Ideas of Modern Physics

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Ideas of Modern Physics
The view which I am so bold to put forth
considers radiation as a high species of
vibration in the lines of force which are known
to connect particles, and also masses of matter
together. It endeavours to dismiss the aether but
not the vibrations. Michael Faraday, 1846
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Review

• Examples of wave superposition
• Interference in time
• Interference in space
• Evidence for the wave character of light

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Today

• The nature of light
• Applications
• Limitations of optical instruments
• A deep mystery

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Light speed

• Around 1670, Danish astronomer Olaus Roemer
  discovered that eclipses of Jupiter's moons
  appears delayed by some twenty minutes when the
  Earth is furthest from Jupiter relative to when
  the Earth is closest to Jupiter. He interpreted
  the delay as light travel time and inferred the
  speed of light.

http//mail.hep.wisc.edu/duncan/phys107/jupitermo
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Light speed on earth

• Light speed was measured on Earth, by Armand
  Fizeau and Foucault in 1849. A rotating toothed
  wheel was used to chop a beam of light sent to a
  mirror 8.7 km distant. During the travel time to
  the mirror and back (about 6e-5 s), the teeth had
  rotated so as to block the returning light. More
  sensitive and accurate measurements were made by
  A. Michelson using a rotating mirror with
  measurement of the angular rotation of the mirror
  during light travel over distances as large as 70
  km.

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Theories of light

• Newton in a long tradition viewed light as a
  stream of particles.
• Optical systems based on lens and mirrors in the
  16th century
• Youngs results indicated that light was a wave.
• Maxwells theory provided a basis for physical
  models of the interaction of light with matter.

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Properties of light

• Light reflects from a surface with reflection
  angle equal to incident angle.
• Light refracts at a boundary between transparent
  media in a way characterized by Snells Law.
• These empirical laws are the basis for design of
  optical devices.

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Wave theory of optics

• EM wave shakes electrons which are attached to
  atoms as if by springs so oscillate at the wave
  frequency.
• Electrons radiate a reflected wave and a
  transmitted wave which interfere with the
  incident wave.
• Transmitted wave is effectively delayed so
  transmitted light appears to slow down. In a
  metal, interference is destructive so nearly
  complete (mirror)reflection.

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Refractive index

• The effective speed of light in a transparent
  medium is v c/n where n is the refractive
  index.
• n depends on the amplitude of the electron motion
  so depends on frequency.
• The refraction angle depends on n so on frequency
  hence a prism separates colors

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Optical instruments

• The eye uses a lens combination to collect
  diverging light waves from each point of an
  object and focus them to an image point on an
  array of light detectors.

Ideal
Myopia gtblurred vision
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Diffraction

• A wave passing thru an aperture large compared to
  a wavelength is collimated.
• A wave passing through an aperture small compared
  to a wavelength emerges in all directions - it
  diffracts.

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Circular aperture or a disk

• Fresnel used wave theory to explain a remarkable
  fact - that an interference pattern of rings
  appears in the shadow of a circular object and a
  bright spot in the center!

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Fraunhofer Diffraction

• Light of wavelength L passing through an opening
  of diameter d is spread out by an angle L/d.

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Diffraction limits

• Our eyes use L5e-7 m and d5e-3 m so can not
  resolve objects separated by angles less than
  about 1e-4 radians.
• The density of receptors on the retina matches
  the intrinsic limit.

15
High resolution telescopes

• The Hubble has a large d and is above the
  atmosphere which variably refracts light.
• It stares in the same direction for hours on end
  to accumulate light from faint sources.

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Fundamental issue

• What is the medium (aether) if any supporting
  electromagnetic waves?
• Is there some content to matter-less vacuum, to
  empty space?
• Albert Michelson set out to observe the aether by
  its affect on light speed.

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The idea of the experiment

• If the earth moves thru a medium thru which light
  moves at speed c, along the direction of the
  earths motion, light should appear from earth to
  move more slowly.

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The idea of the interferometer

• Send light equal distances along two directions -
  one along the earths motion and one
  perpendicular. Compare travel times by bouncing
  the light from mirrors, superposing the beams,
  and studying the interference. A tiny delay in
  one beam will change the interference pattern.
  Rotate the entire experiment.

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The Michelson interferometer

• A beam spliter sends light to two mirrors (M1,M2)
  and recombines the beams. From 2, one sees
  superposed mirror images and interference
  fringes.

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Null results

• The interferometer is sensitive to tiny time
  delays corresponding to a shift in space of less
  than one wavelength. No fringe change under
  rotation implied no change in light speed and no
  aether.

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The mystery

• How can measured light speed not depend on the
  motion of the observer?
• The answer would demand a revision of many
  fundamental ideas.