1. The Wacky World of Quantum Physics
Or Waves acting like Particles?

2. Quantum Theory in General
Applies to subatomic particles (protons, neutrons electons, etc.)
States that
all things have both a wave and particle nature
Energy comes in small packets called quanta rather than being continuous
Cannot know both the position and momentum (mass x velocity) of a particle to 100 % certainty (Heisenberg Uncertainty Principle)
Today will show light has a particle nature
Questions: In general, what does the quantum theory say about subatomic particles? What is a quantum? What is the quantum of energy?
Quantum of energy=photon
Quantum of charge= electron
Quantum of gravity=graviton
Quantum of people = person
Cannot know position and momentum because experiment to find one affects other.
Difficult Questions:
1) A red and blue beam of light are of the same intensity 9brightness). Which contains the greater number of photons?
2) Which color light is used in photography darkrooms and why?

3. The Photon Particle or packet of light Massless particle
Different colors have different amounts of energy/photon
E=hf where h=6.6x10-34J s=4.14 x eV (Planck’s constant)
Red light has less energetic (smaller) photons than blue light.
Questions: What is a photon? What do we mean when we say a photon is a quantum of energy or light? Do all photons contain the same energy? Does the photon fit better with the particle or wave nature of light? Write the equation relating the energy of a photon to its frequency.
Difficult Questions:
1) A red and blue beam of light are of the same intensity 9brightness). Which contains the greater number of photons?
2) Which color light is used in photography darkrooms and why?

4. Check Question
Two lasers have exactly the same energy in their beams. One laser is blue and the other is red. Which beam has the most photons?
A) Red beam
B) Blue beam
C) Both beams have the same number of photons

5. Photoelectric Effect
Photons below threshold frequency of the metal do not cause photoelectrons to be produced no matter the intensity
Photons at the threshold frequency of the metal do cause photoelectrons to be produced in proportion to the intensity
Photons higher than the frequency of the metal cause photoelectrons to be produced with greater KE
Above can be explained if photons of light are colliding w/ metal=>light is a particle
Question: What results when light is shown on a photoelectric metal that is a) below the threshold frequency; b)at the threshold frequency c) above the threshold frequency? Is this photoelectric effect evidence of the particle or wave nature of light?

6. Check Question
A piece of metal gives off photoelectrons when light of frequencies higher than blue light is shone on it. Which of the following will give off the most photoelectrons?
A) Bright violet light
B) Dim violet light
C) Dim UV light
D) Dim X ray light

7. Compton Effect
Measured energy of light before and after hitting metal finding the energy decreased
Measured energy of photoelectron and found it equaled lost energy of light
Shows energy is conserved
Also showed momentum is conserved
Energy and momentum conservation are particle properties=> light is a particle
KE 1
KE 3
KE 2
KE 1 = KE 2 + KE 3

8. Blackbody Radiation
All objects above absolute zero emit light at all frequencies
Classical mechanics predicted that intensity of re-emmited radiation should increase with frequency w/o limit
Experiment disagreed
Viewing energy as packets (particles) can explain graph
Classical theory predicted that the longer the wavelength (lower the frequency) the fewer modes of vibration and energy release that were possible. Think of a small particle. It can release energy by resonating at its natural frequency but also at lower frequencies which are whole multiples of that frequency. Large particles will also resonate at their natural frequency but since this frequency is lower there are fewer frequencies which are whole multiples of that frequency (they are also further apart on the frequency chart).
Remember resonance goes from high to low
For example particles that vibrate at 10 hz can also be made to vibrate at 5 hz, 2 hz and 1 hz. Particles w/ a nf of 5 hz also vibrate at 1 hz, and particles which vibrate at 1 hz only vibrate at that frequency.
BUT IT DOESN’T WORK in Experiment
Theres a prefered frequency for every temperature like only certain size particles are vibrating.
Einstein says this is another case where energy (EM radiation is acting like particle and only certain size packets are exciting the atom

9. Partial Development of Film
Partial development of photographs
As expose photo we see individual light particles causing developing rather than wave developing entire picture at once
Light is acting like a particle

10. Light is both a particle and a wave
Acts like wave when travelling through vacuum or vacuum between particles
Reflection
Refraction
Interference
Diffraction
Acts like particle when interacting with matter
Photoelectric effect
Compton Effect
Black Body Radiation
Partial developing of film

11. Ultraviolet Catastrophe
As heat material it will glow different colors
Red->Orange->Yellow
Colors correspond to light being put out at higher frequencies
No matter how hot you get little or no blue or ultraviolet light (ultraviolet catastrophe)