The wave theory of light was unable to explain something known as the photoelectric effect

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The wave theory of light was unable to explain
something known as the photoelectric effect
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A simple example of the photoelectric effect is
the protective beam across the bottom of a remote
control garage door opener.How does it work?
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The puzzling part was

• Not all light caused a flow of electrons off of
  the metal.
• If light was below a certain frequency, it had
  no impact.
• If it was above a certain frequency, there was a
  flow of electricity.

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Scientists could not explain why light had to be
a minimum frequency to cause the photoelectric
effect.
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Enter a scientist by the name of Albert Einstein
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Einstein expanded an idea put forth by another
physicist, Max Planck
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Planck had proposed that hot objects do not emit
electromagnetic energy continuously, as would be
expected if it was in the form of waves.
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Rather, Planck thought objects emit energy in
small, specific amounts called quanta.

• A quantum is the minimum amount of energy that
  can be gained or lost by an atom
• This was the beginning on quantum mechanics

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Planck proposed this relationship between energy
and frequency

• E hv
• E is energy in Joules
• h is Plancks constant
• v is frequency

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In 1905, Einstein put forth the idea that
electromagnetic radiation has a dual
wave-particle nature.

• In other words, light could be thought of as a
  stream of particles

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He called these particles photons.

• A photon is a particle of EMR having zero mass
  and carrying a quantum of energy

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Einstein explained the photoelectric effect by
suggesting that EMR is absorbed by matter only in
whole numbers of photons.

• If the frequency is too low, the energy of a
  photon is not enough to knock electrons loose.

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Einstein was awarded the Nobel prize for physics
in 1921 for his paper on the photoelectric effect
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Lets observe the hydrogen emission spectrum
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Explaining the hydrogen emission spectrum
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First, how it is different from the spectrum seen
when looking at sunlight?
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Some terminology

• Ground state-
• The lowest energy state of an atom
• Excited state-
• A higher energy state than the ground state

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The specific lines/colors seen from the hydrogen
spectrum have particular wavelengths and
frequencies.
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Once we determine the frequency, we can use Eh?
to find the energy given off by the photons.

• What does that indicate about the energy levels
  in hydrogen?

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It suggests that the electron only can possess
specific energy states.
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Now scientists tried to put a model together that
would fit this observation.
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The Bohr model was incomplete

• It only worked for hydrogen (one electron)
• It didnt explain the chemical behavior of atoms

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The Bohr model

• The electron can only circle the nucleus in
  certain fixed orbits.
• The orbit closest to the nucleus is the lowest
  energy level
• The further the orbit is from the nucleus, the
  higher the energy level.

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Quiz
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What is the photoelectric effect?
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T or FAll light shone on a piece of metal will
cause electrons to be knocked off
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Energy of a photon is related toa)
wavelengthb) frequencyc) speed
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When electrons are in their lowest energy state
they are said to be in the

• Ground state