1. IS LIGHT A WAVE OR A PARTICLE?
MODERN PHYSICS
IS LIGHT A WAVE OR A PARTICLE?

3. 1. THE HISTORY

4. “Corpuscular Theory” of Light
Isaac Newton and the
“Corpuscular Theory” of Light
“Light consists of particles that follow my Laws of Motion.”
“Different colors of light are represented by different ‘sizes’ of particles (corpuscles) traveling at different speeds.”

5. Christian Huygens and the
“Wave Theory of Light”
He believed that light was a wave, and that it exhibited wave-like properties.

6. So, who is right? BOTH MODELS CAN EXPLAIN REFLECTION It’s a WAVE!
It’s a particle!
It’s a WAVE!
BOTH MODELS CAN EXPLAIN REFLECTION

7. So, who is right? BOTH MODELS CAN EXPLAIN REFRACTION It’s a WAVE!
It’s a particle!
It’s a WAVE!
BOTH MODELS CAN EXPLAIN REFRACTION

8. So, who is right? BOTH MODELS CAN EXPLAIN DIFFRACTION It’s a WAVE!
It’s a particle!
It’s a WAVE!
BOTH MODELS CAN EXPLAIN DIFFRACTION

9. In 1801, an English physicist named Thomas Young performed the double-slit experiment.
Because he believed that light was composed of waves, Young reasoned that some type of interaction would occur when two light waves met.
But why?
This is just a random picture of a guy.

10. 1. The intensity (brightness) of an electromagnetic wave is proportional to the square of the amplitude.
2. The intensity (brightness) fluctuates with time. Typical fluctuation is about 1015 Hz. We can see at most about 102 Hz.
3. When two waves meet, we can analyze the result through the principle of superposition.

11. d = distance between slits, n = order number of the maxima (1,2,3…)
We did this!
d = distance between slits,
n = order number of the maxima (1,2,3…)
His double slit experiment showed that light beams interfere, something that is possible only with waves.
TWO PARTICLES CANNOT BE IN THE SAME PLACE
AT THE SAME TIME!!
(Try the same experiment with cars...)

12. So, who is right? AND SO, FOR OVER ONE HUNDRED YEARS,
I STILL THINK I’M RIGHT!
I TOLD YOU!
AND SO, FOR OVER ONE HUNDRED YEARS,
LIGHT WAS GENERALLY ACCEPTED TO BE A WAVE.

13. James Clerk Maxwell, in the 1860’s, stated that light was a wave.

14. His equations described light as electromagnetic waves, produced by the oscillation of charged particles.

15. THE ATOM SHOULD NOT EXIST!
PROBLEMS THAT COULD NOT BE RESOLVED USING “CLASSICAL PHYSICS IN THE EARLY 20TH CENTURY.”
THE ATOM SHOULD NOT EXIST!
According to Maxwell’s equations, an electron oscillating around the nucleus should give off E/M radiation, losing energy, and eventually spiral into the nucleus!

16. 2. LINE SPECTRA
The fact that light emitted from a single element (e.g. hydrogen) appears not as a continuous spectrum, but as a combination of seemingly random lines?

17. Blackbody radiation issues!
3. THE “ULTRAVIOLET CATASTROPHE?
The predicted continual increase in radiated energy with frequency ("ultraviolet catastrophe") did not happen. Nature knew better.
Classical theory (Rayleigh-Jeans formula) suggested as (still matched at long wavelengths)

18. Still more issues! 4. HOW DO YOU EXPLAIN THE “PHOTOELECTRIC EFFECT?”
Predictions using classical physics are in sharp disagreement with the observations made when light hits the surface of a metal and emits electrons.

19. In 1900 Max Planck made a profound discovery in modern physics / Quantum Theory.
He showed (from purely formal / mathematical foundations) that light must be emitted and absorbed in discrete amounts (quanta) if it was to correctly describe observed phenomena (i.e. Blackbody radiation).
(Kept the part where higher frequencies meant higher energies.)

20. The constant of proportionality turns out to be a VERY IMPORTANT number in quantum physics. It is called
PLANCK’S CONSTANT E f
= h

21. E f
= h
h = kgm2/ s
or, x Js

22. E f
= h
It’s the “non-zeroness” of Planck’s Constant that rescues us from the inconsistencies of classical physics:
The existence of atoms
The spectra of atoms
The “Ultraviolet Catastrophe”
The Photoelectric Effect
h = kgm2/ s
or, x Js

23. E f
= h
The hf bundle of energy can only exist as nhf where n is a whole number. You can have multiples of hf (3hf, 817 hf, 670 hf, but you cannot have 0.5hf, 1.33 hf, 23, hf.
h = m2 kg / s
or, x

24. The Stability of Atoms
Electrons lose energy in the form of photons. Because photons must have specific energies, once e- is below the minimum energy to make a photon, it cannot lose any more energy.

25. 1905 Albert Einstein, used Planck’s idea to explain the Photoelectric Effect by proposing that light behaves as if it were a particle, in that light energy is concentrated in quantized bundles called PHOTONS.

