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“My Nobel Prize”
by Albert Einstein
“My Nobel Prize”
by Albert Einstein
Discovery: Heinrich Hertz and Phillip Lenard
Back in 1887…
Hertz clarified Maxwell’s electromagnetic theory of light:
– Proved that electricity can be transmitted in
electromagnetic waves.
– Established that light was a form of electromagnetic
radiation.
– First person to broadcast and receive these waves.
The Spark Gap Generator
First observed the effect while working with a
spark-gap generator ~ accidentally, of course
Illuminated his device with ultraviolet light:
– This changed the voltage at which sparks
appeared between his electrodes!
Hertz’s Spark Gap Generator:
Lenard Goes Further…
His assistant,
Phillip Lenard,
explored the
effect further. He
built his own
apparatus called
a “phototube” to
determine the
nature of the
effect:
Lenard’s Photoelectric Apparatus:
The Experiment:
By varying the voltage on a negatively charged grid
between the ejecting surface and the collector plate,
Lenard was able to:
– Determine that the particles had a negative charge.
– Determine the kinetic energy of the ejected particles.
Lenard’s Findings:
• Thus he theorized that this voltage must be equal to
the maximum kinetic energy of the ejected particles,
or:
KEmax = eVstopping
Perplexing Observations:
» The intensity of light had no effect on
energy
» There was a threshold frequency for
ejection
Classical physics failed to explain this,
Lenard won the Nobel Prize in Physics in 1905.
WHAT PHYSICISTS THOUGHT
BEFORE EINSTEIN’S 1905 PAPER
ON THE PHOTOELECTRIC EFFECT
PREDICTION #1:
Increasing the intensity of the light
(making the light brighter) will cause the
same number of electrons to fly off the
plate, but each electron will have more
energy.
OBSERVATION #1:
Making the light brighter causes a
greater number of electrons to
come flying off the plate, but each
electron has the same energy.
PREDICTION #2:
Changing the frequency of the light
(changing its color) will cause a
greater number of electrons to
come flying off, but have no effect
on each electron’s energy.
OBSERVATION #2:
Changing the color of the
light has no effect on the
number of electrons flying
off, but each electron has
more or less energy,
depending on the color.
OBSERVATIONS THAT LED
EINSTEIN TO EXPLAIN THE
PHOTOELECTRIC EFFECT:
OBSERVATIONS THAT LED
EINSTEIN TO EXPLAIN THE
PHOTOELECTRIC EFFECT:
Each type of metal used had its own “threshold frequency”.
(In other words, light with frequencies below this threshold
had no effect whatsoever.)
OBSERVATIONS THAT LED
EINSTEIN TO EXPLAIN THE
PHOTOELECTRIC EFFECT:
Light with frequencies above the threshold resulted in
electrons being emitted from that metal.
OBSERVATIONS THAT LED
EINSTEIN TO EXPLAIN THE
PHOTOELECTRIC EFFECT:
Once that threshold was crossed, the energy of the emitted
electrons (“photoelectrons”) increased as the frequency of
the light was increased.
BLACKBODY RADIATION
AND
“THE ULTRAVIOLET
CATASTROPHE”
ACCEPTED PHYSICS THEORY AT THE TURN OF THE 20TH
CENTURY STATED:
“ELECTROMAGNETIC ENERGY RADIATED BY AN OBJECT
(E.G. , BBQ GRILL)
MUST BE EQUALLY DIVIDED AMONG ALL POSSIBLE
ELECTROMAGNETIC FREQUENCIES.”
(THIS WOULD MAKE IT A PROBLEM TO GRILL YOUR
FOOD, OR SIT IN FRONT OF A FIREPLACE!)
BUT. . . A NEW THEORY PROPOSED BY. . .
Max Planck (1858 - 1947)
“ENERGY COULD NOT BE RADIATED CONTINUOUSLY ALONG THE
E/M SPECTRUM, AT ANY POSSIBLE FREQUENCY.
IT CAN ONLY BE EMITTED IN DISCRETE, DISCONTINUOUS
PACKETS (WHICH PLANCK CALLED “QUANTA”).”
Max Planck (1858 - 1947)
“DIFFERENT FREQUENCIES REQUIRE
DIFFERENT NUMBERS OF ENERGY
PACKETS.”
Max Planck (1858 - 1947)
“DIFFERENT FREQUENCIES REQUIRE
DIFFERENT NUMBERS OF ENERGY
PACKETS.”
