Dual Nature of Light

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Dual Nature of Light
Is light a wave or a particle?
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Wave Properties

• -Diffraction
• -Interference
• -Polarization

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Diffraction
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Constructive Destructive Interference
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Polarization
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Energy
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Wave E increases with A2/I2.
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Studies of Wave E

• Planck color (f, l) vs. T.
• As T inc. , f inc, (l decr)

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Radiation TemperatureHot Objects Emit Waves
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Intensity/Brightness
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Classical physics could not accurately predict f
vs. Temperature
Problem
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Max Planck related f to T. Light (EM) E, is
quantized--it can only take on certain whole
number values. E comes in little "chunks" of f
x a constant now called Planck's constant, h

• EM radiation waves
• chunks
• quanta
• photons

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Can calculate E in EM waves units quanta or
photons based on frequency.

• E hf.
• h is Planks constant 6.63 x 10-34 Js.
• E is energy in Joules
• f is frequency of radiation

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Show that if E hf,E hc. l

• For waves, v fl.
• Rearrange f v/l.
• Vacuum/air EM v c (3 x 108m/s).
• f c/l.
• E hf f c/l.
• E hc. l

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Ex 1. Each photon of a certain color light has an
energy of 2.5 eV. What is the frequency of and
color of the light?
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SolutionE hf f E/h convert eV to
Joules.(2.5 eV)(1.6 x 10-19J/eV) 6.03 x 1014
Hz 6.626 x 10-34 J s
Green Light
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2. The energy of a certain photon is 2.9 eV. What
type of wave is it? Be specific.

• 2.9 eV x 1.6 x 10-19 J 4.64 x 10-19 J.
• eV

E hf
(4.64 x 10-19 J) (6.63 x 10-34 Js) f
Violet Light
f 7 x 10 14Hz
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Finish Ex Sheet

• Hwk Text Read 830 833 Do pg 833 1-4 and 839
  2, 856 2-4, 9.

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Do Now.

• Planks Formula Sheet from yesterday.
• Solve problem 2. Show work

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Do Now A photon of light has energy 2.072 eV.
What color is it?

• 2.072 eV (1.6 x 10-19 J/eV).
• 3.3152 x 10-19 J
• E hf.
• (3.3152 x 10-19 J) (6.63 x 10-34 Js)f
• f 5.00 x 1014 Hz.
• Orange

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So Energy of EM Radiation comes in chunks, E
hf, maybe its not waves.

• All objects above 0 K radiate EM waves as E.
• Hotter more total E higher freq. (different
  color)
• Energy quantized, E hf (J).

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Other evidence

• Photoelectric effect.

http//phet.colorado.edu/en/simulation/photoelectr
ic
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More evidence for E particlesPhotoelectric Effect
When EM waves shine on a metal surface, the E in
wave may be absorbed by e- in metal. It may have
enough E to kick out surface e-. Materials
that emit e- are photoemissive. The ejected e-
are called photoelectrons.
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Phet observations
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http//phet.colorado.edu/en/simulation/photoelectr
ic

• Use higher Amplitude/Intensity/brightness
• more e- fly off w same speed.
• Current increases (A, C/s)
• Increased f
• e- fly off faster w higher KE.

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Classical Mechanics cannot explain why increasing
A or exposure time does not increase
photoemission. After all

• Boat would be tossed higher faster with
  increased wave amplitude.
• But ejected e- not faster.

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Einstein EM wave E is quantized photons.

• The collision of a photon with e- causes e-
  ejection.
• Increasing f, increases E (p) of each photon, so
  photoelectron has more KE (faster)
• Increasing Intensity (A) increases number of
  photons hitting more e- so more fly out higher
  current!
• Envision EM as little chucks. High f are heavier.

• http//phet.colorado.edu/en/simulation/photoelectr
  ic

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Photoemission only works with metals with weakly
bound e-.

• Photo-emissive metals have
• Threshold Frequency fo.
• Work Function, Wo.

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Threshold frequency fo lowest f that will free
an e-. Light frequencies below the fo eject no
e-, no matter how intense or bright the light.
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Light frequencies above the fo eject e-, no
matter how low the A (how dim).
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A metal has a threshold frequency fo in the blue
light range.

• What will happen if very bright red light is
  shone upon the metal?
• No e- will be emitted
• more e- will be emitted
• The emitted e- will have less energy.

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Einstein confirmed EM waves/photons have E hf.
Very high f give e- more KE. e- flies out
faster.
High f vs. Low f.
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2. A metal has a threshold frequency fo in the
blue light range.Predict what will happen to e-
if UV light is shone upon the metal?

• nothing
• b) the emitted e- will have more energy (KE)
• c) more e- will be emitted with the same energy.

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Increasing the I/A/brightness, increases the
number of photonsincreases rate of e- emission
- the current more e- ejected, but each e-
wont gain any extra E/speed.
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A metal has a threshold frequency in the blue
light range.

• 3. What will happen to photo e- if the blue light
  is made twice as bright?
• nothing
• b) the emitted e- will have more energy (KE)
• c) more e- will be emitted with the same energy.

