Download Lecture 16 - Eunil Won

Eunil Won Dept. of Physics Korea Univ
Ch16 Electromagnetic Radiation
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Eunil Won Dept. of Physics Korea Univ
Interference
Interference pattern occur when waves combine
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Eunil Won Dept. of Physics Korea Univ
Diffraction
Diffraction is the tendency of waves to bend around
objects or spread out after going through an opening
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Eunil Won Dept. of Physics Korea Univ
Light as electromagnetic wave
Present understanding of all electromagnetic waves :
very wide spectrum (Maxwell’s Rainbow)
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Eunil Won Dept. of Physics Korea Univ
Light as electromagnetic wave
Radio wave is
transparent in
the air Æ
transmission is
possible
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Eunil Won Dept. of Physics Korea Univ
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.
f =
c
λ
3.0 ×108 m/s
=
300m
= 1.0 ×106 Hz = 1000 kHz
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Eunil Won Dept. of Physics Korea Univ
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
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Eunil Won Dept. of Physics Korea Univ
Photoelectric Effect
If a beam of light is directed onto a clean metal surface, the light cause
electrons to leave that surface Æ difficult to explain if light has
wave nature…
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Eunil Won Dept. of Physics Korea Univ
Photoelectric Effect
E = hf ⇒ h = 6.63 ×10
−34
J ⋅s
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Eunil Won Dept. of Physics Korea Univ
Photoelectric Effect
More 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 current
V=V
(stopping potential)
stop
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)
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Eunil Won Dept. of Physics Korea Univ
Photoelectric Effect
2nd Photoelectric Experiment: now we vary the frequency of the incident light
and measure Vstop
Photoelectric effect does not occur
below a certain cutoff frequency f0
(cannot explain it with wave nature)
(cutoff wavelength)
To just escape from the target, emust 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
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Eunil Won Dept. of Physics Korea Univ
Compton Scattering
Scattered x rays show the change in wavelength (difficult
to explain if x rays have wave nature)
Part of the photon momentum
is delivered to the electron?
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Eunil Won Dept. of Physics Korea Univ
Compton Scattering
In 1916, Einstein extended his concept of light quanta: a quantum of light
has linear momentum
(photon momentum)
Scattered x rays showed a shift in
wavelength (Compton shift)
: a fraction of momentum is transfered
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Eunil Won Dept. of Physics Korea Univ
Light as Probability Wave
A fundamental
mystery:
Light can be a wave in classical physics
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
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Eunil Won Dept. of Physics Korea Univ
Matter Wave
Matter 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/s
X-ray and electron diffraction
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Eunil Won Dept. of Physics Korea Univ
Example 17.2
h
6.63 ×10 −34 J ⋅ s
Bowling ball : λ =
=
= 1.8 ×10 −35 m
mv (5.0kg)(7.5m/s)
6.63 ×10 −34 J ⋅ s
h
−10
=
1.2
×
10
m
Electron : λ =
=
−31
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mv (9.11×10 kg)(5.0 ×10 m/s)
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Eunil Won Dept. of Physics Korea Univ
Heisenberg’s Uncertainty Principle
The 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 :-)
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Eunil Won Dept. of Physics Korea Univ
Barrier Tunneling
electron 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)
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Eunil Won Dept. of Physics Korea Univ
The Scanning Tunneling Microscope (STM)
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)
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