# Quantum Mechanics Blackbody Radiation, Photoelectric Effect by mps12334

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```									Physics 102: Lecture 22

Quantum Mechanics:
Photoelectric Effect,
Wave-Particle Duality
Hour Exam 3
• Monday, Nov 16
• Covers
–   Lectures through Lecture 21
–   Homework through HW 10
–   Discussions through Disc 10
–   Labs through Lab 8

• No review session 
Recap.
• Interference: Coherent waves
– Full wavelength difference = Constructive
– ½ wavelength difference = Destructive
• Multiple Slits
– Constructive d sin(q) = m l      (m=1,2,3…)
– 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. (see Lab 8)
• Single Slit:
– Destructive: w sin(q) = m l      (m=1,2,3…)
– Resolution: Max from 1 at Min from 2                50
State of Late 19th Century Physics
• Two great theories
– Newton‟s laws of mechanics, including gravity
– Maxwell‟s theory of electricity & magnetism,
including propagation of electromagnetic waves
• But…some unsettling experimental results
calls into question these theories
– Einstein and relativity
– The quantum revolution
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.

5
Three Early Indications of Problems
with Classical Physics
• Photoelectric effect
• Wave-particle duality
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.)
Visible Light: ~0.4mm to
0.7mm

Higher temperature: peak intensity at shorter l
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?
Blue! Lowest wavelength is highest energy.
E = hf = hc/l
The least energy?
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)
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

# photons Energy/second     Power        Power
                            
second    Energy/photon Energy/photon    hf

Red light has less energy/photon so if they
both have the same total energy, red has to
have more photons!
33
Nobel Trivia
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 didn‟t receive a Nobel prize.

12
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.)

25
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?

Demo
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?
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=hf              h=6.626 X 10-34 Joule sec

• Maximum kinetic energy of released electrons
– K.E. = hf – W0

30
Quantum Physics and the Wave-
Particle Duality
I. 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
II. 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!
Young‟s Double Slit w/ electron

• JAVA

d

Source of
monoenergetic
L
electrons
2 slits-
separated       Screen a distance
by d            L from slits        41
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?

43
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!
45
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.

I’m not kidding it’s true!                   46
Schrödinger's Cat
• Place cat in box with some poison. If we
don‟t look at the cat it will be both dead and
alive!

Poison

46
More Nobel Prizes!
• 1906 J.J. Thompson
– Showing cathode rays are particles (electrons).
• 1937 G.P. Thompson (JJ‟s son)
– Showed electrons are really waves.
• Both were right!

47
Quantum Summary
• Particles act as waves and waves act as
particles

• Physics is NOT deterministic

• Observations affect the experiment
– (coming soon!)

49
See you Wednesday!

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