E.G.:
LOW-FREQUENCY LIGHT REQUIRES FEW PACKETS OF
LOW ENERGY.
ULTRAVIOLET WAVES REQUIRE TREMENDOUS
NUMBERS OF HIGH-ENERGY PACKETS.
EINSTEIN
suggests:
“THOSE DISCRETE PACKETS
ACTUALLY REPRESENT
‘PARTICLES OF LIGHT’!!”
THESE “PARTICLES”
WERE CALLED
“PHOTONS.”
THEREFORE -
“THOSE DISCRETE PACKETS
ACTUALLY REPRESENT
‘PARTICLES OF LIGHT’!!”
THEREFORE –
LIGHT CAN BE PICTURED NOT AS A WAVE, BUT AS A
WAVE-LIKE PARTICLE WITH A DISCRETE AMOUNT
OF ENERGY, based on its frequency.
(E α f; or, E = hf)
h = Planck’s Constant = 6.625 x 10-34 J-s
RESCALED GRAPH:
ENERGY AS A FUNCTION OF FREQUENCY
ALL GRAPHS OF eVs vs f HAVE THE SAME SLOPE AS ABOVE!!!!
REWRITE AS:
KEmax = hf - W
total energy
absorbed by the
photoelectron
(NOTICE “y = mx + b” FORM)
“work function”
of the metal
(energy required to
free the electron)
NEW PREDICTION #1:
MAKING THE LIGHT BRIGHTER
SIMPLY MEANS MORE PHOTONS
OF A GIVEN ENERGY ARE
STRIKING THE METAL PLATE.
THIS MEANS MORE COLLISIONS
BETWEEN PHOTONS AND
ELECTRONS, AND MORE
ELECTRONS WILL FLY OFF –
EACH WITH THE SAME ENERGY.
NEW PREDICTION #1:
MAKING THE LIGHT BRIGHTER SIMPLY MEANS MORE PHOTONS OF A GIVEN ENERGY
ARE STRIKING THE METAL PLATE.
THIS MEANS MORE COLLISIONS BETWEEN PHOTONS AND ELECTRONS, AND MORE
ELECTRONS WILL FLY OFF - EACH WITH THE SAME ENERGY.
OBSERVATION:
MATCHES
PREDICTION!
NEW PREDICTION #2:
CHANGING ONLY THE COLOR OF THE LIGHT WILL CHANGE THE ENERGY
CONTENT OF EACH PHOTON, BUT NOT THE NUMBER OF PHOTONS
HEADING TOWARD THE PLATE. AS A RESULT,
THE SAME NUMBER OF COLLISIONS WILL RESULT IN
THE SAME NUMBER OF EJECTED ELECTRONS –
BUT THOSE ELECTRONS WILL HAVE
MORE ENERGY.
NEW PREDICTION #2:
CHANGING ONLY THE COLOR OF THE LIGHT WILL CHANGE THE ENERGY CONTENT
OF EACH PHOTON, BUT NOT THE NUMBER OF PHOTONS HEADING TOWARD THE
PLATE. AS A RESULT, THE SAME NUMBER OF COLLISIONS WILL RESULT IN THE
SAME NUMBER OF EJECTED ELECTRONS - BUT THOSE ELECTRONS WILL HAVE
MORE ENERGY.
OBSERVATION:
MATCHES
PREDICTION!
Einstein’s Interpretation
A new theory of light:
• Electromagnetic waves carry discrete energy packets
• The energy per packet depends on wavelength,
explaining Lenard’s threshold frequency.
• More intense light corresponds to more photons, not
higher energy photons.
This was published in his famous 1905 paper:
“On a Heuristic Point of View About the Creation and Conversion of Light”
1905:
THE STAGE IS NOW SET FOR
THE SWITCH FROM
1905:
I’m NOT HAPPY about
this!!!!!!!!!!!
NEWTONIAN MECHANICS
1905:
Relax Albert,
everything will be
just fine!
I’m not so sure about
this, Neils!
NEWTONIAN MECHANICS
TO
QUANTUM MECHANICS!!!
Quantum leap for quantum mechanics
• Wave-particle duality set the stage for 20th century
quantum mechanics.
• In 1924, Einstein wrote:
“…There are therefore now two
theories of light, both
indispensable, and - as one must
admit today despite twenty years
of tremendous effort on the part
of theoretical physicists - without
any logical connection.”
*This work won Einstein his Nobel Prize in 1922.*
…and so, QUANTUM PHYSICS was born
HAPPY BIRTHDAY
QM!!!!