Quantum Qualities 1 frequency, wavelength and momentum of photons and electrons.
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Transcript of Quantum Qualities 1 frequency, wavelength and momentum of photons and electrons.
Quantum Qualities
2
In 1833, Michael Faraday observed evidence of electrons when he observed a gas discharge glow in an evacuated container. The container was a sealed glass tube with two electrodes.
In 1858, Julius Plucker proved the glow was charged. The glow cast a shadow and responded to a magnetic field. Casting a shadow meant the glow was comprised of rays, and responding to a magnet meant the glow was charged.
In 1876, Eugen Goldstein named the glow “cathode rays”.
Quick Trip Way Down Memory Lane
Quantum Qualities
3
In 1896, Jean-Baptist Perrin captured cathode rays and measured their charge, proving that the rays carry charge.
In 1885, Johann Balmer, a Swiss schoolteacher developed an equation that describes the photographs of emission light spectra of the elements (rainbow light patterns with dark spaces among the colors).
In 1883, Thomas Alva Edison observed that a sealed light bulb deposited carbon inside the bulb and modestly called it the Edison Effect.
Quick Trip Way Down Memory Lane
In 1890’s, Max Planck used spectroscopic data to create a blackbody radiation law.
Quantum Qualities
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In 1897, J. J. Thompson proved that:
• cathode rays were particles.
• the charged particles emitted by a heated cathode were the same as cathode rays.
• cathode rays acted as charged particles in both electric and magnetic fields.
• Later he measured the charge to mass ratio of the charged particles, later determined to be electrons.
Hear J. J. Thompson describe the relative size of an electron.
From the soundtrack of the film, Atomic Physics Copyright J. Arthur Rank Organization, Ltd., 1948,
Quick Trip Way Down Memory Lane
Quantum Qualities
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It was harder back then to get Edison’s bulbs to glow and certainly harder to keep them from burning out.
But, it was still possible to direct a light beam to a prism and then see the diffracted light on a screen on the other side of the grating.
The 19th Century Version of a Diffraction Experiment
Quantum Qualities
6
It was harder back then to get Edison’s bulbs (discharge tubes) to glow and certainly harder to keep them from burning out.
But, it was still possible to direct a light beam (from a discharge tube) to a prism (grating), and then see the diffracted light on a screen on the other side of the grating.
Filter
Current
On
OffVoltage
-1000 0 +1000
Voltage Generator
Discharge Tube
DiffractionGrating Screen
1
3
Today’s Version of a 19th Century Diffraction Experiment
Quantum Qualities
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Johann Balmer found a mathematical series that predicts the wavelengths of the spectra lines when hydrogen is the gas in the glowing bulb.
657
n
m
486
n
m
434
n
m
410
n
m
375
n
m
2
2 4
k
364.56 nmk
He put different integers in the “k” position of the equation and then noticed that his answers matched the wavelengths of the spectra lines.
Unfortunately, Balmer did not have any idea why the equation worked or why there were colored lines in the first place.
Developing a Model: Matching Observations to Theory
Quantum Qualities
8
Any object that absorbs all the light that strikes it is called a blackbody.
Light Source
Max Planck performed his “blackbody radiation” experiment to figure out why the spectra lines occurred.
Blackbody Radiation
Quantum Qualities
9
A blackbody will emit (radiate) light as it heats up
Max Planck used this spectroscopic data to create a blackbody radiation law.
Screen
1
3
Blackbody Radiation
Quantum Qualities
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Screen
1
3
• Planck believed that the separation of light into discrete lines meant that light energy was quantified as discrete energy levels. • If an unbroken spectrum had been observed, it would have meant a continuous energy distribution.
• Planck started with Balmer’s result and “quantized” the photon emissions from various blackbody radiators.
Blackbody Radiation
Quantum Qualities
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Plank’s experiments produced a description of energy emitted from the blackbody as a function discrete frequency values. Frequency can be calculated from the wavelength,
f = c/ = (speed of light wave)/(wavelength of light)Planck’s Radiation Hypothesis states that:
Blackbody Radiation
The emitted or absorbed energy was quantized with the smallest possible value being when n =1 and that:
All other permitted values of the energy are integral multiples of h f.
