Transcript Wave-Particle Duality

Wave-Particle Duality
Light quanta revisited
and introduction to matter waves
Review of Diffraction
• When l of wave is larger than obstacle/slit then
more diffraction occurs
• If l of wave is small, you need a small
obstacle/slit to observe diffraction
• To diffract X-rays (very short l), you need
obstacles/slits the scale of atoms in crystals
• Long l light diffracts easily (passes around) the
molecules in the air, whereas short l light is easily
scattered - thus the blue sky and red/orange
sunset/sunrise
When short l light
(why not long l?)
interacts with free
charges (not bound to
atoms), momentum
and energy must be
conserved. By giving
some of its energy to
the charge, the
photon’s frequency
________, while its l
________.
Momentum is
conserved separately in
x- and -y dimensions.
Young’s Double-slit experiment with light
• Demonstrated that light was wave
• Interference pattern produced; diffraction
occurred:
l > distance between slits produced greater
diffraction
• Waves can interfere with one another
LINK
But… what about
• Blackbody spectrum
• Photoelectric effect
These phenomena proved that light
comes in packets (“quanta” or
“photons”)… so, what’s the deal?
deBroglie Matter Waves
• Since light exhibits particle-like behavior,
perhaps matter could exhibit wave-like
behavior…
h
l
p
h is Planck’s
constant and p
is momentum
Davisson-Germer
• Scattering a beam of electrons off of nickel
target
• Accident caused nickel to crystallize
• Diffraction of the electron beam was
observed – l of electron “wave” much
smaller than visible light, so need very small
spacing of crystal “slits” for diffraction
LINK
Light vs. Electron microscopes
• Light microscopes are limited by the l of visible
light. If object is smaller, waves diffract around
it and it can’t be seen
• Electron waves are much, much shorter than
visible light waves, so they can resolve much
smaller objects
Summary
• Waves can exhibit particle-like properties
– Blackbody spectra
– Photoelectric effect
– Compton scattering
• Matter can exhibit wave-like properties
– Electron diffraction
Reconciliation of the viewpoints
The “wave” for both light and
matter is a probability wave…
It describes where the photons
or electrons are allowed to be…
L
Where is the
electron in the box?
Double-slit experiment for electrons
• When electrons fired
toward a slit for which
d < l, a diffraction
pattern is seen
• If current is lowered so
that only 1 electron is
allowed at a time pattern is STILL
SEEN!
How does the electron “know” where to
go?
If method is used to “look” for
electron to see which slit it passed
through, the pattern disappears!
You see the two-slit “shadow.” The
process of “looking” at the electron
interferes with its position and
momentum. It “collapses” the
electron probability wave so that
only one possibility exists - not
both simultaneously.
If you look
If you don’t