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PHY134 Introductory Astronomy

PHY134 Introductory Astronomy . Light and Matter. Summary. Waves are a mathematical description of reaction of medium to local perturbation Exhibit interference and Doppler effect Energy conservation determines brightness of source at distance. Another Force.

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PHY134 Introductory Astronomy

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  1. PHY134Introductory Astronomy Light and Matter

  2. Summary • Waves are a mathematical description of reaction of medium to local perturbation • Exhibit interference and Doppler effect • Energy conservation determines brightness of source at distance

  3. Another Force • Dominant force in most of physics: electromagnetism • Coulomb force can be attractive or repulsive • Opposite charges attract so most objects neutral • Charge is conserved • A charge creates and is affected by electric field

  4. Magnets – and Light • Moving charges create and are affected by magnetic fields (Ørsted 1820) • Changing magnetic field creates electric field (Faraday 1831) • Changing electric field creates magnetic field (Maxwell 1861) • Leads to propagating waves with velocity • Coincides with speed of light (Fizeau-Foucault 1850) • Light is an electromagnetic wave!

  5. Electromagnetic Spectrum • Electromagnetic waves can have any wavelength • What we see is limited by our eyes which are adapted to transparency of atmosphere • What the Universe produces is not. Observing the Universe in many bands produces additional data

  6. Heat Radiation • A hot object radiates • For dense dark objects radiation completely characterized by temperature – blackbody radiation • Hotter objects are blue Wien 1893 • Hotter objects radiate more. Stefan-Boltzmann 1879 flux at object

  7. Example: Our Sun • Measure Solar constant • Compute luminosity • Sun radius • Luminosity is so • Set to find • Use Wien • This is green

  8. When Light Meets Matter • Dense objects absorb light energy or reflect it. • How much absorbed can depend on wavelength – dyes. Can learn composition from reflected spectrum • Light scatters off tenuous matter (Rayleigh 1871) • Scattering decreases with wavelength: blue scatters more than red

  9. Scattering on Earth • Atmosphere scatters blue light making sky glow blue and Sun appear yellow • When we get more scattering – when Sun low in sky – lose green to scattering leaving Sun red

  10. Scattering and Refraction Moon halo Rainbow

  11. Line Spectra • Fraunhofer 1814: Sun’s spectrum has gaps • Kirchoff-Bunsen 1859: Tenuous gas emits line spectrum • Atoms and molecules emit/absorb at characteristic wavelengths when heated or ionized • Line spectrum yields chemical composition • At higher pressure and density lines broadened

  12. Inside the Atom • Rutherford 1909:Structure of the Atom is Keplerian • Heavy nucleus of positive charge of size • Orbited by light electrons of negative charge in orbits of size • Atoms can bind by trading, sharing, or deforming their electrons. • Chemistry is the science of electronic rearrangement • Elements immutable because nucleus not affected

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