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Energy Frequency
EM waves can be described as quanta or photons.
The E carried by photons is Ephoton
hf or Ephoton hc/l. (for photon traveling
at speed of light).This E can be absorbed by
photo-emissive materials.
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The min. frequency to free e- is fo.The min
energy needed to free an e- is called work
function Wo, or F.Metals have low Wo.
Wo hfo.
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If photon f is higher than fo.

• E photon greater than Wo.
• Any photon E left over after the work function,
  goes into KE of e-.

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4. A certain metal has a work function (Wo) of
1.7 eV. If photons of energy 3.0 eV are
absorbed by the metal

• a) No e- will be emitted at that energy.
• b) More e- will be emitted than would be at the
  Wo.
• c) Higher KE e- will be emitted than would be at
  Wo.

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Classical (wave) vs. Modern (particle) theory
different predictions

• Wave

• Photon Theory

• Metal needs time to absorb energy (like boiling
  water on a stove), eventually e- will be ejected.
• Higher amplitude/intensity waves (brighter), will
  give photo e- more E.

• Photons are particles that collide with e- so no
  time needed for e- to absorb E.
• High f photons have more E, ejected e- come out
  faster more KE.
• High amplitude/brighter more photons of EM so
  can eject more e- but with same E.

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Summary

• EM waves as chunks of energy/photons travel at c.
• Calculate the Energy J E hf, or
• E hc/l.
• Evidence for photons from Photoelectric Effect
  Experiment
• f not A responsible for KE of ejected e-.
• High f high E, photon.
• High A high number of photons.
• Photo-emissive materials have
• fo min f to eject e- (Hz)
• Wo min E to eject e- (J)

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Read Txt 834-837Photoelectric Effect
Questions
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Graph of Photoelectric Experiment

• KE of photoelectron vs. frequency.

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max KE of photo e- vs. f for metal. As f of EM
wave increases, KE increases, slope h. F (work
function), is minimum energy needed to eject e-.
Work function
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State the work function threshold frequency of
this metal
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5. A particular metal has a threshold frequency
fo, of 5 x 1014 Hz. What is its work function
Wo in J eV?

• Wo hfo.
• 3.3 x 10-19 J
• 2.07 eV

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• Ephoton hf is the total E available.
• Absorbed photon E splits between Wo KE photo
  e-, so total E of absorbed by e- is
• Epho Wo KE.
• The maximum KE of ejected e- is
• KEelc Epho Wo.
• Dont forget Wo hfo.

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6 Photoelectric EffectLight having f 1 x
1015 hz shines on a sodium surface. The
photoelectrons have a maximum KE of 3 x 10-19
J.Find the threshold frequency for sodium.
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Photon Photoelectron.Etot Wo KE.Etot
KE Wo.hf KE hfo.
fo (hfphoton KEmax) (h)
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change eV to Joules (1.86 eV) (1.6 x 10-19
J/eV) 2.85 x 10-19 J fo (hfphoton
KEmax)/(h) (6.63 x 10-34 Js)(1 x 1015 hz) -
(2.85 x 10-19 J) (6.63 x 10-34 Js) fo 5.5
x 1014 Hz. Below this frequency no electrons
will be ejected.
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In 1913-1914, R.A. Millikan did a series of
extremely careful experiments involving the
photoelectric effect. He found that all of his
results agreed exactly with Einstein's
predictions about photons, not with the wave
theory. Einstein actually won the Nobel Prize
for his work on the photoelectric effect, not for
his more famous theory of relativity.
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Some experimental results, like this one, seem to
prove that light consists of particles others
insist, that it's waves. We can only conclude
that light is somehow both a wave and a
particle--or that it's something else we can't
quite visualize, which appears to us as one or
the other depending on how we look at it.
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Reg Hwk Intro Photoelectric Effect Prac Packet

• Hwk Text 834 837
• Finish photo elec packet
• Do Regents Packet

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Light Fantastic BBC part 3 58 min

• http//www.youtube.com/watch?vVuGjo9oNqao

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Review of photoelec w german accent 4.11

• http//www.youtube.com/watch?vGpcWc5KLVRo
• Photoelectric Effect Explained 6 min
• http//www.youtube.com/watch?v0qKrOF-gJZ4

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Particle Properties of Waves extend to
conservation of energy and momentum.

• Photons may give up all or part of their energy
  in collisions, but the sum of the momentums and
  energy before must equal the sum after.

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Compton Effect
If light behaves like a particle, then a
collision btw photon e- should be similar to
billiard balls colliding. Photons must have
momentum (p), energy. In collision of photons
with particles (like e-), conservation of energy
conservation of momentum apply.
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If the photon gives only part of its energy
momentum to an e-, its momentum decreases after
the collision by the same amount as absorbed by
the electron. Therefore, the frequency or
energy of the photon decreases. The wavelength
increases. pbefore pafter.E photon before
KEelc after. E photon afterhfi KEelc
after hff photon after
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pphoton hf/c h/l. The wavelength of
the photon increases after collision.
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Matter has wave-like properties.1924 Louis
DeBroglie suggested that since waves had particle
properties, matter might have wave
properties.It turns out that matter does have
wave properties which are inversely related to
the momentum of the particle.
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For matter l h/p or l h/mv. Since
the mass of most objects is so large, the
wavelengths would be very small not
measurable.Electrons, however, do show
diffraction other wave characteristics.