# Visible Spectrum

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Visible Spectrum

Visible Spectrum• Prism splits Visible light into Visible Spectrum• Newton discovered that white light is made upof many colors in 1666• Newton thought light was corpuscles/particles• Young and others showed light was a wave (inthe ether?)

Electric Field• Electrons & Protons are thenegative & positively chargedelementary particles• They are surrounded by anelectric field• Which changes as they move

Electromagnetism• Changing electric fields (currents) make magnetic fields• Changing magnetic fields make electric currents• Changes in one induce changes in the other Maxwell 1860• All electromagnetic disturbances travel at the Speed of Light

Waves Have a Frequency f, Period P,Wavelength λ (lambda) & Amplitude• Frequency countswaves• Frequency = 3 cycles/sec or 3 Hertz• Has 1/3 sec. period• Period of 1 second• Frequency of 1 cycle/sec= 1 Hertz• Has three timeswavelength λ• Both have same heightor Amplitude

Electromagnetic Waves Move atthe Speed of Light c• C= λ f The distance between waves times the number of waves• frequency f =ν=nu• If c=300,000km/sec and the ‘The Q’ has frequency of100Megahertz, what is the wavelength λ?

Microwaves• Microwave ovens usewavelengths of 30cm• 15meter JCM Telescope studiesinterstellar dust gas, solar system& distant galaxies• Atacama Large Millimeter Arraymost expensive ground basedtelescope

8Tables XVII.3 and 4. The sheer complexity of these answers to absurdly simplequestions is a consequence of different usages by various authors of the meaning of“magnetic moment” and of departure from standard SI usage.TABLE XVII.2ANSWERS TO QUESTIONS 1 – 4 IN CGS EMU AND SI UNITSThe answers to the first four questions are identicalτ = 1 dyn cm = 10 −7 N mp 1 = 1 dyn cm Oe −1 = 4π × 10 −7 N m (A/m) −1p 2 = 1 dyn cm G −1 = 10 −3 N m (T) −1p 3 = 1 G cm 3 = 10 −10 T m 3p 4 = 1 Oe cm 3 = 10 −3 /(4π) A m 2p 5 = 4π G cm 3 = 4π × 10 −10 T m 3p 6 = 4π Oe cm 3 = 10 −3 A m 2

Ultraviolet Region• FUSE studies very hot stars and gas• SOHO studies the sun

X-ray Region• First discovered by Roentgen in 1895• Wife’s hand with wedding ring• Produced by gas at 10 million Kelvin• NuSTAR X-ray telescope launchedJune 2012

Cosmic Rays• Very high energy subatomic particles• Raining down on us from all directions –no big worry• Unknown production mechanism• AMS-02 installed on ISS May2010

Temperature• Temperature is a measure of the average kinetic energyof the atoms in gas, liquid or solid = speed of atoms• Animation is of Helium atoms at 20C & high pressure.• Speed of 1500 km/hr slowed down two trillion fold withsome colored red to make them easy to see

Heat or Thermal Energy• The hotter it is; the faster its atoms move• The amount of heat or thermal energy dependson temperature AND mass/number of atoms• The atoms cease moving at absolute zero =zero Kelvin (record=0.45nK)• Bowtie nebula -272C=1K

Brownian Motion Shows Atoms Exist• Microscopic view of milk fat droplets moving randomly due toimpacts from molecules of water – higher temperature faster motion• Einstein published mathematical explanation in 1905

Blackbody Radiation• A blackbody is opaque = stars, incandescent light bulbs• Radiative properties (spectrum) depend only on temperature• When charged particles are accelerated they emit photons

Blackbody Spectrum• Because the particles are going different speedsthe photons have a range of energies

Wein’s Law• As the temperature goes up there are more collisions & more violent• So more photons AND more energetic photons• Higher temperature shorter wavelength of peak emission=• Higher temperature bluer color

Stefan-Boltzmann Law• E= σ T 4• The amount of energy radiated is proportional totemperature to the fourth power• Twice the temperature give 2X2X2x2=16 times the energy

Blackbody Curve• After attempts by Wien, Stephan-Boltzmann, Rayleigh …• Max Planck finds equation (1900) which matches - BUTenergy must be quantized => Quantum Mechanics

Stellar Energy Distributions• Star temperatures vary from 100,000K to 1500K• Maximum of the curve tells us star’s temperature• Albireo composed of hot blue star & cool red one

Planet Impact?• Star HD172555 has a very bright disk• Disk has spectral signatures ofvaporized rock, lava, gravel and dust• Observations consistent with planetaryimpact

A-toms = Not Cutable• Democritus 420BC popularized thetheory matter was made of Atoms:• Too small to be seen• Indivisible• Surrounded by a void• Solid• No internal structure

Picturesof Atoms• 48 iron atoms oncopper• Made with ascanning tunnellingmicroscope

Electrons• JJ Thompson discovers the electron (1897)• How does a CRT work?

