Quantum Physics in a
Nutshell
Wave-Particle Duality
Classical Physics
• This is most of what you studied in grade
11 physics
• Physics of motion, energies, and
interactions (collisions) of objects in the
everyday world around us
Classical Physics
• Greek Scientists: studied how the Earth moved
in relation to the Sun
– Not just mythology and religion
– Aristotle: Attempt to explain motion and even gravity
– Ptolemy: Astronomy (geocentric views)
• Indian and Chinese physicists also had similar
views but these views were based more on logic
rather than experimentation (even some semiheliocentric views of our Solar system)
Classical Physics
• Muslim physicists
– Ibn al-Haytham (“Alhazen”) believed that light
traveled in rays to our eyes (rather than our
eyes emitting the light as European physicists
believed).
– Nasir al-Din al-Tusi believed that the planets
orbited in ellipses rather than circles
Classical Physics
• Galileo
– Went against the Church
– Heliocentric view of the Solar system
– Centre of gravity
• Newton
– Laws of Motion (explaining motion, inertia,
dynamics)
Maxwell
• Proposed that electromagnetic radiation acted
as a wave
• This theory states that electromagnetism
behaves in a continuous way
• Example: Your hand will get continuously
warmer as it is near a hot object
• He also proposed that all electromagnetic waves
traveled at c (in a vacuum)
• Page 380: Electromagnetic spectrum
Visible Light Spectrum
• At a lower range of these hot temperatures, red light
begins to be emitted. Higher frequency colours are seen
as it gets hotter: we will see blue and eventually white
(all colours being emitted).
Questions
• 1) Why don’t we see our hand getting hotter – we can
only feel it?
• 2) The picture below is taken of stars in the globular star
cluster Omega Centauri by Hubble. What can you say
about the different colours in the picture? Which colour
stars have the most energy?
• 3) We can “see”
thermal energy from a
fireplace. If energy is
radiated by all objects,
why can’t we see this
energy for all objects?
Answers
• 1) The energy is not in the visible range of the
spectrum.
• All objects radiate energy we just can’t see it!
• 2) The colours mean that there are different
wave lengths being emitted. Red stars have
longer wavelengths (smaller frequencies) so
have less energy than blue and white stars.
• 3) Energy is radiated from all objects but not
always visible to us (must be in the visible
range).
Quantum Physics
• Physics after 1900/Modern Physics
• Quantum Theory took almost 3 decades to come about.
It cannot be credited to any one scientist. It is now the
basis for explaining the structure of matter and very
small subatomic objects (electrons, protons, neutrons).
• Note: Quantum physics is an attempt to explain and
predict what we observe in a way that we can
understand. Models and theories support each other
and are supported experimentally, but they may not
actually represent what is really happening. We cannot
see what electrons and photons actually are!
Key Terms and Ideas
• Quantum: a discrete amount of energy (not
continuous)
• Photon: a quantum of light or electromagnetic
wave (“particle of light”)
• Electromagnetic Force: the non-contact force
that occurs between electrically charged
particles. Example: electricity, magnetism, and
light.
Key Terms and Ideas
• Radiation: Electromagnetic Waves (EMR):
wave consisting of oscillating electric and
magnetic fields that move at the speed of light
through space.
• Incandescence: light emitted by an object
heated beyond a specific temperature.
Examples: light in lightbulbs
• Black Body: physical body that absorbs all
incident (incoming) radiation. No reflection.
Quantum Physics
• 1900: Max Planck (German)
• Planck was trying to explain incandescent light
spectrums
• A light spectrum is the unique frequencies of visible light
that blend together to produce the colour emitted by any
incandescent object. Every incandescent object has its
own unique spectrum.
• Since the spectrum of light emitted in incandescence is
made up of specific frequencies, and not a range of
frequencies, it is called a DISCRETE or LINE spectrum
(rather than a continuous spectrum).
Why is this useful?
• Elements give off characteristic colours (chemistry
lab?) and that is how astronomers can determine
what elements are present on a distant star.
• Look at the emission spectra of these 2 elements.
Note how they are discrete and unique.
• Top: Emission spectra of hydrogen.
• Bottom: Absorption spectra of hydrogen:
• Note: It is continuous but has some gaps
– these are frequencies that are not
absorbed.
Quantum Physics
• 1900: Max Planck (German)
• Planck was trying to explain
incandescence and black body radiation
• Ultraviolet Catastrophe: Maxwell’s
electromagnetic wave theory did not
accurately predict the observed spectrum
of light that Planck saw.
