do now (2/21/14):
DESCRIPTION
Do Now (2/21/14):. What does the word “quantized” mean? Where have we seen quantization in Physics? What is the structure of an atom?. Objectives. Define photoelectric effect and evidence of particle properties of light. Define work function. Calculate energy of a photon and an electron. - PowerPoint PPT PresentationTRANSCRIPT
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Do Now (2/21/14):Do Now (2/21/14):
What does the word “quantized” mean?Where have we seen quantization in
Physics?What is the structure of an atom?
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ObjectivesObjectives
Define photoelectric effect and evidence of particle properties of light.
Define work function.Calculate energy of a photon and an
electron.Determine Planck’s constant.
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Particles and Waves Particles and Waves
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Quantum TheoryQuantum Theory
Max Planck (1900) recognized electromagnetic radiation is quantized as E=hf.
1905, Einstein proposed photon theory of light. Supported by work in photoelectric effect.
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Photoelectric EffectPhotoelectric Effect
E = KE + WEnergy of impinging light equals KE of
electron plus the work function.Intensity increases will increase current.Frequency changes affect KE.
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2/28/12
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NewtonNewton
Thought of light as particles
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Maxwell’s Theory Maxwell’s Theory
Light is composed of crossed electric and magnetic fields which make up a wave.
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Experiments show that when light shines on a metal surface, the surface emits electrons.
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Planck’s WorkPlanck’s WorkIn 1900, Max Planck came up with a formula to
explain radiation from objects, but the formula only made sense if the energy of a vibrating molecule was quantized.
What are some other examples of “quantization”?
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Planck’s ConstantPlanck’s Constant
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Einstein’s TheoryEinstein’s TheoryBased on Planck's work,
Einstein proposed that light also delivers its energy in chunks
light consists of particles (quanta) called photons, each with an energy of Planck's constant times its frequency
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PhotonPhoton
a light quantum that is massless, has energy and momentum, and travels at the speed of light
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The Photoelectric Effect The Photoelectric Effect
the emission of electrons produced when electromagnetic radiation falls on certain materials
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Threshold Frequency fThreshold Frequency f00
the minimum frequency of incident light which can cause photo electric emission
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Energy of a photonEnergy of a photon
E=hfh=Planck’s constant
f=frequency
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Electron VoltsElectron Volts
1 eV= 1.6x10-19 J
λ=wavelength
nmeVhc
E
1240
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Example:Example:Calculate the wavelength and the energy of a photon of
light with frequency equal to 1.984 x 1014 Hz.• Calculating the wavelength, from :
c=fλ 3x108=λ (1.984 x 1014 )= 1.51 x 10-6 m
• Calculating the energy of the photon:
E = hf E = 6.628 x 10-34 x 1.984 x 1014
= 1.31 x 10-19 J
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KE of photonKE of photon
hf0=min. energy to release electron
hc
- hc
=hf-hf=-E=KE0
0photon
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Stopping Potential (Stopping Potential (Vo)Vo)
The negative potential at which the photo electric current becomes zero
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Example: Example: The stopping potential of a certain photocell
is 4 V. What is the KE given to the electrons by the incident light?
KE=-W
KE=-qV0
KE=-(1.6x10-19)(4)=+6.4x10-19J
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Work Function Work Function ϕϕ00
Minimum amount of energy which is necessary to start photo electric emission.
It is a property of material. Different materials have different values of work function.
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Einstein’s TheoryEinstein’s Theory
hf = + ½ mv2
hf : energy of each photon
Source: http://www.westga.edu/~chem/courses/chem410/410_08/sld017.htm
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Kinetic energy of emitted electron Kinetic energy of emitted electron vs. Light frequencyvs. Light frequency
Higher-frequency photons have more energy, so they make electrons come out faster; same intensity but a higher frequency increases the max KE of the emitted electrons.
If frequency is the same but intensity higher , more electrons come out (because there are more photons to hit them), but they won't come out faster, because each photon still has the same energy.
if the frequency is low enough, then none of the photons will have enough energy to knock an electron out. If you use really low-frequency light, you shouldn't get any electrons, no matter how high the intensity is. if you use a high frequency, you should still knock out some electrons even if the intensity is very low.
