1. 6.1.1 Photons, Photoelectric Effect, and Particle Nature of Light
Did the Greeks Invent Everything?!?
6.1.1 Photons, Photoelectric Effect, and Particle Nature of Light

2. 585 BCE – 1900 CE
Beginning with the Greeks and continuing until around 1900, the system of “Classical Physics” slowly emerged with these fundamental ideas at its core…
Newton’s Laws define mechanics
Light is a WAVE
Atoms make up matter and are indivisible
In around 1900, physicists were announcing that physics is a COMPLETE SCIENCE and can explain EVERYTHING!

3. Well… almost everything…
Physicists in this era were still unable to explain THREE important phenomenon
Blackbody Radiation
Wave Theory: A blackbody will emit radiation that directly relates to its temperature. The higher the temperature, the more frequencies can be emitted.
Experiment: The intensity of radiation does not keep increasing but forms a curve of intensities below the expected amount.

4. Well… almost everything…
Physicists in this era were still unable to explain THREE important phenomenon
Bright Line Spectrum
Wave Theory: When you heat an element it should radiate a continuous range of colors related to that temperature (spectrum).
Experiment: Each element emits only very specific colors in a ‘bright line spectrum’

5. Well… almost everything…
Physicists in this era were still unable to explain THREE important phenomenon
Photoelectric Effect
Wave Theory: Increasing intensity should give the same number of faster electrons. Increasing frequency should give more electrons per second.
Experiment: Increasing intensity gives more electrons per second. Increasing frequency gives faster electrons or makes the effect stop working.

6. Planck’s Constant
Max Planck invented the concept of the QUANTUM to explain the problem of Blackbody Radiation.
Derived the constant 6.63 x J·s to describe the scale of energy increments allowed in atoms.
Energy going into and out of atoms is QUANTIZED. By why?

7. Photoelectric Effect
Albert Einstein uses the concept of the quantum to explain the Photoelectric Effect.
Invents the idea of the PHOTON which has an energy related to Planck’s Constant and its frequency.
Equation

8. Einstein’s Model
So, we have phenomenon that show that
light MUST be a wave…
ex. DIFFRACTION and INTERFERENCE
…but the PHOTOELECTRIC EFFECT shows that light
MUST be made up of particles…
SO WHICH IS IT?!?
metal plate
beam of light
a brighter beam would mean MORE photons
Each photon has a chance to liberate an electron
but the photon must have enough energy (the right frequency)
For Einstein’s explanation to be correct light is NOT a wave! It must be made of particles. The particles carry a certain amount of energy related to the light’s frequency.
EM Radiation (light) is neither waves nor particles but has properties of BOTH.

9. Photon Properties Massless Carry energy and momentum
Travel at the speed of light
Have particle-like collisions – COMPTON EFFCT
Before
After e-
electron
gains KE
electron
incoming
photon e- v
v = 0
outgoing photon
lower E, lower f longer λ

10. Try it before going to the side with answers!
Example #1
What is the energy of a photon with a wavelength of 4.5 x 10-7 meters? What type of photon is it?
Try it before going to the side with answers!

11. Example #1
What is the energy of a photon with a wavelength of 4.5 x 10-7 meters? What type of photon is it?
Ephoton = ?
λ = 4.5 x 10-7 m
c = 3 x m/s
h = 6.63 x J·s
Ephoton = hf = hc/λ
Ephoton = hc/λ
Ephoton =
(6.63 x J·s)(3 x 108 m/s) / (4.5 x 10-7 m)
Ephoton = 4.4 x J
Referencing the EM Spectrum table, we see that this photon’s wavelength corresponds to
Visible Light

13. Try it before going to the side with answers!
Example #2
If a photon has 7.0 x 10-3 joules of energy, what frequency does it have? What type of photon is it?
Try it before going to the side with answers!

14. Example #2
If a photon has 7.0 x 10-3 joules of energy, what frequency does it have? What type of photon is it?
Ephoton = 7.0 x 10-3 J
f = ?
h = 6.63 x J·s
Ephoton = hf = hc/λ
Ephoton = hf
f = (7.0 x 10-3 J)/
(6.63 x J·s)
f = 1.1 x 1031 Hz
Referencing the EM Spectrum table, we see that this photon’s frequency corresponds to
Gamma Ray

16. Wave/Particle Duality
1924 Louie de Broglie: because light has properties of particles, it might be possible for matter to exhibit the properties of waves.
This was proven to be true by ELECTRON DIFFRACTION experiments demonstrated by Davisson and Germer in 1927.
While particles do seem to have a particle nature, this is only evident for FAST-MOVING, SUB-ATOMIC PARTICLES.

17. ENERGY can be converted into MASS.
Mass/Energy Duality
In 1905, Einstein publishes his paper on Special Relativity.
One of several proposals from this paper is that: MASS can be converted into ENERGY
AND
ENERGY can be converted into MASS.
Equation

18. Mass/Energy Duality
This conversion from mass into energy is how an ATOMIC BOMB functions.

19. Try it before going to the side with answers!
Example #3
How much energy would be released if a 7.0 kilogram mass was completely converted into energy?
Try it before going to the side with answers!

20. Example #3
How much energy would be released if a 7.0 kilogram mass was completely converted into energy?
m = 7 kg
E = ?
c = 3.0 x 108 m/s
E = mc2
E = (7 kg)(3.0x108 m/s)2
E = 6.3 x 1017 J
(The amount of energy the entire USA uses in a day!)

21. End of PRACTICE