Physics 102: Lecture 22, Slide 1 Blackbody Radiation Photoelectric Effect Wave-Particle Duality...

Physics 102: Lecture 22, Slide 1

Blackbody RadiationPhotoelectric Effect

Wave-Particle Duality

• Today’s Lecture will cover textbook sections 27.2 - 27.3, 28.1

Physics 102: Lecture 22


Everything comes unglued

The predictions of “classical physics” (Newton’s laws and Maxwell’s equations) are sometimes completely, utterly 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!


The scale of the problem

Let’s say we know an object’s position to an accuracy x.

How much does this mess up our ability to know its speed?

Here’s the connection between x and v (p = mv):

That’s the “Heisenberg uncertainty principle.” h 6.610-34 J·s

4

hv

m x


Atomic scale effects

Small x means large v since4

hv

m x

Example: an electron (m = 9.110-31 kg) in an atom is confined to a region of size x ~ 510-11 m.

How fast will the electron tend to be moving?

Plug in, using h = 6.610-34 to find

v > 1.1106 m/sec


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.


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


Blackbody Radiation Spectrum

Visible Light: ~0.4m to 0.7m

Higher temperature: peak intensity at shorter


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/

Planck’s constant: h = 6.626 X 10-34 Joule sec


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

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

Blue! Lowest wavelength is highest energy.

E = hf = hc/Red! Highest wavelength is lowest energy.

Hotter stove emits higher-energy photons

(shorter wavelength = orange)


ACT: Colored LightThree 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) Red2) Green3) Blue4) Same

max


ACT: Colored Light

Which bulb’s 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.

max

Visible Light


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


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!

# photons Energy/second

second Energy/photon

Power

Energy/photon Power

hf


Wien’s Displacement Law

• To calculate the peak wavelength produced at any particular temperature, use Wien’s Displacement Law:

T · peak = 0.2898*10-2 m·K

temperature in Kelvin!


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.


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.


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 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

http://web.hep.uiuc.edu/home/tstelzer/102project/pe.htm


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 – K.E. = hf – W0


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?


Photoelectric Effect: Light Intensity

• What happens to the rate electrons are emitted when increase the brightness?– more photons/sec so more electrons are emitted. Rate

goes up.

• What happens to max kinetic energy when increase brightness?– no change: each photon carries the same energy as long

as we don’t change the color of the light


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: Light Frequency

• What happens to rate electrons are emitted when increase the frequency of the light?– as long the number of photons/sec doesn’t change, the

rate won’t change.

• What happens to max kinetic energy when increase the frequency of the light?– each photon carries more energy, so each electron

receives more energy.


Preflights 22.4, 22.6Which 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?

32

1


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 !


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

Screen a distance L from slits

Source of monoenergetic electrons

d

2 slits-separated by d

L


Young’s Double Slit w/ electron

• JAVA

Screen a distance L from slits

Source of monoenergetic electrons

d

2 slits-separated by d

L

http://hal.grasslands.ab.ca/interference/Interfer.htm


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?


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


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!


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.

I’m not kidding it’s true!


Schroedinger’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

HereKitty, Kitty!


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!


Quantum Summary

• Particles act as waves and waves act as particles

• Physics is NOT deterministic

• Observations affect the experiment– (coming soon!)


See you next lecture!

• Read Textbook Sections 27.5, 28.2, and 28.4

Physics 102: Lecture 22, Slide 1 Blackbody Radiation Photoelectric Effect Wave-Particle Duality...

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