Post on 02-Oct-2020
Eunil Won Dept. of Physics Korea Univ
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Ch16 Electromagnetic Radiation
Eunil Won Dept. of Physics Korea Univ
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InterferenceInterference pattern occur when waves combine
Eunil Won Dept. of Physics Korea Univ
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DiffractionDiffraction is the tendency of waves to bend around objects or spread out after going through an opening
Eunil Won Dept. of Physics Korea Univ
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Light as electromagnetic wave
Present understanding of all electromagnetic waves : very wide spectrum (Maxwell’s Rainbow)
Eunil Won Dept. of Physics Korea Univ
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Light as electromagnetic wave
Radio wave is transparent in the air transmission is possible
Eunil Won Dept. of Physics Korea Univ
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Example 16.1
A wavelength of 300 m corresponds to EM waves used for AM radio transmission. Calculate the frequency in kilohertz of AM radio wave.
kHz 1000Hz101.0300m
m/s103.0
6
8
=×=
×=
=λcf
Eunil Won Dept. of Physics Korea Univ
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The Photon, the Quantum of Light
In 1905, Einstein proposed: electromagnetic radiation is quantized and exists in elementary amounts (quanta) called photons
The quantum of a light wave of frequency f has energy: E = hf(energy of single photon)
h: Planck constant: h = 6.63 x 10-34 J s = 4.14 x 10-15 eV s
ex) A lamp with 100 W power (wavelength=590 nm). How many photons are emitted per second?
# of photons per second = power / hf = power x c / h x wavelength
Eunil Won Dept. of Physics Korea Univ
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Photoelectric EffectIf a beam of light is directed onto a clean metal surface, the light cause electrons to leave that surface difficult to explain if light has wave nature…
Eunil Won Dept. of Physics Korea Univ
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Photoelectric Effect
sJ1063.6 34 ⋅×=⇒= −hhfE
Eunil Won Dept. of Physics Korea Univ
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Photoelectric EffectMore serious experimental setup:
First photoelectric experiment
1) incident light causes current
2) apply potential difference V : collector C is slightly negatively charged
3) At certain V, there will be no currentV = Vstop (stopping potential)
Kmax : the kinetic energy of most energetic electrons
Kmax = eVstop
Kmax does not depend on the intensity of the
light source (inconsistent with wave nature)
Eunil Won Dept. of Physics Korea Univ
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Photoelectric Effect
Photoelectric effect does not occur below a certain cutoff frequency f0(cannot explain it with wave nature)
2nd Photoelectric Experiment: now we vary the frequency of the incident light and measure Vstop
(cutoff wavelength)
To just escape from the target, e-must pick up a certain energy (properties of the target material: work function)
Einstein summed up the photoelectric experiments as:
(photoelectric equation)
explains the above plot
Eunil Won Dept. of Physics Korea Univ
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Compton Scattering
Part of the photon momentum is delivered to the electron?
Scattered x rays show the change in wavelength (difficult to explain if x rays have wave nature)
Eunil Won Dept. of Physics Korea Univ
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In 1916, Einstein extended his concept of light quanta: a quantum of light has linear momentum
(photon momentum)
Compton Scattering
Scattered x rays showed a shift in wavelength (Compton shift): a fraction of momentum is transfered
Eunil Won Dept. of Physics Korea Univ
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Light as Probability WaveLight can be a wave in classical physicsA fundamental
mystery: It is emitted and and absorbed as photons (in quantum physics)
How can particles make interference patterns?
Single-photon version
: A single-photon version of double-slit experiment (one photon at a time) -> Astonishingly interference fringes still build up,supporting the probability wave nature
Eunil Won Dept. of Physics Korea Univ
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Matter WaveMatter can behave as wave?
In 1924, Louis de Broglie suggested matter waves( A moving matter has wavelength)
ex) K=120 eV electron
ex) Me running v=1m/sX-ray and electron diffraction
Eunil Won Dept. of Physics Korea Univ
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Example 17.2
m101.85m/s)(5.0kg)(7.
sJ106.63:ball Bowling 3534
−−
×=⋅×==mvhλ
m101.2m/s)10kg)(5.010(9.11
sJ106.63 :Electron 10631
34−
−
−
×=××⋅×==
mvhλ
Eunil Won Dept. of Physics Korea Univ
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Heisenberg’s Uncertainty PrincipleThe position and the momentum of a particle cannot be measured simultaneously with unlimited precision
Do not think that the particle really has a sharply defined position: I’m sure you are confused by now :-)
Eunil Won Dept. of Physics Korea Univ
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Barrier Tunnelingelectron with energy E moving toward to a potential barrier (U0) when E<U0
classical physics: the electron is bounced off all the time
quantum physics: in some cases the electron penetrates the barrier
Transmission coefficient : the probability of tunneling of the electron(If T=0.020, 20 out of 1000 electrons will tunnel through)
Eunil Won Dept. of Physics Korea Univ
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Crystalline quartz changes its dimension when an electric potential is applied (piezoelectricity): tip can be moved precisely
Electrons from the sample can tunnel through to the tip : tunnel current can be measured and used as a microscope (STM)
The Scanning Tunneling Microscope (STM)