Astronomy

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• Electric and Magnetic Fields are perpendicular
• Entire pattern races along at c 2.99 x 108 m/s
• Optical wavelengths l ? 400 (violet) 700
  (red) nm

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The REAL rainbow
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Four Ways of Thinking about EM radiation

• RAY - geometrical optics reflection and
  refraction in mirrors, lenses, telescopes
• WAVE - diffraction optics interference and
  diffraction, amounts reflected, transmitted
• from VIS, to IR, to radio
• PHOTON - absorption and emission spectra of hot
  gases from VIS, to UV, to x-ray, to g-ray
• PHOTON GAS - diffusion, cosmic background

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The atmosphere is mostly opaque except in the
visible, some IR and radio
The Doppler Effect works for light too

• if object moves AWAY viewer light is
  red-shifted
• if object moves TOWARD viewer light is
  blue-shifted
• speed must be pretty large to see significant
  shift
• Dl/l vo/c is the formula (we wont use it)

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The Photon Idea

• Einsteins conception in 1905
• Think of EM radiation as a PARTICLE, not a wave
• Absorbed, Emitted All at Once
• Packet of Energy E hf
• E energy in electron-volts (eV)
• f frequency in Hz
• h Plancks constant 4.14 x 10-15 eV/Hz
• Optical E ? 1.5 eV 2.5 eV (LEDs)
• Typical chemical energy

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Energy, Power, Flux and All That

• Energy the total amount present, like a candy
  bar (units Joules or eV electron-Volts)
• Power the time rate at which energy is
  consumed, or produced P E/t (units J/s
  Watts), like a light bulb
• Flux the power divided by the area over which
  it is smeared F P/A (units W/m2)
• Note book is goofy on this point
• If power is emitted in all directions equally
  (like a star), then the flux drops off with
  distance d like 1/d2 inverse-square law

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Power emitted from a point-like source spreads
over larger and larger areas
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Law of Reflection
Angle of Incidence Angle of Reflection
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Law of Refraction
Rays bend toward (away) from the normal as they
pass from one medium to a more (less) optically
dense medium
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Fig.06.18
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For a telescope on the moon
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Fig.06.23
CCD detectors and x-ray optics
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How the Eye Sees the diverging cone of rays

• Two cones here one from top, one from bottom
• Image is REAL (there IS light there) and
  INVERTED

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The object is so close to the lens that a
magnified VIRTUAL (there is no light there) image
is formed Looks diverging
MORAL Convex lenses are not always converging
The object is sufficiently far from the lens that
a shrunken REAL (there is light there) image is
formed Looks converging
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How a telescope ANGULARLY MAGNIFIES