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?

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.

Particles and Waves Particles and Waves

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.

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.

2/28/12

NewtonNewton

Thought of light as particles

Maxwell’s Theory Maxwell’s Theory

Light is composed of crossed electric and magnetic fields which make up a wave.

Experiments show that when light shines on a metal surface, the surface emits electrons.

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”?

Planck’s ConstantPlanck’s Constant

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

PhotonPhoton

a light quantum that is massless, has energy and momentum, and travels at the speed of light

The Photoelectric Effect The Photoelectric Effect

the emission of electrons produced when electromagnetic radiation falls on certain materials

Threshold Frequency fThreshold Frequency f00

the minimum frequency of incident light which can cause photo electric emission

Energy of a photonEnergy of a photon

E=hfh=Planck’s constant

f=frequency

Electron VoltsElectron Volts

1 eV= 1.6x10-19 J

λ=wavelength

nmeVhc

E

1240

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

KE of photonKE of photon

hf0=min. energy to release electron

hc

- hc

=hf-hf=-E=KE0

0photon

Stopping Potential (Stopping Potential (Vo)Vo)

The negative potential at which the photo electric current becomes zero

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

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.

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

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

Source: http://sol.sci.uop.edu/~jfalward/particlesandwaves/phototube.jpg

Simple Photoelectric Experiment

Photoelectric EffectPhotoelectric Effect

Applications

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

Applications: Night Vision Applications: Night Vision DeviceDevice

http://www.lancs.ac.uk/ug/jacksom2/

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

Photoelectric Smoke DetectorPhotoelectric Smoke Detector

Source: http://www.bassburglaralarms.com/images_products/d350rpl_addressable_duct_smoke_detector_b10685.jpg

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

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>.

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.

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!

#1#1According to Einstein, the energy of a

photon depends on the _________ of the electromagnetic radiation.

A.momentum

B. speed

C. frequency

D. intensity

#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

#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

#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

#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

#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

#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

#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

#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

#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

#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

#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

#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

#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

#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

#1#1http://lrt.ednet.ns.ca/PD/ict_projects/photoelectric

/index.htm

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.

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.

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?

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.

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.

Wave Nature of MatterWave Nature of Matter

Louis DeBroglie

= h/(mv)Electrons vs. macroscopic matter

practicepractice What is the de Broglie wavelength of

a .050gram projectile fired at 180m/s?

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

Do Now (3/3/14):Do Now (3/3/14):

Atomic StructureAtomic Structure

J.J. ThomsonErnest RutherfordNiels BohrEnergy level diagramsE = hf and c=fLowest n has lowest energy. (Most

negative)

Big IdeasBig Ideas Millikan Planck Rutherford DeBroglie Bohr Compton Atomic Spectra Photo-electric Effect Wave particle duality

Atomic StructureAtomic Structure

J.J. Thomson

Millikan

Ernest Rutherford

Cathode Ray and the ElectronCathode Ray and the Electron

F=evB

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

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

Ernest RutherfordErnest Rutherford

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.

The Experiment The Experiment

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.

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

Planck’s constantPlanck’s constant

E=hfE=nhfE= energyn=integer (1,2,3…)h=constant= 6.626 *10-34 J*sf= frequency

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

Atomic StructureAtomic Structure

Niels BohrBohr model of the atomEnergy level diagrams

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.

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.

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.

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.

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.

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.