light, plank, einstein bohr
TRANSCRIPT
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Let there beLet there be
LLiigghhtt!!
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Light
The study of light led to the development ofthe quantum mechanical model.
Light is a kind of electromagnetic radiation.
Electromagnetic radiation includes many kinds
of waves.
All waves move at 3.00 x 108
m/s (abbreviatedwith a c) when in a vacuum (actually 2.9979
x 108 m/s).
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Parts of a WaveParts of a Wave
Wavelength
( )
AmplitudeOrigin
Crest
Trough
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FrequencyFrequency
The number of waves that pass a
given point per second.
Units are cycles/sec or hertz (hz) Abbreviated with the Greek letter nu
()
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Frequency & WavelengthFrequency & Wavelength
Are inversely related
As one goes up the other goes down.
c = Different frequencies are different colors of
light. There is a wide variety of frequencies.
The whole range is called the spectrum
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PracticePractice
What is the wavelength of radiationwhose frequency is 1.50x1015 /s?
Is this longer or shorter than red light?
My favorite AM radio station is1150kHz. What is the wavelength of
this station?
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Introducing Mr. Maxwell
Planck
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Max Planck (1858 1947)Max Planck (1858 1947) Prevailing ideas were that matter was
composed of particles and energy in the form
of light was a wave.
Studying radiation profiles emitted by solidbodies heated to incandescence, Plank found
that the results couldNOT BE EXPLAINED
with the assumption that matter could absorbor emit any quantity of energy.
Planck studied Blackbody radiation for the
answers.
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Blackbody RadiationBlackbody Radiation
When a solid or liquid object is heatedthey will emit light. A toaster or oven
heating element or even a tungsten light is
an example. This is blackbody radiation. It was known that the frequencies of
vibrating atoms in a heated object were
responsible for the emitted radiation, but
Classical physics was not able to explain
the distribution of frequencies.
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Quantization
Max Planck observed that energy emitted by hotobjects only had certain wavelengths.
Instead of light of any energy, only
photons at integer multiples ofa small packet of energy ever
were found.
This minimum packet size was found to be6.626 x 10-34 Js,
which is now known as
Plancks constant and
has the symbol, h.
E=nh
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Introducing Mr. Albert
Einstein
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The Photoelectric EffectThe Photoelectric Effect
It was determined experimentally that lightshining on a metal surface will eject electrons
from the metal surface.
The results will differ as to whether or not light
is a particle or a wave, which we will see in a
minute.
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The Photoelectric EffectThe Photoelectric Effect
Light, although thought of as a wave, didnt behave asif it was a wave in this experiment.
A photoemissive surface would release electrons only
if the light was at or above a specific minimumfrequency.
The intensity or strength of the incident light did not
affect the maximum kinetic energy of the emitted
photoelectrons. However, the rate at which
photoelectrons were released did depend on the
intensity of light.
h h l i
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Even with increased
intensity
The PhotoelectricThe Photoelectric
EffectEffectEvidence for the quantization of light can beseen in the photo electric effect.
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Low energy light
produces no effect.High energy light
ejects an electron
Higher frequency
ejects higher energy
electrons
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The Photoelectric EffectThe Photoelectric Effect
The maximum kinetic energy of the emittedphotoelectrons depended on the frequency of
the incident light (provided it was above the
minimum threshold frequency). None of these behaviors could be explained by
classical physics descriptions of light as a
wave.
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1955)1955) Albert Einstein proposed that electromagnetic
radiation can be viewed as a stream ofparticles known as photons.
Albert Einstein actually won the Nobel Prize
for his work on the photoelectric effect.
In 1913 to 1914, R.A. Milliken did a series ofexperiments that proved all of Einsteins
theories!
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The Photoelectric EffectThe Photoelectric Effect
To conclude the photoelectric effect phenomenonif light
was composed of photons whose energy was equal to hh,
then the energy level being dependent on frequency would
make sense of the whole thing!
Higher frequency photons have more energy, so they should
make the electrons come flying out faster. In other words,
more energy would be transferred to them.
Using higher intensity light should cause more electrons tocome flying out, and using very low frequency light then
none of the photons will have enough energy to knock out
an electron.
