Wave? Particles?? Physics 100 Chapt 22. Maxwell Light is a wave of oscillating E- and B-fields James...
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Transcript of Wave? Particles?? Physics 100 Chapt 22. Maxwell Light is a wave of oscillating E- and B-fields James...
Who’s right??
Waves explain diffraction & interference
Photons explain photoelectric effect & Compton scattering
Impossible to explain interference with particles
With 2 slits openno light goes here
Block off one slit
Now lightcan go here
Impossible to explain PE-effectand Compton scattering with waves
Electron KE (electron Volts)
red
0.5 1.0 1.5
yellow
blue
violet
Make an intereferencepattern with low intensity light
One photon at a time goes through the two-slit apparatus
The interference pattern emerges one dot at a time
Wave-Particle “duality
-Light behaves like a wave when it propagates through space-And as a particle when it interacts with matter
Louis de Broglie
If light behaves as particles, maybe other particles (such as electrons) behave as waves h
p = Photons:
hp =
particles:
hp =
Wave-particle duality is
a universal phenomenon
Ordinary-sized objects have tiny wavelengths
0.2kg30m/s
hp =
hmv
= 6.6x10-34Js
0.2kg x 30 m/s =
6.6x10-34Js6.0 kg m/s = 1.1x10-34m =
Incredibly small
the wavelength of an electronis not so small
9x10-31 kg 6x106 m/s
hp =
hmv
= 6.6x10-34Js
9x10-31kg x 6x106 m/s =
6.6x10-34Js5.4x10-24 kg m/s = 1.2x10-10m =
-
About the size of an atom
Send low-momentum electrons thru narrow slits
See a diffraction pattern characteristic of
wavelength =h/p as predicted by de Broglie
Waves thru a narrower slit
wider
py
py
y
When the slit becomes narrower, thespread in vertical momentum increases
x
y
Heisenberg Uncertainty Principle
y py > h
Uncertaintyin location
Uncertainty in
momentum in
that direction
If you make one of these smaller, the other has to become bigger
Heisenberg tries to measure the location of an atom
For better precision, usea shorter wavelength
But then the momentumchange is higher
x px > h
Localize a baseball
0.2kg x px > h px > hx
Suppose x= 1x10-10m
px > 6.6x10-34Js
1x10-10m
vx >
= 6.6x10-24kgm/s
A very tiny uncertainty
About the size of a single atom
px m
6.6x10-44Js
0.2kg = = 3.3x10-23 m/s
Localize an electron
x px > h px > hx
Suppose x= 1x10-10m
px > 6.6x10-34Js
1x10-10m
vx >
= 6.6x10-24kgm/s
Huge, about 2% of c
About the size of a single atom
px me
6.6x10-24Js
9x10-31kg = = 7x106 m/s
-me=9x10-31kg