astro110-01 lecture 14 light and matter...20/02/09 habbal_astro110-01 lecture 14 13 emission line...

120/02/09 Habbal_Astro110-01 Lecture 14

Astro110-01Lecture 14

Light and Matter


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)


Atomic Terminology• Atomic Number

– number of protons in nucleus• Atomic Mass Number

– number of protons + neutrons


Atomic Terminology (cont’d)• Isotope:

– same # of protons but different # of neutrons (4He, 3He)

• Molecules:- two or more atoms (H2O, CO2)


How do light and matterinteract?

• Emission• Absorption• Transmission:

– Transparent objects transmit light.– Opaque objects block (absorb) light.

• Reflection– Scattering = random reflection


Reflection and Scattering

Mirror reflects lightin a particulardirection

Movie screen scatters light inall directions


Interaction of Light with Matter

Interaction between light and matter determinesthe appearance of everything around us.


Probing Matter through Light:The spectrum


Different ways of visualizing a spectrum


Three Types of Spectra:Kirchhof’s Laws

Continuous

Emission

Absorption


Continuous Spectrum

Emission Line Spectrum Absorption Line Spectrum

Spectra of astrophysical objectsare usually combinations of these

three basic types


Continuous Spectrum

• The spectrum of a common (incandescent)light bulb spans all visible wavelengths,without interruption.


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.


Absorption Line Spectrum

• A cloud of gas between us and a light bulb canabsorb light of specific wavelengths, leaving darkabsorption lines in the spectrum.


How does light tell us whatthings are made of?


Recall the parts of an atom

Atom

Electroncloud Nucleus

proton neutron


Chemical Fingerprints

Each type of atom has a unique spectralfingerprint


Production of Emission Lines


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


Transitions yielding emissionlines

• Downwardtransitionsproduce a uniquepattern ofemission lines.


Transitions yielding absorptionlines

• Because thoseatoms canabsorb photonswith those sameenergies, upwardtransitionsproduce a patternof absorptionlines at the samewavelengths.


Production of Absorption Lines


Chemical fingerprints in aspectrum indicate

which kinds of atoms are present


Example: the Solar Spectrum


How does light tell us thetemperatures of planets and

stars?


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.


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.


Wien’s Law λ max = 2.9 106 / T (K)

astro110-01 lecture 14 light and matter...20/02/09 habbal_astro110-01 lecture 14 13 emission line...

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