Physics 212 lecture 29: Light, Light Speed, and Light Rays

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Physics 212 lecture 29 Light, Light Speed,
and Light Rays

• Light
• WAVE-PARTICLE DUALITY. Light has the
  characteristics of either waves or of particles
  depending upon the situation under which it is
  being studied.
• Wave behavior electric and magnetic fields,
  reflection, refraction, interference,
  diffraction, polarization
• Particle behavior momentum, photoelectric
  effect, quantization
• Think of light as the quantization of a wave in
  individual packets of energy

h Plancks constant 6.63x10-34 Js
Energy comes in this amount per photon
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• Measuring Light Speed (Roemer)

• First estimate 2.3x108 m/s
• Observed eclipse of Io, a moon of Jupiter
• Noticed eclipses did not occur when predicted.
  Why?
• Jupiter farther away or closer at different
  times of year
• Light takes longer or shorter path to reach
  earth

Diagram
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• Measuring Light Speed (Fizeau)

• First estimate made on earth
• Cogged wheel turning in path of a reflected
  light beam
• measure time for light to go to
• mirror and back
• measure time for wheel cog to
• turn just enough to see the
• reflected light
• Equal times for light and cog

Diagram
Note how fast wheel must turn to get this cog
just out of the way before the light returns
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• Ray Approximation of Light

• Light travels in a straight line (ray) without
  spreading out
• This is true if light travels through regions
  of space much wider than its
• wavelength

Diagrams
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Example Ch 35 2 Io eclipses delayed by 22min
when observed 6 months apart at closest, furthest
points. Earth orbit has average r1.5x108km.
Find speed of light.