Planck’s Conclusions
• Planck studied the emission spectrums of
incandescent objects in greater detail and made
the following conclusions:
• The energy of electromagnetic radiation was
directly related to frequency. Higher frequency
waves have more energy.
• Any spectral line (frequency) can vary in
intensity (energy). BUT any variations in energy
were always multiples of some integer “n”.
Planck’s Conclusions
• Formula for energy of radiation:
E = nhf
•
•
•
•
E = Energy (Joules)
h = Planck’s constant (6.626 x 10-34 J/Hz or Js)
f = frequency (Hz)
n = integer (so the values of energy are
DISCRETE)
Poor Planck 
• The idea that energy exists only in discrete
amounts was a revolutionary idea. The smallest
amount of energy possible (hf) is called a
quantum of energy. It is extremely small and is
not significant in everyday situations.
• Planck was not happy though. He believed he
just found the math to support the answer but
not a discovery. He had no basis for suggesting
this concept of a quantum of energy other than
his observations.
Einstein
• 1905: “n” is understood. Einstein argues
the existence of photons are particles of
waves. “n” is the number of photons that
make up a wave of light.
• Ephoton = hf
Example
• A molecule vibrates with a frequency of
8.32 x 1015 Hz. What is the energy of a
photon of this molecule?
• 1.325 x 10 -19 J
Try This
• A light source of frequency 5.0 x 10 14 Hz
produces a spectral line with an energy of
1.657 x 10 -16J. How many photons were
ejected to produce that?
• 50 photons
The Birth of Quantum Physics
• Planck’s discovery lead to Quantum
Mechanics. Excited substances can only
emit light of certain frequencies.
• Quantum physics addresses things like
energy that only exist in discrete
quantities.
The Photoelectric Effect
• - the ejection of electrons from an atom (which flow as
electric current) of a photoemissive surface when struck
by light or other forms of EMR (electromagnetic
radiation).
• A photoemissive material is one that can exhibit this
effect. Example: material used in solar panels.
• The photoelectric effect can be seen in:
– photo sensors in digital cameras
– infrared remote controls
– camera film
The Photoelectric Effect
• The Photoelectric Effect: when light
shines on a metal surface, electrons can
be emitted from the surface (these
electrons are called photoelectrons). This
only happens when you have a high
enough frequency (or more energy).
Videos to Help
• https://www.youtube.com/watch?v=0qKrO
F-gJZ4
• (first 4 min)
• http://aplusphysics.com/courses/honors/vi
deos/PhotoelectricEffect/PhotoelectricEffe
ct.html
Photoelectric Effect
• The maximum kinetic energy of a photoelectron
is the difference of the energy of the photon and
the work function of the metal emitter.
E k (max)  hf  W
• Where Ek(max) is the maximum kinetic energy
of a photoelectron (ejected electron) in Joules
• f is the frequency of the EMR (in Hz)
• W is the work function of a metal (in Joules)
– This is the work needed to eject an electron
• h is Planck’s constant
Photoelectric Effect
E k (max)  hf  W
• Think about this equation for a second:
• hf is what?
• W is what?
Work Functions of Common Metals
• Table page 853
• These are all in eV
(electron volts). We
need them in Joules.
• Conversion ratio:
1 eV = 1.60 x 10-19 J
(on formula sheet)
Example Problem
• Light with a wavelength of 581nm (581 x
10-9) strikes a Cesium metal surface inside
a vacuum tube.
• What is the maximum kinetic energy of the
electrons emitted from the surface?
• HINT: We need the wave speed equation
from grade 11.
• v = λf
Try This
• 1. Photons with a frequency of 4.86 x
1015 HZ are directed toward a nickel
surface in a vacuum tube.
• What will be the maximum kinetic energy
of the electrons emitted from the nickel
surface?
Photon Momentum
• Einstein theorized that energy and matter
were related.
• He thought that photons (energy) may
have some of the same characteristics as
matter.
• Photons have momentum – just like
matter!
The Compton Effect
• In 1923, Arthur Compton observed that when a
high-energy photon strikes a free electron at
rest, the electron gains some of the energy and
momentum of the photon and is scattered off.
The other photon is also scattered. This
confirms the conservation of momentum in the
collision.
• So even tiny particles (photons) have
momentum – just like matter.