Source: http://online.cctt.org/physicslab/content/PhyAPB/lessonnotes/dualnature/photoelectric.asp
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Source: http://sol.sci.uop.edu/~jfalward/particlesandwaves/phototube.jpg
Simple Photoelectric Experiment
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Photoelectric EffectPhotoelectric Effect
Applications
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ApplicationsApplications The Photoelectric effect has numerous applications, for
example night vision devices take advantage of the effect. Photons entering the device strike a plate which causes electrons to be emitted, these pass through a disk consisting of millions of channels, the current through these are amplified and directed towards a fluorescent screen which glows when electrons hit it. Image converters, image intensifiers, television camera tubes, and image storage tubes also take advantage of the point-by-point emission of the photocathode. In these devices an optical image incident on a semitransparent photocathode is used to transform the light image into an “electron image.” The electrons released by each element of the photoemitter are focused by an electron-optical device onto a fluorescent screen, reconverting it in the process again into an optical image
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Applications: Night Vision Applications: Night Vision DeviceDevice
http://www.lancs.ac.uk/ug/jacksom2/
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Photoelectric Effect ApplicationsPhotoelectric Effect Applications Photoelectric Detectors In one type of
photoelectric device, smoke can block a light beam. In this case, the reduction in light reaching a photocell sets off the alarm. In the most common type of photoelectric unit, however, light is scattered by smoke particles onto a photocell, initiating an alarm. In this type of detector there is a T-shaped chamber with a light-emitting diode (LED) that shoots a beam of light across the horizontal bar of the T. A photocell, positioned at the bottom of the vertical base of the T, generates a current when it is exposed to light. Under smoke-free conditions, the light beam crosses the top of the T in an uninterrupted straight line, not striking the photocell positioned at a right angle below the beam. When smoke is present, the light is scattered by smoke particles, and some of the light is directed down the vertical part of the T to strike the photocell. When sufficient light hits the cell, the current triggers the alarm.
Source: http://chemistry.about.com/cs/howthingswork/a/aa071401a.htm
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Photoelectric Smoke DetectorPhotoelectric Smoke Detector
Source: http://www.bassburglaralarms.com/images_products/d350rpl_addressable_duct_smoke_detector_b10685.jpg
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ApplicationsApplications
Solar panels are nothing more than a series of metallic plates that face the Sun and exploit the photoelectric effect. The light from the Sun will liberate electrons, which can be used to heat your home, run your lights, or, in sufficient enough quantities, power everything in your home.
Source: www.futureenergy.org/ picsolarpannelsmatt.jpg
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Work CitedWork Cited
Amar, Francois G. The Photoelectric Effect. 25 Sep 2003. Section of Chemistry 121 for fall 03. 11 May 2006 <http://chemistry.umeche.maine.edu/~amar/fall2003/photoelectric.html>
Blawn, Jeramy R. and Colwell, Catharine H. Physics Lab: Photoelectric Effect. 10 Jun 2003. Mainland High School: Online Physics Labs. 11 May 20006 <http://online.cctt.org/physicslab/content/PhyAPB/lessonnotes/dualnature/photoelectric.asp>
Helmenstine, Anne Marie. Photoelectric & Ionization Smoke Detector. 25 Feb 2006. About.com. 11 May 2006 <http://chemistry.about.com/cs/howthingswork/a/aa071401a.htm>
Einstein, Albert. “Concerning an Heuristic Point of View Toward the Emission and Transformation of Light.” American Journal Of Physics 5 May 1965: 137.
Nave, Rod. HyperPhysics. 19 Aug. 2000. Georgia State University. 06 May 2006 <http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html> .
Thornton T., Stephen, and Rex, Andrew. Modern Physics for Scientists and Engineers. Canada : Thomson Brooks/Core, 2006
Photoelectric Effect. 24 Apr. 2006. Wikipedia Free Encyclopedia. 05 May 2006. <http://en.wikipedia.org/wiki/Photoelectric_effect>.
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Do Now (2/25/14):Do Now (2/25/14):
In your own words, describe the photoelectric effect. Use the words “work function,” “threshold frequency,” “electron,” and “photon,” at least once in your paragraph.
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White Board Competition!White Board Competition!
Work in groups For each correct question, make a tally in
the upper right hand corner of your board. BE HONEST!!!
The teams with the most points at the end will receive extra credit!
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#1#1According to Einstein, the energy of a
photon depends on the _________ of the electromagnetic radiation.
A.momentum
B. speed
C. frequency
D. intensity
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#2#2The work function of iron is 4.7 eV.
What is the threshold wavelength of iron?
A.2.60 nm
B. 260 nm
C. 470 nm
D. 2600 nm
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#3#3The stopping potential, V0, that prevents
electrons from flowing across a certain photocell is 6.0 V. What is the kinetic energy in J given to the electrons by the incident light?
A.9.6 x 10-19 J
B.1.60 x 10-19 J
C.6.9 x 10-19 J
D. 6.4 x 10-19 J
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#4#4When light is directed on a metal surface, the
kinetic energies of the electrons
A.vary with the intensity of light
B.vary with the speed of light
C.vary with the frequency of the light
D.are random
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#5#5The threshold frequency for photoelectric
emission in copper is 1.1 x 1015 Hz. What is the maximum kinetic energy in eV of the photoelectrons when light of frequency 1.5 x 1015 Hz is directed on a copper surface?