E
n
=
n x h x f
Quantum Qualities
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n = an integer
Remember that:
f = frequency of light associated with the heat energy
h = 6.63 x 10-34
Joule·seconds
En= is the dependent variable assigned to theenergy absorbed or emitted
hf is h multiplied by frequency and has units of energy (Joules).
Blackbody Radiation
E
n
=
n x h x f
Quantum Qualities
13
Quick Trip a Shorter Distance Down Memory Lane
In 1905, Albert Einstein extended Planck’s concept to light and used it to explain the photoelectric effect.
In 1913, Niels Bohr proposed a model for the hydrogen
In 1911, Robert Millikan measured the charge of an electron.
atom that combined ideas about the nucleus and electrons that explained the spectra colors and the Balmer mathematical series as well.
Quantum Qualities
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Eescape
=
is the energy an electron has when it leaves the surface of a metal.
E escape
(Shine a light on a metal and electrons will escape from the metal surface.)
hf
hf is the energy of the light shining on the metal
hfw-
is the energy used to move the electron away from the metal atom. (The work done to free the electron from the metal surface.)
hfw
Einstein said that when hf is greater than hfw an electron will leave the metal surface.
Einstein and the Photoelectric Effect (1905)
Light Source
Photons
Photoelectric Effect
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Current
On
Off
-1000 0 +1000
Voltage Generator
Electrode
Voltage
He selected the gravitational field and the electric field.
Gravity will pull oil drop down.
Electric field can pull oil drop up.
(Put a charged oil drop in two force fields and balance the forces so the oil drop does not move.)
Fg is the gravity force pulling the charged oil drop down.
Fe = -qE
Remember that:
Fg = mg
Fe is the electric force pulling the charged oil drop up. m is the mass of the the charged oil drop up. q is the amount of charge on the oil drop up. g and E are the gravity and electric field strengths, respectively.
Millikan Oil Drop Experiment (1911)
Quantum Qualities
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Current
-1000 0 +1000
Voltage Generator
Voltage
An oil drop with a surface charge
• Electric field between electrodes can be adjusted to suspend the drop.• The mass of the oil drop is known.
•The oil drop floats because the force on the drop in the electric field exactly counteracted the force on the drop in the gravity field.
On
Off
Electrodes
m gq E
electron 1.602x1
0
-19Coulombs
(Put a charged oil drop in two force fields and balance the forces so the oil drop does not move.)
Millikan Oil Drop Experiment (1911)
Quantum Qualities
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initial finalΔE = E - E = h × f
h = Planck’s constant
f = frequency of the photon
Bohr Hydrogen Atom Model (1913)
• Electrons reside in orbits and can absorb specific quantities of energy to move to another orbit.
• If an electron returns to its original orbit, the specific amount of absorbed energy could be returned to the world as a photon of light.
• The wavelength and frequency of that light could be calculated using the equation Balmer developed in 1885.
• The energy of that light could be calculated using the equation Planck developed in 1900.
PhotonE = h x f
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• Photons, packets of light with energy = hf, are an example of mass less particles.
• The particle wave duality of light can be shown by combining Planck’s Law and Einstein’s hypothesis.
• Planck stated that an atom can only emit and absorb energy of specific values.
initial finalE = E - E = h f • Einstein stated that the frequency times the wavelength of a photon is equal to the speed of light. f = c
Putting the two equations together yields .c
E = h
Particle Wave Duality
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The particle-wave nature of photons can be described using the work of Planck and Einstein.
photon
h cE = h f = = p c
This equation models the energy of a photon that is released when an electron returns from an outer orbit to its original orbit (closer to the nucleus).
