Physics 102: Lecture 22, Slide 1

Quantum Mechanics:Blackbody Radiation, Photoelectric Effect,

Wave-Particle Duality

Physics 102: Lecture 22

Physics 102: Lecture 22, Slide 2

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

Lecture 28

Lectures 22-25

“Classical physics”

Physics 102: Lecture 22, Slide 3

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.

Physics 102: Lecture 22, Slide 4

Three Early Indications of Problems with Classical Physics

• Blackbody radiation• Photoelectric effect• Wave-particle duality

Physics 102: Lecture 22, Slide 5

Hot objects glow (toaster coils, light bulbs, the sun).

As the temperature increases the color shifts from Red (700 nm) to Blue (400 nm)

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

Physics 102: Lecture 22, Slide 6

Blackbody Radiation Spectrum

Visible Light: ~0.4μm to 0.7μm

Higher temperature: peak intensity at shorter λ

Wien’s Displacement Law:λmaxT = 2.898x10-3 m·K

Physics 102: Lecture 22, Slide 7

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/λPlanck’s constant: h = 6.626 x 10-34 Joule sec

Physics 102: Lecture 22, Slide 8

Preflights 22.1, 22.3A 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

Physics 102: Lecture 22, Slide 9

ACT: Nobel Trivia

For which work did Einstein receive the Nobel Prize?

1) Special Relativity E=mc2

2) General Relativity Gravity bends Light3) Photoelectric Effect Photons4) Einstein didn’t receive a Nobel prize.

Physics 102: Lecture 22, Slide 10

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.)

metal

light e–

Physics 102: Lecture 22, Slide 11

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?

Rate increases

Nothing

metal

lighte–

Physics 102: Lecture 22, Slide 12

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?

Increases

Nothing, but goes to 0 for f < fmin

metal

e–

lightNo e–

e–

Physics 102: Lecture 22, Slide 13

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 electronsK.E. = hf – W0

hf

W0

KE

e–

Physics 102: Lecture 22, Slide 14

ACT: PhotonA red and green laser are each rated at 2.5mW.

Which one produces more photons/second?

1) Red 2) Green 3) Same

Physics 102: Lecture 22, Slide 15

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 (blackbody radiation)– Collision with electrons in photo-electric effect

BOTH Particle AND Wave

Physics 102: Lecture 22, Slide 16

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!

Physics 102: Lecture 22, Slide 17

Young’s Double Slit w/ electron

Screen a distance L from slits

Source of monoenergetic electrons

d

2 slits-separated by d

L

Jönsson – 1961

Physics 102: Lecture 22, Slide 18

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?

Physics 102: Lecture 22, Slide 19

Young’s Double Slit w/ electron

Source of monoenergetic electrons

d

L

Merli – 1974Tonomura – 1989

Same pattern for photons

One electron at a time

Interference pattern = probability

Physics 102: Lecture 22, Slide 20

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

Physics 102: Lecture 22, Slide 21

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!

Physics 102: Lecture 22, Slide 22

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

Physics 102: Lecture 22, Slide 23

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!

Physics 102: Lecture 22, Slide 24

Quantum Summary

• Particles act as waves and waves act as particles

• Physics is NOT deterministic

• Observations affect the experiment