How to make EM radiation

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How to make EM radiation

• Fundamentally Jiggle electric charges at
  frequency n and they will radiate at that
  frequency.
• Your power circuits run at 50 Hz and radiate at
  50 Hz hence the annoying hum when you amplify
  an open circuit.
• How to jiggle at n c/410-7 7.51014 Hz?
  Heat stuff and it will jiggle its electrons.
• This is how incandescent bulbs work.

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Blackbody Radiation

• Heating a sufficiently dense object produces
  radiation determined completely by the
  temperature.
• The hotter the bluer. Temperature (oK) measures
  energy of electrons in matter.
• Wiens (1893) law
• lmax(m) 0.0029/T(oK)
• The hotter the brighter. Stefan-Boltzmann (1879)
• F sT4
• s 5.6710-8 (J/S) m-2 K-4

Color of a star tells us its surface temperature
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Some Calculations

• A stars radius R determines the radiating
  surface area
• A 4pR² (m²).
• Total power radiated is P AF (in Joules per
  second or W(atts)).
• So using Stefan-Botlzmann for F we have
• P 4pR²s T4
• For Sun R? 6.96108 m T? 5800K
• Power radiated is
• L? 4pR²s T4 3.91026 W
• So maximum emission is at
• 0.0029/T? 510-7 m 500 nm
• This is blue. Why does it look yellow? Soon..

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Spectral Lines

• Fraunhofer (1814) Spectrum of sunlight has gaps.
• Kirchoff-Bunsen (1859) Heated gas emits
  discrete line spectrum.
• Each element absorbs/emits a characteristic line
  spectrum.

Line spectra tell us chemical composition.
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Scattering

• Another possibility light can scatter off dust,
  moisture, and impurities.
• Blue light scatters most hence blue sky.
• Red scatters least hence red sunsets.

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Doppler Effect

• Relative motion of source and observer shifts
  wavelength (and frequency).
• Doppler (1842)
• l- l0 l0 (v/c)

• Observing shifted spectra tells us objects
  velocity

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Using Mirrors to See
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Cassegrain Telescope
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Bigger is Better
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Radio Telescopes
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Using the spectrum

• Objects in the universe emit radiation all over
  the spectrum.
• We see visible wavelengths because Earths
  atmosphere lets them through.
• More information can be found by looking at other
  wavelengths.
• Most of these require high-altitude or
  space-based observations, and advanced detectors.

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Quantum Physics

• An amazing theory of Modern Physics in 50 minutes

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Fin de Siecle Physics

• From Newton (1670) to about 1900 the Universe was
  rational, comprehensible, predictable,
  controllable.
• Mathematical equations describe laws by which
  nature evolves.
• Technology changes life
• Structure of matter begins to make sense
• The complete answer is around the corner

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Some Problems appear

• Light as a wave works well to explain most
  phenomena, especially Youngs two-slit
  interference.
• Photoelectric effect Light can eject electrons
  from a metal, with energy
• E hn W
• E independent of intensity of light.

The constant h 6.610-34 J-sec was introduced
by Planck (1900) together with photons to
explain problems in details of blackbody spectrum

• Einstein (1905) Light energy comes in packets
  (quanta) photons with energy hn. These are
  absorbed by electrons in PE effect. A wave is
  sometimes particles??

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Matter and Radiation

• Rutherford (1910) Atom has small, dense,
  positive nucleus surrounded by negative
  electrons.
• Problem Electrons jiggle, how is it stable?
• How to understand line spectra?
• Balmer (1885)
• n Rc (1/m2 1/n2)

• Bohr (1915) Electrons can occupy only certain
  orbits with E -hRc/n2 and jump by emitting or
  absorbing one photon.

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Wave Mechanics

• de Broglie (1920) Bohrs electron energies are
  like frequencies of harmonics. Makes sense if
  electron is a wave (?).
• Davisson-Germer (1927) Find two-slit
  interference with electrons.
• Two slit experiment with light
• Two-slit experiment with electrons
• Is it a particle or a wave? Yes.
• Which slit did the electron go through?

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Heisenberg Uncertainty Principle

• If we put a detector near one slit, we know which
  slit the electron went through.
• This has to destroy the interference.
• Heisenberg Measurement of position introduces
  random velocity smearing out the interference
  pattern. Cannot in principle measure position
  and velocity together.

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What Waves Now?

• The wavefunction is the object whose evolution is
  determined by the laws of physics (equations).
• Intensity of the wave determines probability of
  finding the particle.
• Physics is probabilistic.
• Bohr atom replaced by Schrodinger Atom in which
  orbitals become wavefunctions.

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So what do we know?

• As an added bonus, learn Pauli exclusion
  principle two electrons cannot occupy the same
  state. This explains chemistry and the periodic
  table
• We also understand enough about microscopic
  physics to drive 20th century technology.
• We can also understand how stars work.
• But we do not know what an electron is..