Transcript Dual Nature of Light - Red Hook Central Schools

Do Now: List 2 wave behaviors of light and EM
radiation.
State the behavior that is unique to transverse
waves.
• -Diffraction
• -Interference
• -Polarization
Diffraction
Constructive &
Destructive Interference
Polarization
Particle Nature of EM Radiation and Light
Wave Energy vs Amplitude
Heat is E
Hot Objects Emit light of different colors
Intensity/Brightness
Classical physics could not predict
f vs. T
Max Planck related color / f to T.
EM E, is quantized—comes in chunks related to f /
color.
• EM chunks
• quanta
• Photons
Charge is also quantized. What is the
smallest amount of charge?
• 1e
• 1.6 x 10-19 C
• All charged objects have an integer x 1.6
x 10-19 C.
E in EM waves units quanta or photons
based on frequency.
E = hf.
h is Plank’s constant 6.63 x 10-34 Js.
E is energy in Joules
f is frequency of radiation
Show that if E = hf,
E = hc.
l
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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
1. Each photon of a certain color light has
an energy of 4 x 10-19 J. What is the
frequency of and color of the light?
• E = hf
f = E/h
• 4 x 10-19 J
6.63 x 10-34 J s
= 6.03 x 1014 Hz
Green Light
2. The energy of a certain photon is 4.64 x 10-19
J. What type of wave is it? Be specific.
E=
hf
(4.64 x 10-19 J) = (6.63 x 10-34 Js) f
f = 7 x 10 14Hz
Violet Light
• E = hf gives E in Joules.
• Often EM waves have very small energies.
• Define a new unit for very small Energy.
• eV from elementary charges in potential
differences.
Electron-volts, eV
• The electron-volt: tiny unit of work & E.
• The eV, is the work & E required to push 1 e- (or p+)
through a voltage of 1V.
• W or DPE = qV
• (1.6 x 10-19 C)(1V) = 1.6 x 10-19 J = eV.
• 1.6 x 10-19 J = eV
• Find in table
To find eV given elementary charges (e- or p+) &
voltage:
(# e )(# V ) = eV.
If 1 e- is pushed across 1V then (1e)(1V)=
1 eV of work is done e- gains 1 eV energy.
If a charge of 2e- is pushed across a 1V pd then (2e )(1V)
= 2eV.
If 2e- pushed across 6V then work is 12 eV.
16
What if 3e- move across 12 V?
36 eV
To find eV (# elm charges) (voltage)
17
3. How many joules of energy are
represented by 6.9 x 1029 eV.
• 6.9 x 1029 eV x 1. 6 x 10-19 J. = 1.1 x 1011 J
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eV
4. A field does 3.3 x 10-7 J of work on an
e-. How many eV is that?
• 3.3 x 10-7 J x 1eV = 2.1 x 1012 eV
•
1.6 x 10-19 J
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Finish Ex Sheet
• Hwk : Text Read 830 – 833 Do pg 833 #1-4
and 839 #2, & 856 # 2-4, 9.
Do Now.
• Plank’s Formula Sheet from yesterday.
• Solve problem 2. Show work
Do Now: A photon of light has energy =
2.072 eV. What color is it?
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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
Planck EM wave E = hf
Comes in chunks
• All objects above 0 K radiate EM waves as E.
• Hotter = more total E And higher freq. (different color)
• Energy quantized, E = hf (J).
More evidence for E particles
Photoelectric Effect
EM waves shine on a metal surface, the EM E
may be absorbed by e- in metal.
Photon E may knock out e-.
Materials that emit e- are photoemissive.
The ejected e- are called photoelectrons.
• 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.
http://phet.colorado.edu/en/simulation/photoelectric
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.
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/photoelectric
Photoemission only works with metals with
weakly bound e-.
Photo-emissive
metals have:
• Threshold
Frequency fo.
• Work Function,
Wo, f.
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.
Light frequencies above the fo eject e-, no
matter how low the A (how dim).
A metal has a threshold frequency fo in the
blue light range.
1. What will happen if very bright, high
intensity red light is shone upon the
metal?
a) No e- will be emitted
b) more e- will be emitted
c) The emitted e- will have less energy.
High f vs. Low f.
Einstein confirmed EM waves/photons
have E =hf.
Very high f give e- more harder bump more KE.
e- flies out faster.
*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?
a) nothing
b) the emitted e- will have more energy (KE)
c) more e- will be emitted with the same energy.
I/A/brightness, increases the number of photons
increases rate of e- emission - the current;
more e- ejected, but each e- won’t gain any extra
E/speed.
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?
a) nothing
b) the emitted e- will have more energy (KE)
c) more e- will be emitted with the same KE.
This is a higher photocurrent
Review: E & f
EM waves can be described as quanta or photons. The E
carried by photons or waves is:
Ephoton = hf or
Ephoton = hc/l.
This E can be absorbed by photo-emissive materials.
If the photon has enough E, an e- might be ejected.
The min. frequency to free e- is fo.
The min energy needed to free an eis called work function Wo, or F.
Metals have low Wo.
Wo = hfo.
If photon f is higher than fo.
• E = hfo photon has greater E than Wo.
• Any photon E left over after the work
function, goes into KE of e-.
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.
Classical (wave) vs. Modern (particle) theory
different predictions
• Wave
• Photon Theory
• Metal needs time to
• Photons are particles that
absorb energy (like
collide with e- so no time
boiling water on a stove),
needed for e- to absorb E.
eventually e- will be
• High f photons have more
ejected.
E, ejected e- come out
faster – more KE.
• Higher
• High amplitude/brighter
amplitude/intensity waves
= more photons of EM so
(brighter), will give photo
can eject more e- but with
e- more E.
same E.
Summary:
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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)
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
Read Txt 834-837 and watch youtube clips
on webpage
Photoelectric Effect Questions
Answer questions 856 #10-11, 14-18.
• 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.
• Don’t forget Wo = hfo.
6: Photoelectric Effect:
Light 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.
Photon Photoelectron.
Etot =
Wo + KE.
Etot – KE = Wo.
hf – KE = hfo.
fo = (hfphoton – KEmax)
(h)
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.
Graph of Photoelectric
Experiment
• KE of photoelectron vs. frequency.
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
Example:
For the metal below, state the:
1.
Threshold frequency
2.
Work function in Joules.
3.
Sketch the curve for a metal with a work function of 1.4 eV.
4.
Calculate the threshold frequency for the second metal.
eV
0.4
0.0
0.4
0.8
1.0
1.2
1.4
2
3
4
5
x 1014 Hz
6
7
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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.
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.
Reg Hwk Intro Photoelectric Effect Prac
Packet
• Hwk Text 834 – 837
• Finish photo elec packet
• Do Regents Packet
Light Fantastic BBC part 3
58 min
• http://www.youtube.com/watch?v=VuGjo9
oNqao
Review of photoelec
• Photoelectric Effect Explained 6 min
• http://www.youtube.com/watch?v=0qKrOF
-gJZ4
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.
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.
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 after
hfi = KEelc after + hff photon after
pphoton
= hf/c =
h/l. The
wavelength of the photon increases after
collision.
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.
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.