What’s an Atom?-Nucleus & Electron(s)• Rutherford (1911) fired helium nuclei at gold foil and avery few bounced straight back• Nucleus is 100,000 times smaller than the atom• Nucleus of atom is like pinhead in a Stadium

Periodic Table of Elements• Chemical properties determined by number of electrons• Which match number of protons• Increasing by 1 proton/electron for each element

Neutrons & Isotopes

Bohr Atom• Tiny positive nucleuscontains most of mass• “Orbited” by negativelycharged electron(s)• Held in orbit by Coulomb=Electromagnetic Force

Quantum Mechanics• Because electron orbits; it accelerates; it should radiate photons• But energy is quantized so only certain orbitals/energies allowed(just like stairs, piano keys, bookshelf)• Electron can be in lowest energy (=ground state) can raised to higherenergy (=excited state)

Photon Emission/Absorption• The electron jumps to ahigher energy level whena photon is absorbed =absorption• Excited state• The electron jumps to alower energy when itemits a photon =emission• Ground state

EmissionMatchesAbsorptionSpectrum

Kirchoff’s Laws• Continuous – solid, liquid or dense gas will radiate at all wavelengths• Emission - a low density gas will emit light at specific wavelengths• Absorption - results from a continuous spectrum passing through alow density gas resulting in dark spectral lines

Electron Cloud• Because an electron is a wave as well as a particle we can notsay exactly where it is due to its quantum mechanical nature• Heisenberg’s uncertainty principle “you can’t know everythingabout anything”

Light is a Particle: A Photon• Einstein showed light is quantized (1905).• Each quantum of light is called a photon• Planck`s constant relates the energy of a photon to frequency. E=hf• Photons are like money; Radio photons are pennies, X-rays-dollars

Electromagnetic Spectrum• All electromagnetic radiation travels at the speed of light• Energy of a photon depends on frequency/wavelength• Light has both wave and particle properties• Electrons and protons have both wave & particle properties• Everything has wave properties so: Heisenberg uncertaintyprinciple “You can not know anything exactly”

Hydrogen Emission Lines• Difference in energy level determines photon energy=Wavelength• Visible lines are Balmer series; H-Alpha, H-Beta, H-Gamma• Ultraviolet = Lyman series and Infrared = Paschen series• If electron gains enough energy to leave atom it is Ionized

PhotonAbsorptionandEmission• Electron canusually only stayin excited state fora nanosecond• Electrontransitions cantake place due tocollisions

Electron Shells• More protons requires more electrons• More populated energy levels• More possible transitions• More complicated spectra

Spectra ofMolecules• Carbon Monoxide has• A) Electronic transitions• B) Rotational transitions• C) Vibrational transitions• Spectrum of Hydrogen molecule in (a)• Spectrum of Hydrogen atoms in (b)

Solar Spectrum• Fraunhofer discovered lines in solar spectrum 1817• D=Sodium, C&F&h=Hydrogen, H&K=Calcium• Different elements have a different set of lines like a bar code

Annie Jump Cannon• Originated the modern stellarclassification scheme in 1901• Based on strength of H lines• Found to be temperature sequenceClassified 400,000 stars forthe Henry Draper Catalogs

Balmer Thermometer• Balmer lines originate at the n=2 energy level• If it is too cool then all atoms in n=1 level• If it is too hot then all hydrogen atoms are ionized• Use Calcium, Helium and molecules as well

Cecilia Payne-Gaposchkin• Found the abundances of the different elements of the stars• First PhD in astronomy from Harvard/Radcliffe 1925• First woman full professor at Harvard and chair in 1956

Composition of StarsLine strength and line profile depend on abundance of elementAnd temperature (which energy levels are populated)Sun is composed of:Element MassHydrogen 71%Helium 27%All the Rest 2%Most other stars SAME as sun!