A.2.65 eV
B. 2.12 eV
C. 1.66 eV
D. 1.03 eV
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#6#6What will likely happen if a light whose
frequency is below the threshold frequency hits a clean metal surface?
A. no electron will be ejected from the metal
B. fewer electrons will be ejected from the metal
C. more electrons will be ejected from the metal
D. ejected electrons will have higher kinetic energy
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#7#7What is the work function of a
metal whose threshold frequency is 3.5 x 1015 Hz?
A.2.32 x 10-18 J
B. 3.11 x 10-18 J
C. 3.65 x 10-18 J
D. 4.01 x 10-18 J
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#8#8What is the maximum wavelength of
light that will cause photoelectrons to be emitted from sodium if the work function of sodium is 2.3 eV?
A.1.75 x 10-7 m
B. 3.44 x 10-7 m
C. 5.40 x 10-7 m
D. 5.88 x 10-7 m
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#9#9What will the maximum kinetic energy
of the photoelectrons be if 200-nm light falls on a sodium surface (work function is 2.3 eV)?
A.2.96 x 10-19 J
B. 4.73 x 10-19 J
C. 5. 21 x 10-19 J
D. 6.26 x 10-19 J
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#10#10When 230-nm light falls on a metal, the current
through the photoelectric circuit is brought to zero at a reverse voltage of 1.64 V. What is the work function of the metal?
A.4. 39 x 10-19 J
B. 5.38 x 10-19 J
C. 6.01 x 10-19 J
D. 7.11 x 10-19 J
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#11#11The current in a photoelectric effect experiment
decreases to zero when the retarding voltage is raised to 1.25 V. What is the maximum speed of the electrons?
A.6.63 x 105 m/s
B. 5.53 x 105 m/s
C. 4.78 x 105 m/s
D. 4.19 x 105 m/s
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#12#12What is the maximum speed of an electron ejected
from a sodium surface whose work function is 2.28 eV when illuminated by light of wavelength 450 nm?
A.3.25 x 105 m/s
B. 4.10 x 105 m/s
C. 4.85 x 105 m/s
D. 5.25 x 105 m/s
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#13#13Light is incident on the surface of metallic sodium,
whose work function is 2.3 eV. The maximum speed of the photoelectrons emitted by the surface is 1.2 x 106 m/s. What is the wavelength of the light?
A.1.95 x 10-7 m
B. 2.42 x 10-7 m
C. 2.86 x 10-7 m
D. 3.01 x 10-7 m
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#14#14Ultraviolet radiation (wavelength 250 nm) falls on
a metal target and electrons are liberated. If the maximum kinetic energy of these electrons is 1.00 x 10-19 J, what is the lowest frequency of electromagnetic radiation that will initiate a photocurrent on this target?
A.1.05 x 1015 Hz
B. 1.35 x 1015 Hz
C. 1.65 x 1015 Hz
D. 1.78 x 1015 Hz
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#15#15Photons of wavelength 220 nm on a metal target
and liberate electrons with kinetic energies ranging from 0 to 61 x 10-20 J. Determine the threshold wavelength of the metal.
A.1.68 x 10-7 m
B. 1.95 x 10-7 m
C. 2.06 x 10-7 m
D. 6.77 x 10-7 m
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#1#1http://lrt.ednet.ns.ca/PD/ict_projects/photoelectric
/index.htm
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Photoelectric EffectPhotoelectric Effect When light shines on a surface (metal), electrons are
emitted from the surface. E = KEe + W0
Energy of impinging light equals KE of electron plus the work function.
Light Intensity increases will increase current (# of electrons).
Frequency changes affect KEe.
Contributes to the theory of light as a particle. The photons absorbed are “packets” of light energy.
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Work FunctionWork Function
The minimum energy required is called the work function, W0
If hf < W0 then no electrons are emitted
The lower the energy required to expel the electron, the faster the electron will be moving away from the surface.
This makes it more likely be able to escape from the material entirely.
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practicepractice What is the work function when
monochromatic light of frequency 4.5x1015Hz releases the least tightly held electrons from a metal with a maximum KE of 13.10eV?
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Compton EffectCompton Effect
Short wavelength light (x-rays) scattered from materials had a lower frequency than the incident light.
Wave nature of light would not have shown this shift in wavelength. Explained only through particle explanations.
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Wave Particle DualityWave Particle Duality
Apparently conflicting observations of wave nature and particle nature of light.
Principle of Complementarity (Niels Bohr)
E=hf is a nice bridge since it incorporates both particle and wave properties.
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Wave Nature of MatterWave Nature of Matter
Louis DeBroglie
= h/(mv)Electrons vs. macroscopic matter
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practicepractice What is the de Broglie wavelength of
a .050gram projectile fired at 180m/s?
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Photons and MatterPhotons and Matter4 possible interactions of photon with matter:
– Scattering (Compton effect) with lower frequency but same speed (c).
