Blackbody Radiation, Photoelectric Effect, WaveParticle Duality

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Blackbody Radiation, Photoelectric Effect,
Wave-Particle Duality
Physics 102 Lecture 22
Demo
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Hour Exam 3

• Monday, April 14
• Covers
• lectures through Lecture 20 (last Mondays
  lecture)
• homework through HW 10
• discussions through Disc 10
• Review, Sunday April 13, 3 PM, 141 LLP

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These objects are just resolved
Two objects are just resolved when the maximum of
one is at the minimum of the other.
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Resolving Power
To see two objects distinctly, need qobjects gt
qmin
qobjects
qobjects is angle between objects and aperture
qmin
qmin is minimum angular separation that aperture
can resolve
D
sin qmin qmin 1.22 l/D
y
d
Improve resolution by increasing qobjects or
decreasing qmin
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Resolving Power
How does the maximum resolving power of your eye
change when the brightness of the room is
decreased. 1) Increases 2) Constant 3)
Decreases
When the light is low, your pupil dilates (D can
increase by factor of 10!) But actual limitation
is due to density of rods and cones, so you dont
notice an effect!
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Recap.

• Interference Coherent waves
• Full wavelength difference Constructive
• ½ wavelength difference Destructive
• Multiple Slits
• Constructive d sin(q) m l (m1,2,3)
• Destructive d sin(q) (m 1/2) l 2 slit only
• More slits brighter max, darker mins
• Huygens Principle Each point on wave front acts
  as coherent source and can interfere.
• Single Slit
• Destructive w sin(q) m l (m1,2,3)
• Resolution Max from 1 at Min from 2

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Everything comes unglued

• The predictions of classical physics (Newtons
  laws and Maxwells equations) are sometimes
  completely, utterly WRONG.
• classical physics says that an atoms 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,

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The source of the problem

• Its 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!

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The scale of the problem

• Lets say we know an objects position to an
  accuracy Dx.
• How much does this mess up our ability to know
  its speed?
• Heres the connection between Dx and Dv (Dp
  mDv)
• Thats the Heisenberg uncertainty principle. h
  ? 6.6?10-34 Js

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Atomic scale effects

• Small Dx means large Dv since

Example an electron (m 9.1?10-31 kg) in an
atom is confined to a region of size Dx 5?10-11
m. How fast will the electron tend to be
moving? Plug in, using h 6.6?10-34 to find
Dv gt 1.1?106 m/sec
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Quantum Mechanics!

• At very small sizes the world is VERY different!
• Energy is discrete, not continuous.
• Everything is probability nothing is for
  certain.
• Particles often seem to be in two places at same
  time.
• Looking at something changes how it behaves.
• If you arent confused by the end of this
  lecture, you werent paying attention!

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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.)
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Blackbody Radiation Spectrum
Higher temperature peak intensity at shorter l
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Blackbody RadiationFirst 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 Plancks
constant h 6.626 X 10-34 Joule sec
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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?
Blue! Lowest wavelength is highest
energy. E hf hc/l
80 correct!
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 (lower
wavelength orange)
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ACT Photon

• 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!
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Nobel Trivia

• For which work did Einstein receive the Nobel
  Prize?
• 1) Special Relativity Emc2
• 2) General Relativity Gravity bends Light
• 3) Photoelectric Effect Photons
• 4) Einstein didnt 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.)

Demo
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Photoelectric Effect Light Intensity

• What happens to the rate electrons are emitted
  when increase the brightness?
• What happens to max kinetic energy when increase
  brightness?

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Photoelectric Effect Light Frequency

• What happens to rate electrons are emitted when
  increase the frequency of the light?
• What happens to max kinetic energy when increase
  the frequency of the light?

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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 h6.626 X 10-34 Joule sec
• Maximum kinetic energy of released electrons
• K.E. hf W0

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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

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Are Electrons Particles or Waves?

• 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!
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Youngs Double Slit w/ electron

• JAVA

Source of monoenergetic electrons
L
Screen a distance L from slits
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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?

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ACT Electrons are Particles

• If we shine a bright light, we can see which
  hole the electron goes through.
• (1) Both Slits (2) Only 1 Slit

But now the interference is gone!
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Electrons are Particles and Waves!

• Depending on the experiment electron can behave
  like
• wave (interference)
• particle (localized mass and charge)
• If we dont look, electron goes through both
  slits. If we do look it chooses 1.

Im not kidding its true!
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Schroedingers Cat

• Place cat in box with some poison. If we dont
  look at the cat it will be both dead and alive!

Here Kitty, Kitty!
Poison
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More Nobel Prizes!

• 1906 J.J. Thompson
• Showing cathode rays are particles (electrons).
• 1937 G.P. Thompson (JJs son)
• Showed electrons are really waves.
• Both were right!

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Quantum Summary

• Particles act as waves and waves act as particles
• Physics is NOT deterministic
• Observations affect the experiment
• (coming soon!)

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See you Wednesday!