lecture12_08 - LECTURE 12 Types of spectra The Bohr Model...

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LECTURE 12 Types of spectra The Bohr Model of the Atom The Balmer Series The Doppler Effect Inverse Square Law of Radiation
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Newton discovered the colours of the solar spectrum by passing sunlight through a prism.
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However, the angular size of the Sun causes sharper features in the spectrum to be smoothed out (Newton saw only the continuum of the solar spectrum). To resolve the spectrum better, one needs a slit spectrograph. Joseph Fraunhofer (1814) used such a spectrograph . ..
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Spectrograph used by Kirchoff and Bunsen about 1823. Light source (burning gas) Collimator Telescope Prism Slit
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Of course, modern spectrographs are more sophisicated. This is a spectrograph used by undergrad students on small telescopes, such as those on the roof of McLennan Labs. It's similar to the spectrograph in my lab.
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Sketch of solar spectrum coloured by Fraunhofer (c. 1814). http://www.fmc.uni-karlsruhe.de/~timo/spektro1.html
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Types of Spectra Kirchhoff’s Laws A continuous spectrum contains an entire range of wavelengths rather than separate, discrete wavelengths. In 1814 Fraunhofer analyzed the solar spectrum and found a number of dark lines across the continuous spectrum . The dark lines are caused by absorption . In the mid-1800s it was discovered that if gases are heated until they emit light, a spectrum made up of bright lines appears. It was also found that each chemical element has its own distinctive pattern of lines.
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Kirchhoff’s Laws (cont.) In the 1860s Kirchhoff formulated a set of rules that summarize how the three types of spectra are produced: (a) A hot, dense glowing object (a solid or dense gas) emits a continuous spectrum . (b) A hot, low-density gas emits light of only certain wavelengths—a bright line spectrum . (c) When light having a continuous spectrum passes through a cool gas , dark lines appear in the continuous spectrum.
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Fig. 4-14 Why does each element have its own unique pattern of defined spectral lines ? Why does the solid hot glowing body NOT show lines?
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The Bohr Model of the Atom 1. The connection between Kirchoff’s laws and the nature of matter was not made until 1913, when Niels Bohr proposed a new model of the atom. 2. Three postulates of the Bohr atom: (a) Electrons in orbit around a nucleus can have only certain specific energies. (An electron is the negatively charged particle that orbits the nucleus —the central massive part—of an atom.) (b) An electron can move from one energy level to another changing the energy of the atom. (c) The energy of a photon determines the frequency of light that is associated with the photon.
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The negatively- charged electrons orbit around the positively-charged nucleus , like an electrostatic solar system. .. but the orbits are not any old orbits, but discrete ones. Nucleus
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lecture12_08 - LECTURE 12 Types of spectra The Bohr Model...

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