Title: Blackbody Radiation Photoelectric Effect Wave-Particle Duality
1Blackbody RadiationPhotoelectric
EffectWave-Particle Duality
Physics 1161 Lecture 22
2Everything 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,
3The 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!
4Quantum 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.
5Blackbody 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.)
6Blackbody Radiation Spectrum
Higher temperature peak intensity at shorter l
7Blackbody 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
8Preflights 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
9Preflights 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)
10Three 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?
- Red
- Green
- Blue
- Same
11Three 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?
- Red
- Green
- Blue
- Same
Colored bulbs are identical on the inside the
glass is tinted to absorb all of the light,
except the color you see.
12A red and green laser are each rated at 2.5mW.
Which one produces more photons/second?
- Red
- Green
- Same
13A 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!
- Red
- Green
- Same
14Wiens 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!
15For which work did Einstein receive the Nobel
Prize?
- Special Relativity E mc2
- General Relativity Gravity bends Light
- Photoelectric Effect Photons
- Einstein didnt receive a Nobel prize.
16For which work did Einstein receive the Nobel
Prize?
- Special Relativity E mc2
- General Relativity Gravity bends Light
- Photoelectric Effect Photons
- Einstein didnt receive a Nobel prize.
17Photoelectric 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.)
18Photoelectric Effect
19Light Intensity
- Kinetic energy of ejected electrons is
independent of light intensity - Number of electrons ejected does depend on light
intensity
20Threshold 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 -
21Difficulties 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
22Photoelectric 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
23If hf for the light incident on a metal is equal
to the work function, what will the kinetic
energy of the ejected electron be?
- the kinetic energy would be negative
- the kinetic energy would be zero
- the kinetic energy would be positive
- no electrons would be released from the metal
24If hf for the light incident on a metal is less
than the work function, what will the kinetic
energy of the ejected electron be?
- the kinetic energy would be negative
- the kinetic energy would be zero
- the kinetic energy would be positive
- no electrons would be released from the metal
25If hf for the light incident on a metal is less
than the work function, what will the kinetic
energy of the ejected electron be?
- the kinetic energy would be negative
- the kinetic energy would be zero
- the kinetic energy would be positive
- no electrons would be released from the metal
26Photoelectric 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
27Preflights 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
28Preflights 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
29Is 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 !
30The 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.
31Are 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!
32Interference 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.
33Single 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
34How 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.
35How 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?
36Modern 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.
37Wavicle?
- 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
38Electrons?
- 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
39Electrons 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?
40Electrons 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.
41Electrons 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.
42Quantum Summary
- Particles act as waves and waves act as particles
- Physics is NOT deterministic
- Observations affect the experiment