Blackbody Radiation Photoelectric Effect Wave-Particle Duality

1
Blackbody RadiationPhotoelectric
EffectWave-Particle Duality
Physics 1161 Lecture 22

• sections 30-1 30-4

2
Everything comes unglued

• The predictions of classical physics (Newtons
  laws and Maxwells equations) are sometimes
  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,

3
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!

4
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.

5
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
7
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/? Plancks
constant h 6.626 X 10-34 Joule sec
8
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
9
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
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)
10
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 bulbs
filament is hottest?

1. Red
2. Green
3. Blue
4. Same

11
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 bulbs
filament is hottest?

1. Red
2. Green
3. Blue
4. Same

Colored bulbs are identical on the inside the
glass is tinted to absorb all of the light,
except the color you see.
12
A red and green laser are each rated at 2.5mW.
Which one produces more photons/second?

1. Red
2. Green
3. Same

13
A red and green laser are each rated at 2.5mW.
Which one produces more photons/second?
Red light has less energy/photon so if they both
have the same total energy, red has to have more
photons!

1. Red
2. Green
3. Same

14
Wiens Displacement Law

• To calculate the peak wavelength produced at any
  particular temperature, use Wiens Displacement
  Law

T ?peak 0.289810-2 mK
temperature in Kelvin!
15
For which work did Einstein receive the Nobel
Prize?

1. Special Relativity E mc2
2. General Relativity Gravity bends Light
3. Photoelectric Effect Photons
4. Einstein didnt receive a Nobel prize.

16
For which work did Einstein receive the Nobel
Prize?

1. Special Relativity E mc2
2. General Relativity Gravity bends Light
3. Photoelectric Effect Photons
4. Einstein didnt receive a Nobel prize.

17
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.)

18
Photoelectric Effect
19
Light Intensity

• Kinetic energy of ejected electrons is
  independent of light intensity
• Number of electrons ejected does depend on light
  intensity

20
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

21
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 lights frequency

22
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
• hf KE W0

23
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

24
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

25
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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Photoelectric summary table

• Wave Particle Result
• Increase Intensity
• Rate Increase Increase Increase
• KE Increase Unchanged Unchanged
• Increase Frequency
• Rate Unchanged Increase Increase
• KE Unchanged Increase Increase

Light is composed of particles photons
27
Preflights 22.4, 22.6
Which drawing of the atom is more correct?
This is a drawing of an electrons p-orbital
probability distribution. At which location is
the electron most likely to exist?
1
3
2
28
Preflights 22.4, 22.6
Which drawing of the atom is more correct?
This is a drawing of an electrons p-orbital
probability distribution. At which location is
the electron most likely to exist?
1
3
2
29
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 !

30
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.
31
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!
32
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.

33
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

34
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.

35
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?

36
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.

37
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

38
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

39
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?

40
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.

41
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 of them.

42
Quantum Summary

• Particles act as waves and waves act as particles
• Physics is NOT deterministic
• Observations affect the experiment