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Light & Matter

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Emma vL

on 10 August 2016

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Transcript of Light & Matter

Analogous to gravity
Models of light
Particle
Packets or (quanta) of energy called
photons
Wave
Every point spreads out in a wave
Youngs double slit experiment
Particle model prediction:
Wave model prediction:
http://www.walter-fendt.de/ph14e/doubleslit.htm
http://science.sbcc.edu/physics/flash/2%20slit%20interference.swf
Photoelectric effect
http://www.kcvs.ca/site/projects/physics_files/photoelectric/photoelectricEffect.swf
Gif
Example
A torch emits red light of wavelength 620 nm and power 2 mW.
What is the momentum of a single photon?
If the photon is reflected of a perfect mirror, what is its change in momentum?
The torch beam is focused onto the mirror, What is the average force on the mirror in 1s?
Checkpoints
623-626, 635-636, 639-640, 645, 654-655, 658, 679-680
Light emission spectrum
Light absorption spectrum
Hydrogen emission spectrum
Iron emission spectrum
Light travels at c in a vacuum
Single and double slit patterns
Effect of the ratio lambda/w
Diffraction
Path difference =
2
Path difference =
P.D
P.D
1st minimum
1st maximum
2nd minimum
2nd maximum
Central maximum
1st minimum
2nd minimum
2nd maximum
1st maximum
Calculate the path difference at each dark/light band shown
Example
Effect of wavelength, d and L
The
path difference
to the second maximum is 723 nm longer than the
path difference
to the first minimum.
What is the wavelength of the light used?
633-634, 641-644, 647, 649-650, 659, 662-665, 675-678, 685-688
Checkpoints
Checkpoints
694-695
617-622, 627-628, 636, 640-642, 644, 647-649, 650-651, 652-654, 656-657, 662-670, 677-680
Checkpoints
2014
Continuous supply of energy
Packets of energy
More intensity = more energy
More intensity = more photons
not
more energy
Energy related to frequency
only
Wave model prediction
Particle model prediction
The Ek of the electrons depends
only
on the frequency of light
Electrons will always be ejected, it will just take longer in dimmer light
The Ek of the electrons depends
only
on light intensity
No time delay
More intensity = more electrons emitted
Wave nature of electrons
Calculate the momentum of the electrons.

What is the energy in eV of the electrons?

Why can electrons of de Broglie wavelength 1.67x10 m be used for this experiment, rather than visible light of wavelength 400 nm?
Support your answer with calculations.
Electrons with a de Broglie wavelength of 1.67x10 m are used to investigate the spacing of atoms in a nickel crystal. The atomic spacing in the nickel crystal is 2.15x10 m.
Example
-10
-10
-10
tonight:
672-676
week:
rest
Checkpoints
Energy levels of atoms
de Broglie:
Electrons can only exist in orbits which form standing waves!!!!
Energy levels of atoms
END!!
"Physics makes us all its bitches''
- Of Montreal
Continuous supply of energy
More intensity = more energy
Packets of energy
More intensity = more photons
Energy related to frequency ONLY
contradictory models, so which is correct?
Wave properties
How many photons leave the torch each second?
Refraction of light
Particle properties
V
Electric fields
A pair of charged parallel plates have a potential difference of 10,000 volt between them, and are 3.0 mm apart.
d
Calculate the magnitude of the electric field between the plates (in V m-1)
Calculate the work done on an electron crossing from the negative plate to the positive plate. Give your answer in J and in eV
(As per sound)
Effect of wavelength & gap size single slit
-w
Gradient = h
x-int = f
y-int = -W
0
Ek = hf-W
equation of lines:
x-int = V
c
Light of wavelength 450 nm is used to eject electrons from potassium. The graph of current vs voltage is shown below.
What is the energy of a photon of 450 nm in eV?
What is the work function of potassium in eV?
Sketch the graph for what happens when:
a) the intensity is doubled
b) the wavelength is doubled
Example
What is the maximum kinetic energy of the photoelectrons?
6.7 eV
8.8 eV
8.8 eV
3.9 eV
What happens to an incident photon of 6 eV?
Which best represents the ‘standing wave’ state of an electron in an atom where the circumference is equal to four wavelengths?
Example
(electron is freee!)
Mercury atom energy levels
Hydrogen atom energy levels
A photon of wavelength 478 nm is emitted. Which two energy levels has the electron jumped between?
An electron is on the n=4 state. List the possible photon energies that could be emitted as it decays back to the ground state.
0 eV
10.2 eV
12.1 eV
12.8 eV
13.1 eV
13.6 eV
Light and matter

