Properties of light

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Light and Matter
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Purpose

• Clarify a few topics within the study design
• Give an overview of the importance of the topic
• Show you demos and animations that may be useful
  in your teaching

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Outline

• Nature of light Wave ideas, particle ideas, and
  the connection
• Production of light Wave ideas, particle
  ideas and the connection
• Matter Wave ideas, particle ideas and the
  connection

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Two familiar models

• WAVES
• (water waves)
• Properties
• Energy is spatially spread out
• Diffracts (bends) and interferes (adds)
• Not Discrete

• PARTICLES
• (billiard balls)
• Properties
• Energy and momentum in a concentrated space
• Discrete

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Wavelike nature of light

• Waves interfere (add and subtract

• Waves diffract (bend)

Waves interfere and diffract. Light interferes
and diffracts. Therefore light is a wave?
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Wavelike nature of light

• Waves interfere (add and subtract

• Waves diffract (bend)

Waves interfere and diffract. Light interferes
and diffracts. Therefore light is a wave? NO!
More correct to say Light has wavelike
properties
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Particlelike nature of light

• Experiments which show particle nature
• Photoelectric effect
• Compton effect
• Any experiment with single photons

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Particlelike nature of light

• Experiments which show particle nature
• Photoelectric effect

Photoelectric effect note The wave and particle
models BOTH predict electrons will come off the
surface of the metal
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Connection between particlelike and wavelike
nature of light
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Connection between particle and wavelike nature
of light

• Probability wave?
• Wave packet?
• Even closer Mathematical function

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Connection between particle and wave nature

• Wave model
• Light is electromagnetic waves
• Colour tells us the frequency of the wave

• Particle model
• Light is packets of energy called photons
• Colour tells us the energy of the photon

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Production of light

• Heating dense things up e.g. sun, lightbulbs
• Exciting the molecules in a low pressure gas
  using heat or electricity

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Wavelike production of light

• Light / Electromagnetic waves are created by
  oscillating charges
• The vibration of charges in an object cause it to
  radiate light.
• The hotter the object the faster the average
  oscillation, and the higher the average frequency

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Particlelike production of light

• Light is a particle created by energy level jumps
  in atoms
• Atomic spectra (applet from 2000)

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Particlelike properties of lightBohrs model of
the atom

• Electrons only occupy certain energy levels.
• A photon is emitted when an electron moves to a
  lower energy level.
• There are set energy levels, so there are set
  energies and therefore frequencies and
  wavelengths the photons can have.

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Connection between particle and wave nature of
light production

• Atomic spectra
• atoms separated
• Continuous spectra
• atoms close together

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Connection between particle and wave nature of
light production

• Atomic spectra
• atoms separated
• Continuous spectra
• atoms close together
• In between? BROAD LINES IN THE SPECTRA

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Particlelike nature of matter

• Experiments that show the particlelike nature of
  matter
• J.J. Thomson discovery of the electron he
  measured charge mass.

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Particlelike nature of matter

• Experiments that show the particlelike nature of
  matter
• Millikan measured electron charge

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Particlelike nature of matter

• Experiments that show the particlelike nature of
  matter
• Both Millikans and Thomsons experiments showed
  discrete particlelike properties
• Also we can fire and detect single electrons.

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Wavelike nature of matter

• de Broglies idea SYMMETRY!
• LIGHT AND MATTER ALL HAVE WAVELIKE AND
  PARTICLELIKE PROPERTIES AND OBEY
• EXPERIMENTAL EVIDENCE
• Electron interference patterns.
• Wavelike nature of matter also fits in nicely
  with Bohrs model for the Hydrogen atom.

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Wavelike nature of matter Bohr / de Broglie atom

• Only orbits/energy levels that have the waves
  constructively interfering are possible
• This equation is NOT enough to determine that
  only fixed energy levels are possible.
• There are two common but incorrect pictures

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Wavelike nature of matter Bohr/de Broglie atom
Incorrect picture no 1All orbits have the same
wavelength but different radii
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Incorrect picture no 2All orbits have the same
radii but different wavelengths
Wavelike nature of matter Bohr/de Broglie atom

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Wavelike nature of matter Bohrs actual model

• NOT fixed wavelength. NOT fixed radius.
• Bohr actually said that the ANGULAR MOMENTUM was
  quantised

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Wavelike nature of matter Bohrs actual model

• Assumptions
• Circular orbit with coulomb force centripetal
  force
• The condition of constructive interference
• De Broglies wavelength
• Some algebra later

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Wavelike nature of matter

• Bohr / de Broglie model worked amazingly well for
  Hydrogen but not for other atoms. A more
  accurate model requires Schrodinger!

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Comparing wavelengths

• Compare the momentum of photons and of particles
  of the same wavelength
• For matter AND light
• LIGHT
• MATTER (much slower than c)

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Philosophical stuff

• Wave or particle?
• Neither! Something that doesnt fit into our
  experience

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Some current questions

• How does the wavefunction collapse into a
  particle when it gets detected?
• What kinds of wave-particles are there and how do
  they interact with each other?
• Can all these wave-particles and their
  interactions be derived from a simple unified
  theory?
• Is gravity caused by a wave-particle?
• Are the fundamental building blocks not
  wave-particles, but oscillating strings?

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My favourite websites with animations

• www.colorado.edu/physics/2000
• http//phet.colorado.edu/web-pages/simulations-bas
  e.html
• http//www.msu.edu/user/brechtjo/physics
• http//www.walter-fendt.de/ph11e
• http//vsg.quasihome.com/interfer.htm