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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)