or emitted by the atom is associated with the changes in the motion of the orbiting electron The first theory of the atom to provide an explanation of hydrogen’s observed spectral lines is as follows: o Known as theBohr model of the atom There is a state of lowest energy – the ground state – which represents the “normal” condition of the electron as it orbits the nucleus There is a maximum energy that the electron can have and still be part of the atom. Once the electron acquires more than that maximum energy, it is no longer bound to the nucleus, and the atom is said to be ionized; an atom missing one or more of its electrons is called an ion Most important, between those two energy levels, the electron can exist only in certain sharply defined energy states, often referred to as orbitals Each orbital has a precise energy, required for an electron to remain their In the atomic realm, such discontinuous behaviour is the norm The orbital energies are said to b quantized o The rules of quantum mechanics, the branch of physics governing the behaviour of atoms and subatomic particles, are far removed from everyday experiences Atoms do not always remain in their ground state www.notesolution.com o An atom is said to be in aenxcited state when an electron occupies an orbital at a greater-than-normal distance from its parents nucleus An atom in such a state, also has a greater-than-normal amount of energy Atoms can become excited in one of two ways: o By absorbing some energy from a source of electromagnetic radiation or by colliding with some other particle – another atom, for example However, the electron cannot stay at a higher orbital forever; the ground state is the only level where it can remain indefinitely Radiation as Particles Because electrons can exist only in orbitals having specific energies, atoms can absorb only specific amounts of energy as their electrons are boosted into excited states o Likewise, atoms can emit only specific amounts of energy as their electrons fall back to lower energy states Thus, the amount of light energy absorbed or emitted in these processes must correspond precisely to the energy difference between two orbitals The atoms quantized energy levels require that light be absorbed and emitted in the form of distinct “packets” of electromagnetic radiation, each carrying a specific amount of energy o We call these packetp shotons A photon is, in effect, a “particle” of electromagnetic radiation The idea that light some
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