need for quantum physics problems remained from classical mechanics that relativity didnt explain...
Post on 18-Jan-2018
238 Views
Preview:
DESCRIPTION
TRANSCRIPT
Need for Quantum PhysicsNeed for Quantum Physics Problems remained from classical Problems remained from classical
mechanics that relativity didn’t explainmechanics that relativity didn’t explain Blackbody RadiationBlackbody Radiation
• The electromagnetic radiation emitted by a The electromagnetic radiation emitted by a heated objectheated object
Photoelectric EffectPhotoelectric Effect• Emission of electrons by an illuminated metalEmission of electrons by an illuminated metal
Spectral LinesSpectral Lines• Emission of sharp spectral lines by gas atoms in Emission of sharp spectral lines by gas atoms in
an electric discharge tubean electric discharge tube
Development of Quantum Development of Quantum PhysicsPhysics
1900 to 19301900 to 1930• Development of ideas of quantum mechanicsDevelopment of ideas of quantum mechanics
Also called wave mechanicsAlso called wave mechanics Highly successful in explaining the behavior of atoms, Highly successful in explaining the behavior of atoms,
molecules, and nucleimolecules, and nuclei Quantum Mechanics reduces to classical Quantum Mechanics reduces to classical
mechanics when applied to macroscopic mechanics when applied to macroscopic systemssystems
Involved a large number of physicistsInvolved a large number of physicists• Planck introduced basic ideasPlanck introduced basic ideas• Mathematical developments and interpretations Mathematical developments and interpretations
involved such people as Einstein, Bohr, Schrinvolved such people as Einstein, Bohr, Schrödinger, ödinger, de Broglie, Heisenberg, Born and Diracde Broglie, Heisenberg, Born and Dirac
Photoelectric EffectPhotoelectric Effect When light is incident on certain metallic When light is incident on certain metallic
surfaces, electrons are emitted from the surfaces, electrons are emitted from the surfacesurface• This is called the This is called the photoelectric effectphotoelectric effect• The emitted electrons are called The emitted electrons are called photoelectronsphotoelectrons
The effect was first discovered by HertzThe effect was first discovered by Hertz The successful explanation of the effect The successful explanation of the effect
was given by Einstein in 1905was given by Einstein in 1905• Received Nobel Prize in 1921 for paper on Received Nobel Prize in 1921 for paper on
electromagnetic radiation, of which the electromagnetic radiation, of which the photoelectric effect was a partphotoelectric effect was a part
Photoelectric Effect SchematicPhotoelectric Effect Schematic When light strikes E, When light strikes E,
photoelectrons are photoelectrons are emittedemitted
Electrons collected at Electrons collected at C and passing through C and passing through the ammeter are a the ammeter are a current in the circuitcurrent in the circuit
C is maintained at a C is maintained at a positive potential by positive potential by the power supplythe power supply
Photoelectric Current/Voltage Photoelectric Current/Voltage GraphGraph
The current increases The current increases with intensity, but with intensity, but reaches a saturation reaches a saturation level for large level for large ΔV’sΔV’s
No current flows for No current flows for voltages less than or voltages less than or equal to –ΔVequal to –ΔVss, the , the stopping potentialstopping potential• The stopping potential The stopping potential
is independent of the is independent of the radiation intensityradiation intensity
Features Not Explained by Features Not Explained by Classical Physics/Wave TheoryClassical Physics/Wave Theory
No electrons are emitted if the No electrons are emitted if the incident light frequency is below incident light frequency is below some some cutoff frequencycutoff frequency that is that is characteristic of the material being characteristic of the material being illuminatedilluminated
The maximum kinetic energy of the The maximum kinetic energy of the photoelectrons is independent of the photoelectrons is independent of the light intensitylight intensity
More Features Not ExplainedMore Features Not Explained The maximum kinetic energy of the The maximum kinetic energy of the
photoelectrons increases with photoelectrons increases with increasing light frequencyincreasing light frequency
Electrons are emitted from the Electrons are emitted from the surface almost instantaneously, even surface almost instantaneously, even at low intensitiesat low intensities
Einstein’s ExplanationEinstein’s Explanation A tiny packet of light energy, called a A tiny packet of light energy, called a photonphoton, ,
would be emitted when a quantized oscillator would be emitted when a quantized oscillator jumped from one energy level to the next lower jumped from one energy level to the next lower oneone• Extended Planck’s idea of quantization to Extended Planck’s idea of quantization to
electromagnetic radiationelectromagnetic radiation The photon’s energy would be E = hThe photon’s energy would be E = hƒƒ Each photon can give all its energy to an electron Each photon can give all its energy to an electron