Particle Wave Duality
It can be used to relate the energy of a photon to that photon’s:
•Frequency: Ephoton = h x f
•Wavelength: Ephoton = (h x c)/
•Momentum: Ephoton = p x c
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Your Great Grandfather’s Time
By 1920, engineers and scientists understood that:• photons existed• if photons ever stopped moving, they would have no
mass (a photon’s rest mass equals zero).
Since photons do move:
• their mass is related to the energy needed for movement as defined by Einstein’s famous equation!
•their mass is “relativistic” mass, m using E = mc2
•they have momentum, p , like all moving objects with mass using Pr = mrc
Quick Trip not so Long Ago
Quantum Qualities
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Your Great Grandfather’s Time
Quick Trip not so Long Ago
In 1923, Louis De Broglie understood the properties of photons:
• Some photons can be seen by humans because they have a wave frequency (color) that is detected by human eyes and/or human instruments.
PhotonE = n(h)(v)
• all photons move and have a relative mass connected to the energy, Ephoton, they possess.
r r=p (m )c
• all photons obey Planck’s quantum energy equation:
• They, like all moving objects with mass, have momentum, pr
Quantum Qualities
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He also believed that:
• all things including photons have wave properties.
• unlike photons, most things have a rest mass.
Thus, a baseball’s mass is not zero when it is not being thrown but it does have a wave frequency and a wavelength associated with it when it is in motion.
Momentum and frequency values with corresponding wavelengths.
(m )object
(v )object velocity
hPlanck’s constant any object =
In 1923, Louis De Broglie understood the properties of photons:
Your Great Grandfather’s Time
Quick Trip not so Long Ago
Quantum Qualities
23
• since the mass of a baseball (0.15 kg) is so large, the wavelength associated with a moving baseball is too short for humans to detect.
• since the mass of an electron (9.11 x10-31 kg) is so small, the wavelength associated with a moving electron is long enough for human instruments to detect.
Your Great Grandfather’s Time
Quick Trip not so Long Ago
He also believed that:
In 1923, Louis De Broglie understood the properties of photons:
(m )object
(v )object velocity
hPlanck’s constant any object =
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In 1926, Erwin Schrodinger:
• wondered why De Broglie’s relationship only quantitatively worked when the moving particles where in a force-free environment.
• developed a general equation that:• described electrons moving with a wave motion
because they were under the influence of a force environment generated by the positive charge of the nucleus.
• gives the same momentum and frequency (wavelength) values that would be obtained by De Broglie if the electron was moving without the influence of the atom’s nucleus. Schrodinger’s equation is simply called the “wave
equation”. Unfortunately, it only works really well for the hydrogen atom.
Your Great Grandfather’s TimeQuick Trip not so Long Ago
Quantum Qualities
25
When De Broglie’s ideas were first applied to electron behavior, the topic was called quantum mechanics.
When Schrodinger’s ideas were applied to electron behavior, the topic was called wave mechanics.
The Mechanics of Electron/Atom Behavior
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The Mechanics of Electron/Atom Behavior
• the difficulties with quantum mechanics and/or wave mechanics is that there is no way to know both the position and speed of an electron at the same time.
In 1927, Werner Heisenberg suggested that:
• a probability perspective turns out to be a better way to describe electron behavior within an atom.
Heisenberg’s approach is known today as both:
statistical quantum mechanics and/or
statistical wave mechanics.
Quantum Qualities
27
Quantum Qualities
The major step forward in the understanding of light, electrons and atoms was the steady development of the ideas that:
• energy comes into and goes out of atoms but it does so in packets or quanta.
• each packet of energy can be associated with a frequency (wavelength).
• each packet of energy can interact with an electron within an atom.
• an electron will change orbits or even leave the atom if it interacts with the right packet of energy.
• energy packets, quanta, differ from each other by multiple integer values of Planck’s constant.
Quantum Qualities
28
Equation Summary of our Historical Trip
In 1885, Johann Balmer: 2
24
k364.56 nm
kAn equation that described the photographs of light spectra (rainbow light patterns with dark spaces among the colors).
