astro110-01 lecture 14 light and matter...20/02/09 habbal_astro110-01 lecture 14 13 emission line...
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120/02/09 Habbal_Astro110-01 Lecture 14
Astro110-01Lecture 14
Light and Matter
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What are the building blocks ofmatter?
Or the parts of an atom
Atom
Electroncloud Nucleus
proton neutron
mass (proton) ≅ mass (neutron) ≅ 2000 x mass(e)
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Atomic Terminology• Atomic Number
– number of protons in nucleus• Atomic Mass Number
– number of protons + neutrons
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Atomic Terminology (cont’d)• Isotope:
– same # of protons but different # of neutrons (4He, 3He)
• Molecules:- two or more atoms (H2O, CO2)
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How do light and matterinteract?
• Emission• Absorption• Transmission:
– Transparent objects transmit light.– Opaque objects block (absorb) light.
• Reflection– Scattering = random reflection
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Reflection and Scattering
Mirror reflects lightin a particulardirection
Movie screen scatters light inall directions
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Interaction of Light with Matter
Interaction between light and matter determinesthe appearance of everything around us.
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Probing Matter through Light:The spectrum
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Different ways of visualizing a spectrum
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Three Types of Spectra:Kirchhof’s Laws
Continuous
Emission
Absorption
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Continuous Spectrum
Emission Line Spectrum Absorption Line Spectrum
Spectra of astrophysical objectsare usually combinations of these
three basic types
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Continuous Spectrum
• The spectrum of a common (incandescent)light bulb spans all visible wavelengths,without interruption.
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Emission Line Spectrum
• A thin or low-density cloud of gas emits light only atspecific wavelengths that depend on its compositionand temperature, producing a spectrum with brightemission lines.
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Absorption Line Spectrum
• A cloud of gas between us and a light bulb canabsorb light of specific wavelengths, leaving darkabsorption lines in the spectrum.
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How does light tell us whatthings are made of?
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Recall the parts of an atom
Atom
Electroncloud Nucleus
proton neutron
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Chemical Fingerprints
Each type of atom has a unique spectralfingerprint
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Production of Emission Lines
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Chemical FingerprintsEnergy Levels
Each type of atomhas a unique setof energy levels.
Each transitioncorresponds to aunique photonenergy andfrequency (orwavelength).
Energy levels of hydrogen
E = h x f = h / λ
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Transitions yielding emissionlines
• Downwardtransitionsproduce a uniquepattern ofemission lines.
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Transitions yielding absorptionlines
• Because thoseatoms canabsorb photonswith those sameenergies, upwardtransitionsproduce a patternof absorptionlines at the samewavelengths.
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Production of Absorption Lines
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Chemical fingerprints in aspectrum indicate
which kinds of atoms are present
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Example: the Solar Spectrum
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How does light tell us thetemperatures of planets and
stars?
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Thermal Radiation
• Nearly all large or dense objects emit thermalradiation, including stars, planets, and you.
• An object’s thermal radiation spectrumdepends on only one property: itstemperature.
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Properties of Thermal Radiation1. Hotter objects emit more light at all frequencies
per unit area.2. Hotter objects emit photons with a higher
average energy.
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Wien’s Law λ max = 2.9 106 / T (K)