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Mr. Einstein, meet Mr.Planck
E=mc2 E=nh
Ephoton=hm = E
c2 Ephoton=hc
c=
= ___c
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Does this mean that photons have mass?
yes, in some sense, but only when it is moving
But isnt light a wave?
yes, some times it is
m = E
c2Ephoton=hc
m = hc/c2
m = hc
Huh?
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Wave/Particle Duality
Some times you feel like a nut, sometimes you dont.
Wave-Particle Duality
http://hyperphysics.phy-astr.gsu.edu/hbase/mod1.htmlhttp://hyperphysics.phy-astr.gsu.edu/hbase/mod1.html -
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AtomicAtomicSpectrumSpectrum
So how does color tell us aboutSo how does color tell us aboutatoms, I mean really, get on withatoms, I mean really, get on with
it! XDit! XD
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A PrismA Prism White light is made up
of all the colors of the
visible spectrum.
Passing it through a
prism separates it
because the different
wavelengths bend at
different angles whenentering and leaving the
substance.
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If the light is not whiteIf the light is not white
By heating an element withelectricity or heat, a characteristic
color of light is given off. When
this light is passed through a
spectrum, there are distinct
wavelengths of light present and
nothing in between!
This unique fingerprint, can beused to identify elements present in
suns, other planets, unknown
samples, etc.
(H )
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mercury (Hg)
sodium (Na)
neon (Ne)
hi b h i b i lN hi b h i b Ni l
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Nothing bohring about NielsNothing bohring about Niels
BohrBohr
In 1913, Niels Bohr developed a theoreticalexplanation for the observed emission spectra
of the Hydrogen Atom.
Bohr Assumed that electrons were orbiting thenucleus and from classical physics, he
assumed that they traveled in a circular path
much like planets did around the sun.
Bohr borrowed the idea of quantized energy
from our good friend Maxwell Planck.
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Niels BohrNiels Bohr
Bohr proposed that only orbits of certain radii,corresponding to defined energies are
permitted.
An electron orbiting in one of these orbitshas
1. A defined energy state.
2. Will not radiate energy
3. Will not spiral into the nucleus.
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Excited Energy StatesExcited Energy States
If you add energy to an electron it can jump from oneenergy level to another producing an excited state.
1H
Ground StateH: 1st level Excited State
H: 2nd level
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Atomic EmissionAtomic Emission
A photon of light is emitted when the electron returnsto its ground state and releases the previous quantum
amount of energy absorbed.
1H
Ground StateH: 1st level
Excited StateH: 2nd level
There are many energy levelsThere are many energy levels
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There are many energy levelsThere are many energy levels
and many transitions thatand many transitions that
can occurcan occur
1Hn =7 6 5 4 3 2 1
Lyman Series:
(ultraviolet)
Balmer Series:
(visible) Paschen series:
(infrared)
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Review: Bohrs Model
Electrons exist in specific energy levels outside thenucleus, can move from one level to another, andoccupy the lowest energy levels possible understable conditions.
When exposed to energy (electrical current, flame,etc.), electrons absorb a quantum of energy andmove to an excited state. Energy is released in theform of light when the electrons return to theground state.
The further an electron jumps the more energy isabsorbed or released when it returns to its ground
state.
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PropertiesProperties Light has a dual nature. It behaves as both a particle
and a wave. Electromagnetic radiation, which was
previously thought to exhibit only wave properties,
seems to show characteristics of particulate matter
as well and can be considered as a stream ofphotons.
Energy is quantized, it seems to occur only in
discrete units called quanta. Light energy dependson its frequency as shown on the next slide.
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QuantizedThe Quantum of Energy was equal to
E = nhE is the energy of the photon
(nu) is the frequency
h is Plancks constant = 6.626 x 10 -34Joule
secn is an integer (1, 2, 3, etc.)
A Joule is the metric unit of Energy
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Review of JoulesReview of Joules Energy is a force moving through a distance. Force is
equal to mass times acceleration. Therefore Force = ma (mass times acceleration)
Acceleration = meter/s2
Mass is measured in kilograms And so a Force = kilogram meter/s2
Which, incidentally is called aNewton.
Joule = Force times a distance
Therefore, Joule = (kg m2)/s2
And now you have units for Energy!
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The End!!