In 1900, Max Planck: photonE = h f The quantum equation that describes the
relationship between the energy of a wave and the frequency of that wave.
In 1905, Albert Einstein: E escape
= hf hfw
-An equation that connects light energy to electrons leaving an atom.
Quantum Qualities
29
In 1911, Robet Millikan:
In 1913, Niels Bohr:
initial finalE = E - E = h fAn equation that explained the electron orbit quantum nature of hydrogen atom as seen from the hydrogen absorption spectrum and predicted the wavelength of the lines in that spectrum.
An equation that related the charge of an electron to BOTH its mass and the electric field its suspended in.
-mgqelectron
= E
An equation that connects the wavelength of a moving object to its mass and velocity.
(m )object (v )velocity
h object
In 1923, Louis De Broglie:
Equation Summary of our Historical Trip
Quantum Qualities
30
2.) For a photon with a wavelength of 700 nm, calculate the
Two Example Problems
a) frequency, b) the energy, and c) the momentum of the photon.
1.) Calculate the wavelength and the frequency of one photonof light that has an energy of 3.64 x 10-19
Joules.
Objectives: 1. to show the use of quantum equations 2. to illustrate unit manipulations
Quantum Qualities
31
Knowns:
Energy = J-193.64 10Planck’s constant = h =
-346.63 10 J ·s
Speed of light = c =
8 m3.0 10
s
Unknowns: Wavelength = = nmFrequency = f = s -
1
Equations:photon
h cE = h f = = p c
1.) Calculate the wavelength and the frequency of one photonof light that has an energy of 3.64 x 10-19 Joules.
Quantum Qualities
32
Unknowns: Wavelength = = nmFrequency = f = s -
1
Equations:photon
h cE = h f = = p c
1.) Calculate the wavelength and the frequency of one photon
photon
h c=
E
photonEf =
h
photon
h cE = photonE = h f
of light that has an energy of 3.64 x 10-19 Joules.
Quantum Qualities
33
Solution:
-34 8
-19
6.63 10 J s 3.0 10 m/s=
3.64 10 J
Wavelength Value Calculation for Example #1Knowns:
Energy = J-193.64 10Planck’s constant = h =
-346.63 10 J ·s
Speed of light = c =
8 m3.0 10
s
Equations: photonEf=
hphoton
h c=
E
-7 = 5.464 10 m The wavelength of light that has 3.64 x 10-19 J of energy is
Quantum Qualities
34
Solution:
-7 = 5.464 10 m
Units: Wavelength is typically reported in units of nanometers.
91 m = 1 10 nm
-7 9 nm = (5.464 10 m) (1 10 ) = 546.4 nm
m
Answer:
-19A photon with an energy of 3.64 10
J wavelengtoules has h of 54 a 6 nm
Wavelength Unit Conversion for Example #1
9 1 10 nm1 m
Remember:
or
546 nm
Quantum Qualities
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Equations: photonEf=
h
Solution:
-19
-34
3.64 10 Jf=
6.63 10 J s
Frequency Calculation for Example #1
Knowns:
Energy = J-193.64 10
Planck’s constant = h =
-346.63 10 J ·s
Speed of light = c =
8 m3.0 10
s
photon
h c=
E
14 1f = 5.49 10
s
The frequency of light that has 3.64 x 10-19 J of energy is
Quantum Qualities
36
Solution:
14 1f = 5.49 10
s
Units: Frequency is typically given in units of Hertz. 1
Hertz = Hz = second
14 141f = 5.49 10 = 5.49 10 Hz
s
Answer:
-19
14
A photon with an energy of 3.64 10
J frequency of 5.49 oul 10es has a Hz.
Frequency Unit Conversion for Example #1
Quantum Qualities
37
Summary for Problem #1 1.) Calculate the wavelength and the frequency of one
photon -193.64 10of light that has an energy of Joules.
Answer:
14and a frequency of 5.49 10 Hz.
