1. “My Nobel Prize” by Albert Einstein

2. “My Nobel Prize” by Albert Einstein

4. 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.

5. 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!

6. Hertz’s Spark Gap Generator:

7. 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:

8. Lenard’s Photoelectric Apparatus:

9. 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.

10. 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.

11. WHAT PHYSICISTS THOUGHT
BEFORE EINSTEIN’S 1905 PAPER
ON THE PHOTOELECTRIC EFFECT

12. 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.

13. 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.

14. 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.

15. 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.

16. OBSERVATIONS THAT LED EINSTEIN TO EXPLAIN THE PHOTOELECTRIC EFFECT:

17. 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.)

18. OBSERVATIONS THAT LED EINSTEIN TO EXPLAIN THE PHOTOELECTRIC EFFECT:
Light with frequencies above the threshold resulted in electrons being emitted from that metal.

19. 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.

20. BLACKBODY RADIATION
AND
“THE ULTRAVIOLET CATASTROPHE”

21. 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.”

22. (THIS WOULD MAKE IT A PROBLEM TO GRILL YOUR FOOD, OR SIT IN FRONT OF A FIREPLACE!)

23. BUT. . . A NEW THEORY PROPOSED BY. . .

24. Max Planck ( )
“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”).”

26. Max Planck ( )
“DIFFERENT FREQUENCIES REQUIRE DIFFERENT NUMBERS OF ENERGY PACKETS.”

27. “DIFFERENT FREQUENCIES REQUIRE DIFFERENT NUMBERS OF ENERGY PACKETS.”
Max Planck ( )
“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.

28. EINSTEIN
suggests:

29. “THOSE DISCRETE PACKETS ACTUALLY REPRESENT ‘PARTICLES OF LIGHT’!!”
THEREFORE -
THESE “PARTICLES” WERE CALLED “PHOTONS.”

30. “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 = x J-s

31. ALL GRAPHS OF eVs vs f HAVE THE SAME SLOPE AS ABOVE!!!!
RESCALED GRAPH:
ENERGY AS A FUNCTION OF FREQUENCY
ALL GRAPHS OF eVs vs f HAVE THE SAME SLOPE AS ABOVE!!!!

32. REWRITE AS: KEmax = hf - W (NOTICE “y = mx + b” FORM)
total energy absorbed by the photoelectron
“work function”
of the metal
(energy required to free the electron)
(NOTICE “y = mx + b” FORM)

33. 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.

34. MATCHES PREDICTION! NEW PREDICTION #1: OBSERVATION:
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!

35. 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.

36. NEW PREDICTION #2: OBSERVATION: MATCHES PREDICTION!
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!

38. 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”

39. 1905:
THE STAGE IS NOW SET FOR
THE SWITCH FROM

40. 1905:
NEWTONIAN MECHANICS
I’m NOT HAPPY about this!!!!!!!!!!!

41. 1905: NEWTONIAN MECHANICS TO QUANTUM MECHANICS!!!
Relax Albert, everything will be just fine!
I’m not so sure about this, Neils!

42. 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.*

43. …and so, QUANTUM PHYSICS was born

44. HAPPY BIRTHDAY QM!!!!