Each photon carries energy:
Constructive and destructive interference observed.
_
_
568μm
Hydrothermal worms are deep sea creatures, almost as small as bacterium, and are largely found near hydrothermal vents in the ocean.
Electron Microscope
Pollen from a variety of common plants: sunflower (Helianthus annuus), morning glory Ipomoea purpurea, hollyhock (Sildalcea malviflora), lily (Lilium auratum), primrose (Oenothera fruticosa) and castor bean (Ricinus communis). The image is magnified some x500, so the bean shaped grain in the bottom left corner is about 50 μm long.
Turns
out
we
need
both
to
explain
light
...
c =
f
wave equation:
f =
1
T
_
T =
1
f
_
Refraction
E = hf =
h = 6.63x10 J s
h = 4.14x10 eV
-34
-15
1 eV = 1.60x10 J
-19
Joules (J) and Electron volts (eV)
1 J = 6.24x10 eV
18
Two units for energy:
where h is Plancks constant:
low E
high E
incident photon
target
electron
scattered photon
recoil
electron
Compton scattering
momentum of photons:
p =
E
c
_
p = =
hf
c
_
h
_
* h in J s NOT eV s *
do eV later with photoelectric effect?
4.9 eV
Wave model:
Bend towards the normal as it slows down, wavelength decreases
Conservation of momentum:
A win for the wave model
A win for the particle model
photons have no mass but can transfer momentum!!
wave model wins round!
particle model KO'd.
Node (dark):
PD = (n- )
2
_
1
Anti-node (light):
PD = n
n=1,2,3...
n=0,1,2,...
x
_
_
L
d
_
know (qualitatively) what effect changing these variables has on the pattern spacing
Effect of wavelength & gap size
know (qualitatively) what effect changing these variables has on the pattern spacing
x
_
_
w
http://www.walter-fendt.de/ph14e/singleslit.htm
L
w
x
Diffraction: Single Slit/object
Double Slit
Blue Light
Red Light
* depends
only
on f
particle v
s wave
... which will win?
not on course
not on course
not on course
(or w) = less spacing)
(or w) = more spacing)
know (qualitatively) what effect changing these variables has on the pattern spacing
x
_
_
w
red spreads the most, blue the least
Can get pretty rainbows with white light :)
circular hole of width w
Object size (w) too small -> light diffracts around it so less reflects back. Cant see well with light!
So use electrons instead...
Ek = hf - W
Same spacing = same wavelength & momentum
de Broglie wavelength of matter:
:
= =
h
p
_
h
_
mv
_
NB: mass must be sub-atomic to see diffraction effects (ratio of lambda/w > 1) this is why we do not diffract
~
Diffraction of laser through circular hole
photon momentum & energy
Double slit exp.
Checkpoints:
Young's double slit experiment
Young's double slit experiment
Let's sort this s*** out.
The wave nature of matter:
All chapter 16
Any one ever wonder why it has to be like this?
GIF: http://www.colorado.edu/physics/PhysicsInitiative/Physics2000.03.99/images/sgt_gunn.gif
.
Electric Fields
Charged plates
The Electron Volt
Each photon carries energy:
h = 6.63x10 J s
-34
h = 4.14x10 eV s
-15
1 eV = 1.60x10 J
-19
Joules (J) and Electron volts (eV)
1 J = 6.24x10 eV
18
Two units for energy:
E = hf
where h is Planks constant:
* depends
only
on f
.
W = E =
qV
Ek = 0
EPE = qV = q
Ek = q
EPE = 0
V=1
-19
Kinetic energy gained by accelerating an electron across a potential difference of 1 V
Magnitude of Electric field
Must have Ek V eV to make it across
Potential difference of V volts b/n plates.
Fire electrons towards -ve plate:
F = qE
W = Ek = V eV
q = 1.60x10 C
Joules (J) and Electron volts (eV)
1 J = 6.24x10 eV
18
Two units for energy:
-19
Charge on electron/proton:
Ek = V
V = min voltage needed to stop it
c
c
(cut-off voltage)
More intensity = more electrons
higher freq = more Ek = higher cut-off voltage
max
The
minimum
energy needed to escape the metal
Depends only on type of metal
Work function (W)
E = hf
ph
Photon delivers a packet of energy to the electron.
The electron needs a
minimum
of W eV to escape the metal, the rest becomes Ek
max
Ek = V eV
max
c
c
.
Ek <-> EPE
Photoelectrons
V = stopping potential/cut-off voltage
c
E =hf = W
0
Min E needed to escape metal
Threshold frequency
hf < W so electrons can't escape metal
hf > W so electrons escape with E to spare
Graph 1:
Ek vs frequency
Graph 2:
Voltage vs Current
max
ph
Ek = 0
max
* use electron volts for this *
Recall:
NB: E is electric field here
NOT
energy
Electrons released by light energy are called
photoelectrons
(still just plain old electrons tho)
Ek -> EPE
Provides min E needed to escape metal (but nothing extra)
Threshold frequency
E =hf = W
0
ph
Ek = 0
max
http://hyperphysics.phy-astr.gsu.edu/hbase/tables/photoelec.html
Work functions of common metals:
* h in Js not eVs *
p = 2mE
k
http://theeternaluniverse.blogspot.com.au/2010/10/why-pigs-dont-diffract-through-doorways.html
Just passes through.
E = E = E - E
ph
atom
initial
final
c =
f
wave equation:
hc
_
_
E is
electric field
here
E is
energy
here
An electron accelerated across a P.D. of V volts gains V eV of kinetic energy
Ek << V
Ek < V
Ek <<< V
Ek <<<< V
Ek > V
Coherence of light
prolly won't turn up but just in case...
Light must be coherent (in phase) to start with to get interference patterns
Solution:
use lasers (already coherent)
put incoherent light through a single slit first
Sunlight:
many colours (
polychromatic
) and waves not in phase (
incoherent
)
With wavelength filter
Without wavelength filter
Incoherent
Coherent
Motion: relative motion
Prolly not but maybe sneakily on course
LED
: one colour (
monochromatic
) and waves not in phase (
incoherent
)
Laser
: one colour (
monochromatic
) and waves in phase (
coherent
)
Coherence of light
prolly won't turn up but just in case...
Polychromatic,
Coherent
Polychromatic,
Incoherent
Monochromatic,
Coherent
The evidence....
Wave