– Photoelectric effect– Excitation of electron (if energy too small to
ionize)– Pair production-photon creates matter through
production of an electron and a positron
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Do Now (3/3/14):Do Now (3/3/14):
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Atomic StructureAtomic Structure
J.J. ThomsonErnest RutherfordNiels BohrEnergy level diagramsE = hf and c=fLowest n has lowest energy. (Most
negative)
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Big IdeasBig Ideas Millikan Planck Rutherford DeBroglie Bohr Compton Atomic Spectra Photo-electric Effect Wave particle duality
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Atomic StructureAtomic Structure
J.J. Thomson
Millikan
Ernest Rutherford
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Cathode Ray and the ElectronCathode Ray and the Electron
F=evB
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Accurately determined the charge carried by an electron using his oil-drop experiment (1.602x10-19 coulomb)
Proved that this quantity is a constant Experimentally verified Einstein’s photoelectric
equation and made the first direct photoelectric determination of Planck’s constant
Explored the region of the spectrum between ultraviolet and X-radiation, extending the ultraviolet spectrum far beyond the known limit
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Two parallel metal plates acquire charge when electric current is applied.
Atomizer sprays mist of oil droplets, which then fall slowly through a small hole.
Space between plates ionized by radiation and electrons attach themselves to oil droplets, giving them a negative charge
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Ernest RutherfordErnest Rutherford
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Rutherford HistoryRutherford History
Ernest Rutherford, 1st Baron Rutherford of Nelson, OM, FRS (30 August 1871 – 19 October 1937) was a New Zealand chemist who became known as the father of nuclear physics. He discovered that atoms have a small charged nucleus, and thereby pioneered the Rutherford model (or planetary model, which later evolved into the Bohr model or orbital model) of the atom, through his discovery of Rutherford scattering with his gold foil experiment. He was awarded the Nobel Prize in Chemistry in 1908.
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The Experiment The Experiment
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Rutherford ScatteringRutherford Scattering
This experiment showed that the positive matter in atoms was concentrated in an incredibly small volume and gave birth to the idea of the nuclear atom. In so doing, it represented one of the great turning points in our understanding of nature.
It also put a rest to the Thompson model of the atom because of the angle’s at which the particles were scattered away from the nucleus of the atoms was greater than the Thompson model said it could be.
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Quantum theory – Max PlanckQuantum theory – Max Planck
In 1900 Planck postulated that energy is radiated in small, discrete units, which he called quanta.
he discovered a universal constant of nature, Planck's constant. Planck's law states that the energy of each quantum is equal to the frequency of the radiation multiplied by the universal constant.
E=hf
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Planck’s constantPlanck’s constant
E=hfE=nhfE= energyn=integer (1,2,3…)h=constant= 6.626 *10-34 J*sf= frequency
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practicepractice According to Plank’s quantum hypothesis,
which of the following could be the energy of molecular vibrations in a radiating object with a wavelength of λ?
a. 4λhcb. hc/2λc. 4hc/λd. 2λc/he. λhc/2
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Atomic StructureAtomic Structure
Niels BohrBohr model of the atomEnergy level diagrams
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Bohr and Quantum Bohr and Quantum HypothesisHypothesis
Discharge spectrahf=Eu – Ei where Eu is energy of the upper
state.Orbit closest to the nucleus has lowest
energy (most negative). An electron at infinite distance has energy of 0 eV.
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Energy Level DiagramsEnergy Level Diagrams
Minimum energy to remove an electron is binding energy or ionization energy.
13.6eV – energy required to remove an electron from the lowest state E1= -13.6eV up to E=0.
Lyman series, Balmer series, Paschen series for hydrogen atoms. – pg 848.
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The diagram above shows the lowest four discrete energy levels of an atom. An electron in the n = 4 state makes a transition to the n = 2 state, emitting a photon of wavelength 121.9 nm.
(a) Calculate the energy level of the n = 4 state.
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The diagram above shows the lowest four discrete energy levels of an atom. An electron in the n = 4 state makes a transition to the n = 2 state, emitting a photon of wavelength 121.9 nm.
(b) Calculate the momentum of the photon.
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The diagram above shows the lowest four discrete energy levels of an atom. An electron in the n = 4 state makes a transition to the n = 2 state, emitting a photon of wavelength 121.9 nm.
The photon is then incident on a silver surface in a photoelectric experiment, and the surface emits an electron with maximum possible kinetic energy. The work function of silver is 4.7 eV.
(c) Calculate the kinetic energy, in eV, of the emitted electron.
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The diagram above shows the lowest four discrete energy levels of an atom. An electron in the n = 4 state makes a transition to the n = 2 state, emitting a photon of wavelength 121.9 nm.
(d) Determine the stopping potential for the emitted electron.