in the metalin the metal The maximum kinetic energy of the liberated The maximum kinetic energy of the liberated
photoelectron is KE = hphotoelectron is KE = hƒ – Eƒ – EWFWF EEWFWF is called the is called the work functionwork function of the metal of the metal
Explanation of Classical Explanation of Classical “Problems”“Problems”
The effect is not observed below a The effect is not observed below a certain cutoff frequency since the certain cutoff frequency since the photon energy must be greater than photon energy must be greater than or equal to the work functionor equal to the work function• Without this, electrons are not emitted, Without this, electrons are not emitted,
regardless of the intensity of the lightregardless of the intensity of the light The maximum KE depends only on The maximum KE depends only on
the frequency and the work function, the frequency and the work function, not on the intensitynot on the intensity
More ExplanationsMore Explanations The maximum KE increases with The maximum KE increases with
increasing frequencyincreasing frequency The effect is instantaneous since The effect is instantaneous since
there is a one-to-one interaction there is a one-to-one interaction between the photon and the electronbetween the photon and the electron
Verification of Einstein’s TheoryVerification of Einstein’s Theory Experimental Experimental
observations of a observations of a linear relationship linear relationship between KE and between KE and frequency confirm frequency confirm Einstein’s theoryEinstein’s theory
The x-intercept is The x-intercept is the cutoff the cutoff frequencyfrequency
Planck’s LawPlanck’s Law Light not emitted continuously, but in discrete Light not emitted continuously, but in discrete
chunks (photons)chunks (photons)• E = hf, where h = “Planck’s constant”E = hf, where h = “Planck’s constant”• h = 6.626 h = 6.626 10 10–34 –34 J-sec = 4.1 J-sec = 4.1 10 10–15–15 eV–sec eV–sec
Example: FM radioExample: FM radio• f = 100 MHz = 10f = 100 MHz = 1088 Hz Hz• E = hf = 4.1 E = hf = 4.1 10 10–15–15 10 1088 = 4.1 = 4.1 10 10-7-7 eV per photon eV per photon
Example: 600 nm lightExample: 600 nm light• f = c / f = c / = 3 = 3 10 1088 / 600 / 600 10 10-7-7 = 5 = 5 10 101414 Hz = 500 THz Hz = 500 THz• E = hf = 4.1 E = hf = 4.1 10 10–15–15 (5 (5 10 101414) = 2.1 eV per) = 2.1 eV per photon photon
Photon Theory of LightPhoton Theory of Light Chunks of light are called “photons”Chunks of light are called “photons” Light consists of streams of photonsLight consists of streams of photons
• Emitted discretely, using Planck law and E = hfEmitted discretely, using Planck law and E = hf• Propagates as photonsPropagates as photons• Each photon absorbed discretelyEach photon absorbed discretely
Photons have particle Photons have particle andand wave properties wave properties• Wave:Wave: Frequency, wavelengthFrequency, wavelength• Particle:Particle: Energy of single photon is E = hfEnergy of single photon is E = hf
ExamplesExamples Calculating photon energy from wavelengthCalculating photon energy from wavelength
Example: Visible light, Example: Visible light, = 600 nm = 600 nm• E = 1240 / 600 = 2.1 eVE = 1240 / 600 = 2.1 eV
Example: Ultraviolet, Example: Ultraviolet, = 100 nm = 100 nm• E = 1240 / 100 = 12.4 eVE = 1240 / 100 = 12.4 eV higher energy (cell damage) higher energy (cell damage)
Example: X-Ray, Example: X-Ray, = 1 nm = 1 nm• E = 1240 / 1 = 1240 eVE = 1240 / 1 = 1240 eV very high energy (radiation very high energy (radiation
damage)damage)
1240eVnm
hcE hf
E
Wave Properties of ParticlesWave Properties of Particles In 1924, Louis de Broglie postulated In 1924, Louis de Broglie postulated
that that because photons have wave and because photons have wave and particle characteristics, perhaps all particle characteristics, perhaps all forms of matter have both propertiesforms of matter have both properties
Furthermore, the frequency and Furthermore, the frequency and wavelength of matter waves can be wavelength of matter waves can be determineddetermined
de Broglie Wavelength and de Broglie Wavelength and FrequencyFrequency
The The de Broglie wavelengthde Broglie wavelength of a of a particle is particle is
The frequency of matter waves isThe frequency of matter waves isph
mvh
hEƒ
The Davisson-Germer The Davisson-Germer ExperimentExperiment
They scattered low-energy electrons from a They scattered low-energy electrons from a nickel targetnickel target
They followed this with extensive diffraction They followed this with extensive diffraction measurements from various materialsmeasurements from various materials
The wavelength of the electrons calculated The wavelength of the electrons calculated from the diffraction data agreed with the from the diffraction data agreed with the expected de Broglie wavelengthexpected de Broglie wavelength
This confirmed the wave nature of electronsThis confirmed the wave nature of electrons Other experimenters have confirmed the Other experimenters have confirmed the
wave nature of other particleswave nature of other particles
top related