vs
Particle
It's on.
max
=
>
(min voltage needed to stop them)
Now increase V until all PEs stopped...
Must have Ek V eV to make it across
-
Ek = V eV
max
c
V = stopping potential/cut-off voltage
c
(min voltage needed to stop them)
Now increase V until all e stopped...
Ek <-> EPE
-
Image: Douglas Hofstadter
623-626, 635-636, 639-640, 645, 679-680
2015
2016
633-634, 641-644, 647, 649-650, 659, 662-665, 675-678, 685-690
2015
2016
diffraction
2015
2016
693-695
/
/
_
_
An
electron
accelerated across a P.D. of V volts gains V eV of kinetic energy
w
Single slit diffraction/interference!
< w
https://en.wikipedia.org/wiki/Huygens%E2%80%93Fresnel_principle
More:
https://en.wikipedia.org/wiki/Huygens%E2%80%93Fresnel_principle
More:
=
~
same medium => no effect on v, f,
The Electron Volt
Kinetic energy gained by accelerating an electron across a potential difference of 1 V
Ek = 0
EPE = qV = q
Ek = q = 1.6x10 J
EPE = 0
Potential Difference = 1V
i
i
f
f
= 1 eV
The Electron Volt
627-632, 637-638, 646, 651-657, 660-661, 666-674, 679-684,

Checkpoints
2015
What size gap would a 0.1 kg ball traveling at 10 m/s diffract through?
How fast do we need to fire a 10 kg cat at a 1 m wide slit to see it diffract?
Why don't cats diffract through doors then?
2 r = n
a
itute.com
lengageeducation.org.au
tsfx.com.au
vicphysics.org
vcaa.vic.edu.au

Wavelength decreases, velocity decreases, frequency stays the same
. .
Ek = V
Full transcript