Transcript Physics 1161: Lecture 22 Blackbody Radiation Photoelectric Effect

Physics 1161: Lecture 22
Blackbody Radiation
Photoelectric Effect
Wave-Particle Duality
• sections 30-1 – 30-4
Everything comes unglued
The predictions of “classical physics” (Newton’s laws
and Maxwell’s equations) are sometimes WRONG.
– classical physics says that an atom’s electrons should fall into
the nucleus and STAY THERE. No chemistry, no biology can
happen.
– classical physics says that toaster coils radiate an infinite
amount of energy: radio waves, visible light, X-rays, gamma
rays,…
The source of the problem
It’s not possible, even “in theory” to know
everything about a physical system.
– knowing the approximate position of a particle corrupts our
ability to know its precise velocity (“Heisenberg uncertainty
principle”)
Particles exhibit wave-like properties.
– interference effects!
Quantum Mechanics!
• At very small sizes the world is VERY different!
– Energy can come in discrete packets
– Everything is probability; very little is absolutely
certain.
– Particles can seem to be in two places at same time.
– Looking at something changes how it behaves.
Blackbody Radiation
Hot objects glow (toaster coils, light bulbs, the sun).
As the temperature increases the color shifts from
Red to Blue.
The classical physics prediction was completely
wrong! (It said that an infinite amount of energy
should be radiated by an object at finite temperature.)
Blackbody Radiation Spectrum
Visible Light: ~0.4mm to 0.7mm
Higher temperature: peak intensity at shorter l
Blackbody Radiation:
First evidence for Q.M.
Max Planck found he could explain these curves if he
assumed that electromagnetic energy was radiated in
discrete chunks, rather than continuously.
The “quanta” of electromagnetic energy is called the
photon.
Energy carried by a single photon is
E = hf = hc/l
Planck’s constant: h = 6.626 X 10-34 Joule sec
Preflights 22.1, 22.3
A series of light bulbs are colored red, yellow, and blue.
Which bulb emits photons with the most energy?
The least energy?
Which is hotter?
(1) stove burner glowing red
(2) stove burner glowing orange
Preflights 22.1, 22.3
A series of light bulbs are colored red, yellow, and blue.
Which bulb emits photons with the most energy?
Blue! Lowest wavelength is highest energy.
The least energy?
E = hf = hc/l
Red! Highest wavelength is lowest energy.
Which is hotter?
(1) stove burner glowing red
(2) stove burner glowing orange
Hotter stove emits higher-energy photons
(shorter wavelength = orange)
Three light bulbs with identical filaments
are manufactured with different colored
glass envelopes: one is red, one is green,
one is blue. When the bulbs are turned on,
which bulb’s filament is hottest?
1.
2.
3.
4.
Red
Green
Blue
Same
lmax
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Three light bulbs with identical filaments
are manufactured with different colored
glass envelopes: one is red, one is green,
one is blue. When the bulbs are turned on,
which bulb’s filament is hottest?
1.
2.
3.
4.
Red
Green
Blue
Same
lmax
Colored bulbs are identical on the inside
– the glass is tinted to absorb all of the
light, except the color you see.
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A red and green laser are each rated at
2.5mW. Which one produces more
photons/second?
1. Red
2. Green
3. Same
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A red and green laser are each rated at
2.5mW. Which one produces more
photons/second?
1. Red
2. Green
3. Same
Red light has less
energy/photon so if they
both have the same total
energy, red has to have
more photons!
Power
# photons Energy/second
Power
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

second
Energy/photon Energy/photon
hf
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Wien’s Displacement Law
• To calculate the peak wavelength produced
at any particular temperature, use Wien’s
Displacement Law:
T · lpeak = 0.2898*10-2 m·K
temperature in Kelvin!
For which work did
Einstein receive the Nobel
Prize?
1.
2.
3.
4.
Special Relativity E = mc2
General Relativity Gravity bends Light
Photoelectric Effect Photons
Einstein didn’t receive a Nobel prize.
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For which work did
Einstein receive the Nobel
Prize?
1.
2.
3.
4.
Special Relativity E = mc2
General Relativity Gravity bends Light
Photoelectric Effect Photons
Einstein didn’t receive a Nobel prize.
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Photoelectric Effect
• Light shining on a metal can “knock” electrons
out of atoms.
• Light must provide energy to overcome
Coulomb attraction of electron to nucleus
• Light Intensity gives power/area (i.e. Watts/m2)
– Recall: Power = Energy/time (i.e. Joules/sec.)
