Experimental History Leading to Quantum Theory

1
Experimental History Leading to Quantum Theory

• Particles versus Waves

2
Properties of Light1650

• Travels in straight lines in a given material
• Casts sharp shadows
• Comes in colors
• Reflects from smooth surfaces
• Refracts when traveling from one material to
  another
• Research limited to descriptions of how light
  behaved
• In 1660s, question changes to why light
  behaves as it does and to what is the fundamental
  nature of light

3
Model 1Isaac Newton Particle Model

• Light is a stream of small particles
• Model adequately explains reflection refraction
• Colors produced by particles of different shapes
• Behavior of particles governed by law of gravity
• Predicts that light speeds up when going from air
  into a refracting medium

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Model 2Christian Huygens Wave Model

• Light is a wave and each point on a wavefront
  serves as point sources of subsequent wavelets
• Adequately explains reflection refraction
• Colors correspond to different wavelengths
• Predicts that light slows down when going from
  air into refracting medium

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Setting up the Problem

• Is light a particle or a wave?

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Resolving the DilemmaWhat is the speed of light?

• Galileo 1600
• Speed of light is very fast too fast to measure
• Roemer - 1676
• Used occultation of Io by Jupiter
• Speed of light is finite, but very fast
• Fizeau and Foucault 1840s
• Measured speed of light in air and water
• Found it to slow down in water

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New Experimental ResultsYoungs Double Slit
Experiment

• Interference fringes can only be explained with
  the wave model
• Provided first technique for measuring wavelength
  of visible light

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Merger of Optics and EM

• Maxwell unifies experiment work of Faraday,
  Ampere and Oersted into four laws expressed in
  vector calculus notation known today as Maxwells
  equations
• Simultaneous solution results in an equation
  known to describe wave behavior, with the speed
  of the wave in vacuum being 3 x 108 m/s

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Conclusion?

• Newton was wrong
• Light is a wave of electromagnetic origin
• Wavelengths extend beyond the visible into
  ultraviolet and infrared regions
• Waves carry energy
• In 1880s, physics is declared a dead science
  the physical world is totally known
• All thats left are details and applications

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One of the Details

• What determines the colors emitted by an ideal
  radiator as it heats up?
• Max Planck and blackbody radiation
• A solid can be modeled as a matrix of small
  masses connected to nearest neighbors by springs
• Vibrational modes of the springs dictate what
  wavelengths are emitted

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Problems with Classical Explanation

• Classically, vibrational modes form a continuum
• Classical explanation leads to ultraviolet
  catastrophe
• Plancks resolution only discrete, quantized
  vibrational modes are possible
• E nhf, where h is an empirical constant and n
  0, 1, 2

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Another DetailThe Photoelectric Effect

• When a polished metal surface is exposed to
  ultraviolet light, electrons (a.k.a.
  photoelectrons) are emitted from the surface with
  measurable kinetic energy

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Another Problem with Classical Physics

• Experimental results
• Increased UV intensity results in larger number
  of emitted photoelectrons
• Increased UV frequency results in larger kinetic
  energy of photoelectrons

• Classical predictions
• Increased UV intensity results in larger kinetic
  energy of photoelectrons
• Increased UV frequency results in larger number
  of emitted photoelectrons

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Einsteins Explanation

• Light comes in packets, called photons
• Intensity is proportional to the number of
  photons
• Energy of each photon is proportional to the
  frequency E hf
• Result 1921 Nobel Prize

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Further Evidence

• The Compton Effect
• Photons of light carry momentum as well as energy
• Result 1927 Nobel Prize

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The Real Problem

• How can light be both a wave and a particle?
• deBroglies response If a wave can behave like
  a particle, then a particle can behave like a
  wave
• Result 1929 Nobel Prize

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Experimental Confirmation

• Davisson-Germer Experiment
• When electrons pass through a crystalline
  lattice, they interfere to form an interference
  pattern similar to Youngs double slit experiment
  with light
• Result 1937 Nobel Prize

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Whats Going On?

• Are photons like particles or like waves?
• Are particles like waves or like particles?