26. In 1923, Arthur Compton showed what happens when “light” (in this case x-rays) interacts with electrons.
Historically, the “Compton Effect” was for many old-school physicists the final convincing evidence for the reality of quanta.
1. Classical physics predicts that the electron should absorb energy from from the light wave, then re-emit at the same frequency.

27. 2. Experiment shows that the light scatters
off the electron with a lower frequency- just as if the light were a beam of particles that interacts with electrons in the same way that two billiard balls collide.
An incoming photon bounces off an electron, giving up some of its energy and lowering the frequency.

28. A QUANTUM QUANDRY?
IF LIGHT CONSISTS OF PARTICLES, HOW CAN WE EXPLAIN THE RESULTS OF THE DOUBLE-SLIT INTERFERENCE EXPERIMENT?
WHICH SLIT DOES A PHOTON GO THROUGH?

29. If light truly behaves like a particle, then we would expect to find photons traveling straight paths through the slits,
thus forming two separate bright spots on the screen lining up with the slits.

30. Light (a continuous flow of photons), in this case monochromatic green, produces a characteristic interference pattern on a screen.
Interference pattern
green light.

31. This is essentially what Young did in the 19th century, and what I did as a demonstration using red laser light.
Interference pattern
green light.

32. But, if we block one of the slits…
THE INTERFERENCE PATTERN DISAPPEARS!
Interference pattern
green light.

33. HOW DID THE PHOTONS GOING THROUGH THE OTHER SLIT “KNOW” ABOUT THE FIRST SLIT BEING CLOSED?
green light.

34. THE INTERFERENCE PATTERN DISAPPEARS AS WELL!
Strangely enough, if we put photon detectors at each slit, to “catch” photons in the act of going through,
THE INTERFERENCE PATTERN DISAPPEARS AS WELL!
green light.

35. If we “dim” the light, so low, that only one photon at a time an interference pattern gradually builds up!
Interference pattern
A single photon of green light.

36. Which slit will I go through?
Interference pattern

37. Even if light is dimmed so much that one photon per day were to be released, an interference pattern would still emerge!
Watch as photons build up one at a time to eventually form an interference pattern!
I’ll go through BOTH!
Interference pattern

39. SO, IS LIGHT
A PARTICLE OR A WAVE?

40. IT’S BOTH!

41. IT’S BOTH!
IF YOU DON’T TRY TO DETECT IT, LIGHT ACTS LIKE A WAVE AND EXHIBITS INTERFERENCE EFFECTS.
THE BEHAVIOR OF WAVES IS GOVERNED BY MAXWELL’S EQUATIONS.

42. IT’S BOTH!
IF YOU DETECT LIGHT, IT BEHAVES AS IF IT CONSISTS OF PARTICLES.
THERE IS A RELATIONSHIP BETWEEN WAVE AND PARTICLE, BUT IT IS ONLY A STATISTICAL ONE.

43. THE PROBABILITY OF FINDING A PHOTON IS RELATED TO THE WAVE AMPLITUDE; THE STRONGER THE WAVE IS AT SOME POINT, THE MORE LIKELY YOU ARE TO FIND IT THERE.
SO, THE WAVE PICTURE PREDICTS THAT WAVES SHOULD BE STRONG AT CERTAIN POINTS ON THE SCREEN IN A TWO-SLIT EXPERIMENT, AND QUANTUM PHYSICS PREDICTS THAT’S WHERE YOU ARE MOST LIKELY TO DETECT PHOTONS.

44. HOW CAN IT BE? THERE’S NO CONTRADICTION,
SAYS NEILS BOHR’S PRINCIPLE OF COMPLEMENTARITY.
WAVE AND PARTICLE ASPECTS OF LIGHT ARE COMPLEMENTARY.”

45. AND SO, WE HAVE THE ORIGINS OF QUANTUM PHYSICS, ALL BROUGHT ABOUT BY RESOLVING THE QUESTION, “IS LIGHT A WAVE OR A PARTICLE?”
CREDITS GO TO:
Neils Bohr
Albert Einstein
Max Planck
Wolfgang Pauli
Werner Heisenberg
Erwin Schrodinger

46. AND SO, WE HAVE THE ORIGINS OF QUANTUM PHYSICS, ALL BROUGHT ABOUT BY RESOLVING THE QUESTION, “IS LIGHT A WAVE OR A PARTICLE?”
CREDITS GO TO:
Max Born
Paul Dirac
Louis deBroglie

49. There are five main ideas represented in Quantum Theory:
Quantum physics is a branch of science that deals with discrete, indivisible units of energy called quanta as described by the Quantum Theory.
There are five main ideas represented in Quantum Theory:
Energy is not continuous, but comes in small but discrete units.
The elementary particles behave both like particles and like waves.
The movement of these particles is inherently random
It is physically impossible to know both the position and the momentum of a particle at the same time. The more precisely one is known, the less precise the measurement of the other is.
The atomic world is nothing like the world we live in.