Stationary Source• Speed of waves equalsthe wavelength λtimes the frequency f• C=λf

Moving Source• Wavelengths in direction ofmotion are compressed• Wavelengths when sourceis moving away fromobserver are stretched• Speed of source determineshow much stretching

Doppler Effect• V r / C = (λ - λ o ) / λ o = Δ λ/λ o• If the source is receding (moving away) then it is a redshift• If the source is approaching then the light is blueshifted

Relative Motion• Stationary observer sees wave with emitted wavelength• Moving observer sees wavelength decreased due to his relativemotion

Transverse/Radial Velocity• We can measure the radial velocity of objects on the otherside of the universe but• We can measure the transverse velocity of only nearby stars• Thus we can only measure the true velocity of the nearby stars

Discovering Extrasolar Planets• Spectra from largest telescopes have enough precision tosee radial velocity variations of star moving around centerof mass• First was 51 Peg in 1995• Big close planets easiest• Called Hot Jupiters

Car Horn

Energy LevelDiagram of aHydrogen Atom-If an electron gains more than thebinding energy then it willescape from the atom-This process is called ionization- The atom is a positive ion- Denoted by H +- Called a negative ion if it gainsan electron H -

Spectral Classes / Spectral Sequence• Oh Be A Fine Girl/Guy Kiss Me• Spectral Types have subdivisions of 0-9• Pick a star to be representative/standard star

Pressure• Gas is made of tiny atoms & molecules which are in constant motion• The higher the temperature the faster the particles move and• The more often the gas atoms hit the wall the larger the pressure• Bubble nebula – star wind exerts pressure to expand nebulaCoolHot

Pressure/Collisional Broadening• Spectra of Vega & Deneb

Relative Radii of Atoms• Atomic radii are all about 0.1-0.2 nanometer• Smaller than the wavelength of visible light

Radioactive Decay of Carbon 14• Isotopes have same chemical properties• 1 Carbon 14 = 14 C in 10 13 carbon atoms• Half the Carbon 14 will decay in 5700 years• Your body is radioactive (thousands ofdisintegrations/second)

Clocks in Rocks• Clocks start at time of solidifying and reset by melting•238U decays to 234 Th → 206 Pb with half-life of 4.5billion years• We have rock samples of the Earth, Moon, Mars, meteorites• All date to a maximum of ~4.6 Billion years• Time of formation of Solar System

Time to Solar System Formation• Meteorites with chondrules (spherical) formed before Earth• White Calcium Aluminum inclusions were formed 4.57 billion years• Short lived radioactive elements indicate that ~million years elapsedfrom supernova/solar system formation and rock crystallization

Atom• Number of Protons determines the Element• Neutrons hold the nucleus together• Number of electrons determined by protons in nucleus• Chemistry determined by electrons

Transitions of Hydrogen• Difference in energylevels determinesphoton energy• Photon energydepends onwavelength equalsphoton’s color• Balmer lines invisible• Lyman lines in UV• Paschen in infrared

Star’s With Dust Disks• Star radiates light like ahot Blackbody• Some starlight warmsthe disk• Disk radiates light(Infrared) like a coolerBlackbody

Spectral Sequence = Temperature Sequence

Spectrum of a Nebulae

Solar Spectrum• Millions of absorption lines of different elements and ions• Width of lines depends on temperature and abundance

Kirchhoff’s Laws=Kinds of Spectra

Types of SpectraWe usually look at graph of Intensityversus Wavelength• No lines = Continuous• Absorption lines = Dark lines• Emission lines = Bright Lines

HydrogenEnergyLevelDiagram• Lyman Series• Balmer Series• Paschen Series• Ion, Ionized,Ionization andBinding energy• Recombination

(B-V) Color Index• Counting the number of photons which pass through a filter andcomparing to another filter will tell us the temperature of a star• A hot blue star has a (B-V) = -0.1 and (B-V)=1.5 for a cool red star

Carbon Atom in Ground State• Six electrons populatingmany energy levels givesmuch more complexspectrum

Wien’s Law• Wavelength of maximum intensity depends on temperature• λ = 3,000,000/Temp in nanometers & degrees Kelvin• λ = 3,000/Temperature in microns & Kelvin

Image/Line Spectra• We rarely look at bands of color• Usually we graph intensity versus wavelength

Loss ofAtmosphere• First atmosphere of Hydrogenand Helium• Is lost to space because thetemperature / velocity is abovethe escape velocity

Waves• Wavelength λ is distance betweencrests• Frequency f is number of wavesper second which pass a point

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