Download Wave Particle Duality

WAVE-PARTICLE DUALITY
The Nature of Light
HISTORY
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
Before the nineteenth century, not much was
known about light.
It was known to be bright, fast, and came in a
bunch of different of colours.
Scientists wanted to know what the nature of
light was – if it was a particle or a wave.
PARTICLES AND WAVES

Movement of a particle:
A particle will travel in a straight line.
 Interferes with one another by deflecting each other.

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
Eg. Ping pong balls
Movement of a wave
A wave, when sent through a slit or doorway, spreads
around the doorframe and continues on its way.
 Interferes with one another by cancelling out or
heightening the power of the other wave.


Eg. Water waves
THOMAS YOUNG
Wanted to find the nature of light, so he shone a
light through two very small slits.
 If light was a particle, it would make a pattern on
the wall in the shape of the two slits.
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
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Eg. Throwing paint-dipped ping-pong balls through a
window.
If light was a wave, it would make a wave
interference pattern on the wall.
DOUBLE SLIT EXPERIMENT
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Young set up his experiment and shone light onto
photo sensitive paper to see what happened.
When he turned on the light, an interference
(wave-like) pattern appeared.
This experiment proved that light was a wave.
http://homepage.univie.ac.at/Franz.Embacher/KinderUni2005/waves.gif
http://www.quantum.physik.uni-mainz.de/de/bec/gallery/index.html
LIGHT AS A WAVE
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Later in the nineteenth century, James Clerk
Maxwell discovered that light was actually an
electromagnetic wave – a wave of oscillating
magnetic and electric fields.
This was proven experimentally by Heinrich
Hertz.
PHOTOELECTRIC EFFECT
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In the early 1900’s, Einstein used Plank’s idea
that light was emitted in small bursts instead of
a wave to help explain the photoelectric effect.
The photoelectric effect does not work if light is
considered a wave, because the interaction
between the light and the electrons is like that of
a particle giving all of its energy to the electrons.
This proves that light is a particle.
QUANDARY
How can light be both a particle and a wave?
 It’s not. It’s something called a “quantum vector
field”. It sometimes acts like a wave (double slit
experiment) and sometimes acts like a particle
(photoelectric effect).
 Scientists have named this “multiple personality
syndrome” of light (and all matter for that
matter) Wave-Particle Duality.

http://prelectur.stanford.edu/lecturers/hofstadter/images/light-wave.jpg
HEISENBERG’S PRINCIPLE OF
UNCERTAINTY
Adapted from:
Zitzewitz, Paul W. Merrill Physics: Principles and
Problems. Glencoe/McGraw-Hill: Westerville, 1992
HEISENBERG’S PRINCIPLE OF
UNCERTAINTY
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Heisenberg’s Principle of Uncertainty states that you
cannot precisely measure the position and momentum of a
particle at the same time.
This is because in order for you too see a particle- and thus
determine its position- light must strike the particle.
However, when light strikes a particle, its momentum is
changed.
Taken from Merrill Physics: Principles and Problems by Paul W.
Zitzewitz
HEISENBERG’S PRINCIPLE OF
UNCERTAINTY

To illustrate this: let’s say, hypothetically, that you were in
a gym, blindfolded, and had an unlimited supply of
basketballs. In the gym there is also a cat. You are trying
to figure out where the cat is. Since you are blindfolded,
you must throw basketballs at random until you hit the
cat, at which point the cat will probably make a sound,
enabling you to figure out where it is. However, once you
hit the cat, the cat’s speed and direction will probably
change, changing its momentum.
http://www.clipartguide.com/
http://topendsports.com/
http://www.bradfitzpatrick.com/
SCHRÖDINGER REVAMPS THE
THEORETICAL MODEL OF THE
ATOM
Adapted from:
Zitzewitz, Paul W. Merrill Physics: Principles and
Problems. Glencoe/McGraw-Hill: Westerville, 1992
SCHRÖDINGER REVAMPS THE
THEORETICAL MODEL OF THE ATOM

The Bohr model of the atom is the one you have
been learning since grade 9. In the Bohr model,
electrons are found in specific locations around
the nucleus and orbit it much like the planets
orbit the Sun.

This is a Bohr-Rutherford
diagram of a Neon atom.
The nucleus is in the
center, and the electrons
are in a carefully ordered
orbit around the nucleus.
SCHRÖDINGER REVAMPS THE
THEORETICAL MODEL OF THE ATOM
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Then, in 1926, a German scientist by the name of
Erwin Schrödinger created a new, quantum model of
the atom. He used much of de Broglie’s work on the
wavelike properties of particles in the creation of this
model.
Because of Heisenberg’s Principle of Uncertainty, we
cannot know both the position and the momentum of
an electron at a given time. Rather than give exact
location of an electron, the quantum model only gives
the probability that an electron will be in a particular
spot. The area where there is a high probability of
finding an electron is called the electron cloud. The
most probable regions are still the regions predicted
by the Bohr model.
The quantum model of the atom is the basis for
quantum mechanics.
THE QUANTUM MODEL- AN ANALOGY
Taken from Merrill Physics: Principles and Problems by Paul W.
Zitzewitz
HOMEWORK QUESTIONS
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Wave-Particle Duality
I.
II.
III.

Explain in your own words the difference between
the movement of a particle and a wave
Describe Young’s double slit experiment and its
results.
What effect of light proves that light acts like a
particle?
Heisenberg
IV. State, in your own words, Heisenberg’s Principle of
Uncertainty.

Schrödinger
V.
What are some similarities and differences
between the Bohr model of the atom and the
quantum model of the atom?
BIBLIOGRAPHY
Dave. "The Page of Uncertainty." Dave's Physics Shack.
1997. Morningside College. 24 Apr. 2008
<http://my.morningside.edu/slaven/Physics/uncertaint
y/index.html>.
"Interference Movie." The Atomic Lab. 2000. 24 Apr.
2008
<http://www.colorado.edu/physics/2000/schroedinger/b
ig_interference.html>.
Nave, C. R. "Wave-Particle Duality." HyperPhysics.
Georgia State University. 24 Apr. 2008
<http://hyperphysics.phyastr.gsu.edu/hbase/mod1.html>.
Zitzewitz, Paul W. Merrill Physics: Principles and
Problems. Glencoe/McGraw-Hill: Westerville, 1992.
Edwards, Lois. McGraw-Hill Ryerson: Physics.
McGraw-Hill: Toronto, 2003.