Particle Model of Light

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Particle Model of Light
But Mr Hibbert said it was a wave!!!!!!!!!
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We know that light shows wave properties such as

•        Reflection
•        Refraction

•        Diffraction
•       
• Polarization

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The electroscope
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How the Photoelectric effects works

• We charge the electroscope with a negative
  charge.
• We expose the reactive metal to light of a long
  wavelength, e.g. red.
• We observe that there is no effect, however
  bright the light.
• We then expose the metal to short wavelength
  light, e.g. UV.
• This time we see that the gold leaf drops down,
  showing that the electroscope is losing charge.
• It does not matter how bright or dim the UV light
  is.
• No effect was observed when the electroscope was
  positively charged.

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Huh?

• Electrons were being knocked off. Reactive
  metals have outer shell electrons that can be
  removed easily.
• Red light would not show this effect however
  bright it was. So the amplitude of the light
  wave was not important.
• There was a threshold frequency at which this
  phenomenon started to occur. Light waves with a
  frequency higher than this (shorter wavelength)
  always showed the effect, whatever the
  brightness light waves with a lower frequency
  never showed it.
• The more reactive the metal, the lower was the
  threshold frequency.
• This indicated a particle behavior to light.

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In German
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In English
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Why do these results suggest that light is not a
wave?   

• If light were a wave, bright red light   which
  has a large amplitude (and lots of energy) would
  be able to knock off the outermost electrons of
  the metals

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The name is Planck.Max Planck
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• These findings led to the notion of light being
  tiny little packets of wave energy (quantums or
  quantahelp Jamila) called photons.

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Whoohoo its Quantum Physics
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Photon Model of Electromagnetic Radiation

• Light and other electromagnetic radiation is
  emitted in bursts of energy. We say that it is
  quantized.
• The packets of energy, photons, travel in
  straight lines.
• When an atom emits a photon its energy changes by
  an amount equal to the photon energy.
• The energy changes are discrete amounts or
  quanta.

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• E hf
• E energy in J
• h Plancks Constant gt 6.6 ? 1034 Js (joule
  seconds, NOT joules per second)
• f frequency of the radiation in Hz

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This is useful..

• E hf and c f?
•  
• ? E hc
• ?

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Quick question

• What is the photon energy of light wavelength 350
  nm?

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5.65 10-19 J
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• The joule is the SI unit for energy.
• However atomic physicists find the joule far too
  big and clumsy.
• So they use a unit called the electron volt (eV).
  
• The electron volt is the amount of energy used
  when a charge of electronic charge passes through
  a potential difference of 1 volt.
• The charge on an electron is 1.6 10-19 C, so 1
  eV 1.6 10-19 J

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Convert your answer to in the earlier question to
electron volts. 

• 5.65 10-19 J

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THIS IS IMPORTANT

• Electron volts are almost always used in atomic
  and nuclear physics, but before using equations
  like E hf, the energies MUST be converted to
  joules.

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Another Question.

• A photon has an energy of 10.3 eV. 
• What is its wavelength? 
• Where on the electromagnetic spectrum would this
  be? 

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This may help..
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The answer

• 1.2 ? 10-7 m
• This is 120 nm, which is UV light.

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This is cool
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Excited electrons
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