Photoelectric Effect
Light Intensity
• Kinetic energy of ejected
electrons is independent of
light intensity
• Number of electrons ejected
does depend on light intensity
Threshold Frequency
• Glass is not transparent to
ultraviolet light
• Light in visible region is lower
frequency than ultraviolet
• There is minimum frequency
necessary to eject electrons
Difficulties With Wave Explanation
• effect easy to observe with violet or ultraviolet
(high frequency) light but not with red (low
frequency) light
• rate at which electrons ejected proportional to
brightness of light
• The maximum energy of ejected electrons NOT
affected by brightness of light
• electron's energy depends on light’s frequency
Photoelectric Effect Summary
• Each metal has “Work Function” (W0) which
is the minimum energy needed to free
electron from atom.
• Light comes in packets called Photons
E = h f
h=6.626 X 10-34 Joule sec
• Maximum kinetic energy of released electrons
hf = KE + W0
If hf for the light incident on a metal is
equal to the work function, what will the
kinetic energy of the ejected electron be?
1. the kinetic energy would
be negative
2. the kinetic energy would
be zero
3. the kinetic energy would
be positive
4. no electrons would be
released from the metal
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If hf for the light incident on a metal is less
than the work function, what will the
kinetic energy of the ejected electron be?
1. the kinetic energy would
be negative
2. the kinetic energy would
be zero
3. the kinetic energy would
be positive
4. no electrons would be
released from the metal
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If hf for the light incident on a metal is less
than the work function, what will the
kinetic energy of the ejected electron be?
1. the kinetic energy would
be negative
2. the kinetic energy would
be zero
3. the kinetic energy would
be positive
4. no electrons would be
released from the metal
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Preflights 22.4, 22.6
Which drawing of the atom is more correct?
This is a drawing of an electron’s p-orbital probability distribution. At which location is
the electron most likely to exist?
1
2
3
Preflights 22.4, 22.6
Which drawing of the atom is more correct?
This is a drawing of an electron’s p-orbital probability distribution. At which location is
the electron most likely to exist?
1
2
3
Is Light a Wave or a Particle?
• Wave
– Electric and Magnetic fields act like waves
– Superposition, Interference and Diffraction
• Particle
– Photons
– Collision with electrons in photo-electric effect
Both Particle and Wave !
The approximate numbers of photons at each stage are
(a) 3 × 103, (b) 1.2 × 104, (c) 9.3 × 104, (d) 7.6 × 105, (e) 3.6 × 106, and (f) 2.8 × 107.
Are Electrons Particles or Waves?
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Particles, definitely particles.
You can “see them”.
You can “bounce” things off them.
You can put them on an electroscope.
• How would know if electron was a wave?
Look for interference!
Interference Pattern Develops
• Stages of two-slit interference pattern.
• The pattern of individually exposed grains progresses
from (a) 28 photons to (b) 1000 photons to (c) 10,000
photons.
• As more photons hit the screen, a pattern of
interference fringes appears.
Single Slit Diffraction
• If we cover one slit so that photons hitting the
photographic film can only pass through a
single slit, the tiny spots on the film
accumulate to form a single-slit diffraction
pattern
How Do They “Know”
• photons hit the film at places they would not
hit if both slits were open!
• If we think about this classically, we are
perplexed and may ask how photons passing
through the single slit “know” that the other
slit is covered and therefore fan out to
produce the wide single-slit diffraction
pattern.
How Do They “Know?”
• Or, if both slits are open, how do photons
traveling through one slit “know” that the
other slit is open and avoid certain regions,
proceeding only to areas that will ultimately
fill to form the fringed double-slit interference
pattern?
Modern Answer
• modern answer is that the wave nature of
light is not some average property that shows
up only when many photons act together
•
• Each single photon has wave as well as
particle properties. But the photon displays
different aspects at different times.
Wavicle?
• photon behaves as a particle when it is being
emitted by an atom or absorbed by
photographic film or other detectors
• photon behaves as a wave in traveling from a
source to the place where it is detected
• photon strikes the film as a particle but travels
to its position as a wave that interferes
constructively
Electrons?
• fact that light exhibits both wave and particle
behavior was one of the interesting surprises
of the early twentieth century.
• even more surprising was the discovery that
objects with mass also exhibit a dual
waveparticle behavior
Electrons are Waves?
• Electrons produce interference pattern
just like light waves.
– Need electrons to go through both slits.
– What if we send 1 electron at a time?
– Does a single electron go through both
slits?
Electrons are Particles and Waves!
• Depending on the experiment electron
can behave like
– wave (interference)
– particle (localized mass and charge)
• If we don’t look, electron goes through
both slits. If we do look it chooses 1.
Electrons are Particles and Waves!
• Depending on the experiment
electron can behave like
– wave (interference)
– particle (localized mass and
charge)
• If we don’t look, electron goes
through both slits. If we do look
it chooses 1 of them.
Quantum Summary
• Particles act as waves and waves act
as particles
• Physics is NOT deterministic
• Observations affect the experiment