11/15/2010 1 Blackbody Radiation Photoelectric Effect Wave-Particle Duality • sections 30-1 – 30-4 Physics 1161: Lecture 22 Everything comes unglued The predictions of “classical physics” (Newton’s laws and Maxwell’s equations) are sometimes 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! 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: ~0.4μm to 0.7μm Higher temperature: peak intensity at shorter λ
Transcript
11/15/2010
1
Blackbody Radiation
Photoelectric Effect
Wave-Particle Duality
• sections 30-1 – 30-4
Physics 1161: Lecture 22 Everything comes unglued
The predictions of “classical physics” (Newton’s laws
and Maxwell’s equations) are sometimes 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!
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: ~0.4µm to 0.7µm
Higher temperature: peak intensity at shorter λ
11/15/2010
2
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/λ
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?
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/λ
Red! Highest wavelength is lowest energy.
Hotter stove emits higher-energy photons
(shorter wavelength = orange)
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 bulb’s filament is hottest?
1 2 3 4
0% 0%0%0%
1. Red
2. Green
3. Blue
4. Same
λmax
A red and green laser are each rated at 2.5mW. Which one produces more photons/second?
• 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.
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
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.
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?
11/15/2010
6
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.
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
