Download Modern Physics 2-Quantum Optics

3/10/2015
Black body radiation
• A black body absorbs all incident light
and converts the light's energy into
thermal energy (no light is reflected).
Modern Physics 2
Quantum Optics-ch26
• The black body then radiates
electromagnetic waves based solely on
its temperature.
Physics 116
Eyres
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What characterizes a black body?
• We can model a black body as
the surface of a small hole in the
side of a closed container.
• The graph is called a spectral
curve.
Studies of black body radiation
• If we raise the temperature of the box
and again measure the spectrum of the
EM radiation being emitted from the
hole, we find that:
• The total power output from the hole is
now greater.
• The spectral curve rises at all wavelengths.
• The peak of the power per small
wavelength interval shifts to a shorter
wavelength.
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The color
of stars
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• Very hot stars (around 10,000 K) look blue.
• Even hotter stars are generally invisible to
our eyes.
Planck's hypothesis
• Planck hypothesized that charged particles could radiate
energy only in discrete portions called quanta.
• He proposed that each microscopic oscillating charged particle
had some kind of fundamental portion of energy, which was
proportional to the frequency of its oscillation.
• According to Planck, the particle could emit amounts of energy
equal only to multiples of this fundamental portion.
More Blue …… more Red
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Work function
Photoelectric Effect
• The work function is the minimum energy needed to remove a free
electron from a metal.
• The work function has units of energy but is measured in electron volts
because it is typically very small.
• The greater the work function of a metal, the more tightly the free electrons
are bound to the metal and the more energy must be added to separate
them.
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
Photoelectric Effect
Photoelectric Effect
Freq (x 1014 Hz)
KE (x 10-19 J)
KE vs f photoelectric effect
wavelength (nm) Volts (V)
2.11
0.94
6.32
1.50
G
1.87
0.73
G
5.61
1.17
Y
1.72
0.64
Y
5.17
1.02
O
1.67
0.52
O
5.00
0.83
R
1.43
0.34
R
4.29
0.54
UV max
2.30
1.52
UV max
6.90
2.43
UV min
2.73
1.52
UV min
8.20
2.43
Quantum model explanation of the
photoelectric effect
• According to Planck, the energy of a quantum of EM radiation is
proportional to its frequency.
• Using this idea, Einstein developed the following equation:
K = (0.542 x10-33 Js)f - 1.7917 J
3.00
2.50
KE (x10^-19 J)
B
B
Value of Plank’s constant is:
6.63 X 10-34 Js
How do these results
compare to theory?
2.00
1.50
1.00
0.50
0.00
0.00
2.00
4.00
6.00
8.00
10.00
freq (x10^14 Hz)
Cutoff Frequency and Work Function
We can express the cutoff
frequency in terms of the work
function of the metal and
Planck's constant:
If the energy of one quantum is
less than the work function of
the metal, and no photocurrent
is produced.
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Checking Understanding
Example 26.6
• Light and UV radiation shine on
three different metals. Graph lines
representing the stopping potential
versus the incident light or UV
frequency are shown below. The
work functions of the metals are
sodium (Na), 2.3 eV; iron (Fe), 4.7
eV; and platinum (Pt), 6.4 eV. Use
Einstein's hypothesis of light
absorption to explain the results.
In the photoelectric effect experiment, why does red light not cause
the emission of an electron though blue light can?
A. The photons of red light don’t have sufficient energy to eject an
electron.
B. The electric field of the red light oscillates too slowly to eject an
electron.
C. Red light contains fewer photons than blue, not enough to eject
electrons.
D. The red light doesn’t penetrate far enough into the metal
electrode.
Threshold for Tissue Damage
Checking Understanding
In the photoelectric effect experiment, as we increase the
frequency of the light what change do we see?
A. There are more electrons emitted.
B. The emitted electrons are faster.
C. Both A and B.
D. Neither A nor B.
Wave-like and particle-like properties of
photons
Exposure to a sufficient quantity of ultraviolet will redden the skin,
producing erythema—a sunburn. The amount of exposure
necessary to produce this reddening depends on the wavelength.
For a 1.0 cm2 patch of skin, 3.7 mJ of ultraviolet of 254 nm will
produced reddening; at 300 nm, 13 mJ are required.
A. What is the photon energy corresponding to each of these
wavelengths?
B. How many total photons do each of these exposures
correspond to?
C. Explain why there is a difference in the number of photons
needed to provoke a response in the two cases.
Photon momentum
• Photons participate in collisions with electrons inside metals (the
photoelectric effect).
• This suggests that photons must have momentum.
• In 1922, Arthur Compton performed a testing experiment to determine
whether this expression was reasonable.
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The Compton effect and X-ray interference
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