-19A photon with an energy of 3.64 10
Joules has a wavelength of 546 nm
Is this wavelength of light in the visible part of the spectrum?If so, what color is it?
?Yes, a 546 nm light wave is in the visible spectrum and will be green.
Quantum Qualities
38
Known: Wavelength = = 700 nm-34Planck's constant = h = 6.63 10 J s
8 mSpeed of light = c = 3.0 10
s
Unknown:Frequency = f in s
–1
Energy = Ephoton in J
2.) For a photon with a wavelength of 700 nm calculate theenergy, frequency, and the momentum.
Momentum = p in kg-m/s
Equations:
photon
h cE = h f = = p c
Quantum Qualities
39
Equations:photon
h cE = h f = = p c
photonEf =
h
h p =
photon
h cE = photonE = h f
h cp
c
h c
= p c
2.) For a photon with a wavelength of 700 nm calculate the
Unknown:
Frequency = f in s –
1
Energy = Ephoton in J
Momentum = p in kg-m/s
energy, frequency, and the momentum.
Quantum Qualities
40
Units:
-9 -91 10 m
700 nm = 700 nm = 700 10 m1 nm
Wavelength Unit ConversionKnown: Wavelength = = 700 nm
Wavelength = = 700 nmWavelength is usually reported in nm and the speed of light is given in m/s.
(For this problem the wavelength will be converted to meters from nanometers so the velocity can be used in meters/second.)
Unknown:
Frequency = f in s
–1
Energy = Ephoton in J
Momentum = p in kg-m/s
8 mSpeed of light = c = 3.0 10
s
Quantum Qualities
41
photon
h cE =
Energy Calculation
Known: Wavelength = = 700 nm
-34Planck's constant = h = 6.63 10 J s
8 mSpeed of light = c = 3.0 10
s
Unknown: Energy = Ephoton in Joules
Equations: photonEf=
h
h p =
photon
h cE =
Quantum Qualities
42
photon
h cE =
Solution:
-34 8
photon -7
m6.63 10 J s 3.0 10
sE = 7 10 m
-19photonE = 2.8 10 Joules
Energy Calculation
Equations:
Known: Wavelength = = 700 nm
-34Planck's constant = h = 6.63 10 J s
8 mSpeed of light = c = 3.0 10
s
Quantum Qualities
43
Units: Energy is typically reported in units of Joules.
Answer:
-19
A photon with a wavelength of 700 nm
has an energy of 3 10 Joules.
Energy Calculation
Is this wavelength of light in the visible part of the spectrum?If so, what color is it?
?Yes, a 700 nm light wave is in the visible spectrum and will be red.
Quantum Qualities
44
Frequency Calculation
Known: Wavelength = = 700 nm-34Planck's constant = h = 6.63 10 J s
8 mSpeed of light = c = 3.0 10
s
Equations: photonEf=
h
Unknown: Frequency = f in s –
1
Energy =Ephoton in J
Momentum = p in kg-m/s
photon
h cE =
h
p =
Quantum Qualities
45
Equations: cf =
Solution:
8
-7
m3.0 10
sf = 7 10 m
14 1f = 4.3 10
s
Frequency Calculation
Known: Wavelength = = 700 nm-34Planck's constant = h = 6.63 10 J s
8 mSpeed of light = c = 3.0 10
s
h p =
Quantum Qualities
46
Solution:
14 1f = 4.3 10
s
Units: Frequency is typically reported in units of Hertz. 1
Hertz = Hz = second
14 141f = 4.3 10 = 4.3 10 Hz
s
Answer:
14
A photon with a wavelength of 700 nm
has a frequency of 4 10 Hz.
Frequency Calculation
Quantum Qualities
47
Equations:
Momentum Calculation
Known: Wavelength = = 700 nm-34Planck's constant = h = 6.63 10 J s
8 mSpeed of light = c = 3.0 10
s
Unknown:
Frequency = f in s –
1
Energy =Ephoton in J
Momentum = p in kg-m/s
photonEf=
hphoton
h cE =
h
p =
Quantum Qualities
48
Equations:
h p =
Solution:
-34
-7
6.63 10 J sp =
7 10 m
-28 J sp = 9.5 10
m
Momentum Calculation
note: 1 Joule
of energy
(1 kg)
(1 m)
(1 second)
2
2 kg s
2 m
2= 1
This momentum unit is also the same as the units of energy divided by velocity.