• So waves act like matter…
The Particle (Quantum) Nature of
Waves – Summary
• By 1923, the particle nature of waves was
largely accepted by the scientific community due
mainly to 3 pieces of evidence:
• Planck’s discovery that incandescent bodies
emit unique EM spectra that are discrete.
Meaning energy is quantized not continuous as
previously thought.
• Einstein’s explanation of the Photoelectric Effect
– light exists as particles called photons.
• Photon Momentum
The Particle (Quantum) Nature of
Waves – Summary
• These 3 observations demonstrate the EM:
– Exists as particles called photons
– These particles possess kinetic energy and
momentum, which are fundamental properties of
matter
• New Definition of Electromagnetic Wave
– consists of discrete particles of energy called photons
that travel through space in a wave-like fashion,
transferring energy
The Wave Nature of Matter
• 1924 – Louis deBrogli proposes the
opposite is true – if waves have properties
of matter, then matter must have
properties of waves.
• People thought he was crazy until Einstein
endorsed this (and then he received the
Nobel prize).
De Broglie
• (“Broy”)
• https://www.youtube.com/watch?v=JIGIeXK0tg
So What are Matter Waves?
• There are 3 fundamental wave types in the
universe:
– Mechanical waves – they need a medium.
Examples: Sound, water
– Electromagnetic waves – they consist of
photons
– Matter – the wave nature of matter in motion.
Also called deBroglie waves.
Some good videos for summary
• http://www.youtube.com/watch?v=DfPepr
Q7oGc
• http://www.perimeterinstitute.ca/news/schr
-dinger-s-cat-wanted-dead-and-alive
Video part 3
Double-Slit Experiment – Classical
Particles
• Tennis balls (classical particles like sugar)
move as localized particles through slits
• Once they hit the screen they produce the
following distribution:
Double-slit … Again
• If we use water waves, sound, or any
other classical waves they spread out
behind the double slit (diffraction – see the
review notes from Physics 11)
• http://www.launc.tased.edu.au/online/scien
ces/physics/diffrac.html
Double-slit with electrons
• Using small subatomic particles (electrons), it
was found that
– Each single electron hits the detection screen as a
particle
– After many electrons hit, an interference pattern
forms, demonstrating wave behaviour
– The same interference pattern occurs if we fire each
electron singly or more than one at a time
– SAY WHAT? That makes no sense and tons of sense
all at the same time… kind of like time dilation and
length contraction.
So electrons…
• Behave as particles and waves.
• Wave-particle Duality
So what happens with Light?
• Just like electrons, light behaves like
waves and particles
• Light hits the screen as an individual
particle but then produces an interference
pattern (like waves) over time
• https://www.youtube.com/watch?v=DfPepr
Q7oGc
So what happens with all quantum
objects?
• All exhibit wave-particle duality
– Electrons, protons, neutrons, atoms,
molecules
What if we want to see it in more
detail???
• Video Part 4
• Who wouldn’t be interested in the electron
in this experiment
– What is it doing at each key phase?
– How does it decide which slit to go through?
– Does it split into two parts?
– Does it go through both parts at once?
Curiosity Killed the Cat…
• When we look at the electron to see what
it is doing while passing through the
double-slit barrier, we are making a
measurement which perturbs the electron
and destroys the interference pattern.
• This is called measurement disturbance.
Why? Interpretations…
• We can predict the overall behaviour of
the electrons in the double-slit experiment,
but nobody really knows what the
electrons are doing between the source
and the detector.
• There are 4 interpretation to complete this
picture…
Collapse Interpretation
• Thinking of electrons as spread-out waves that collapse
to point-like particles once they are measured
• Pilot Wave
• Thinking of electrons as particles that are guided by an
invisible wave
• Many Worlds
• Parallel universes that come into being when we make
measurements at the quantum level
• Copenhagen Interpretation
• Just think about the results from the measurements
• Who cares about what happens in the middle!
Pilot Wave Interpretation
• Thinking of electrons as particles that are
guided by an invisible wave
Many Worlds Interpretation
• Parallel universes that come into being
when we make measurements at the
quantum level
Copenhagen Interpretation
• Just think about the results from the
measurements
• Who cares about what happens in the middle!
• Come up with a skit/song/way to explain one of
these interpretations and present to the class
• Video Part 5
What if we want to see it in more
detail???
• https://www.youtube.com/watch?v=IOYyC
HGWJq4