=
Quantum Qualities
49
Units:
2
2
kg m s 1 =
s m
-28 -28J s kg m9.5 10 = 9.5 10
m s
Answer: -27
A photon with an wavelength of 700 nm
has a momentum of 1 10 kg m/s.
Momentum CalculationThere are several popular choices for momentum units, but the usual units are (mass)(velocity).
For metric calculations these units would be kg m/s.
12
2
J s kg m s = 1
m s m
kg m1
s
Quantum Qualities
50
Summary for Example Problems 1 and 21.) Calculate the wavelength and the frequency of one photon -193.64 10of light that has an energy of Joules.
Answer:
14and a frequency of 5.49 10 Hz.
-19A photon with an energy of 3.64 10
Joules has a wavelength of 546 nm
2.) For a photon with a wavelength of 700 nm calculate theenergy, frequency, energy and the
momentum.
Answer:
-27a momentum of 1 10 kg m/s.
14a frequency of 4 10 Hz, and
A photon with a wavelength of 700 nm has -19an energy of 3 10 Joules, and
Quantum Qualities
51
De Broglie’s equation connects the wavelength of a moving object to the mass and velocity of that moving object.
(m )
object(v )
velocity
h object =
3a.) What is the wavelength value associated with a 0.15 kg baseballmoving with a velocity of 30 meters/second?
Two De Broglie Wave Equation Examples
3b.) What is the wavelength associated with a 9.11 x10 kg electron
-31
moving with a velocity of 1.47 x 10 meters/second?
7
Objectives: 1. to show the use of quantum equations 2. to illustrate unit manipulations
Quantum Qualities
52
Wavelength CalculationKnowns
:
-34Planck's constant = h = 6.63 10 J s
Unknowns:
bWavelength of baseball = in nm
Equations:
1 mSpeed of baseball = v = 3.0 10
s
7 mSpeed of electron = v = 1.4 10
s
-1Mass of baseball = m = 1.5 10 kg
-31Mass of electron = m = 9.11 10 kg
eWavelength of electron = in nm
object velocity
h = (m )(v )
Quantum Qualities
53
Wavelength CalculationKnowns:
-34h = 6.63 10 J s
Equations:
ball1 m
v = 3.0 10 s
electron7 m
v = 1.4 10 s
ball-1m = 1.5 10 kg electron
-31 m = 9.11 10 kg
bball ball
h = (m )(v )
electron e e
h =
(m )(v )
34
1 1) )b
6.64 x 10 Jsmeters
(1.5 x 10 kg (3.0 x 10 m/s
34
31 7) )e
6.64 x 10 Jsmeters
(9.11 x 10 kg (1.4 x 10 m/s
Quantum Qualities
54
Wavelength Calculation
Equations:
b -341.5 x 10 meters
e -34
-31 7
6.64 x 10 Js
(9.11 x 10 kg) (1.4 x 10 m/s)
e -34
-24
6.64 x 10 Js
(12.8 x 10 kg m/s)
b -34
-1 1
6.64 x 10 Js
(1.5 x 10 kg) (3.0 x 10 m/s)
e -10
6.64 x 10 meters
12.8
Quantum Qualities
55
Wavelength Calculation
b -34baseball wavelength = 1.5 x 10 meters
eelectron wavelength = = -115.19 x 10 meters
3a Answer:
A baseball does travel on a wave path like water but going 30 meters per second the ball goes through one complete wave after it has gone 1.5 x10-34 meters.
3b Answer:
An electron does travel on a wave path like water but going 1.4 x 107 meters per second the electron goes through one complete wave after it has gone 5.19 x10-11 meters.