Welcome to PH 103

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Welcome to PH 103!

• Quick skim through syllabus

Announcements

• Labs begin week two, Science Rm 119
• New lab section 1230-2220 Tues.
• Due to Easter break first labs as follows
• Mon., Tues. meet Mar. 17, 18 (week 2)
• Wed., Thurs. meet Mar. 26, 27 (week 3)
• If not on my list, come after class so I can
  enter you into Qwizdom and WebAssign

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Biographical Info

• Started at Augie 96-98
• Finished B.A. at University of Chicago 01
• Completed Ph.D. at Cornell University last spring
  in optics (ultrafast/nonlinear optics, biomedical
  imaging) mpm pic. next slide
• Hobbies triathlon
• Just had baby in January! Eloise

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Typical histology vs. nonlinear microscopy
Multiphoton image of a follicle within an
unstained, intact ovary from mouse.
Autofluorescence (green) derives from NAD(P)H and
retinol within the tissue. SHG (red) deliniates
the bursa.
HE stained histology of sliced ovary.
Image from Blue Histology, Western University of
Australia
Image from Webb Lab and Nikitin Lab, Cornell
University
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PH 103
Light and Index of Refraction
Dr. James van Howe Lecture 1
March 10, 2008
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Qwizdom Trivia Bonus
What was the first movie where Al Pacino and
Robert Deniro shared a scene?

• Goodfellas
• Godfather Part II
• Heat
• Scarface

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Visible light travels more slowly through an
optically dense medium than through a vacuum. A
possible explanation for this could be that the
light

• Is absorbed and re-emitted by the atomic
  structure of the optically dense medium
• Is absorbed and re-emitted by the nucleus of the
  optically dense medium
• Bounces around randomly inside of the optically
  dense medium
• Loses amplitude as it passes through the
  optically dense medium

Problem 71. from MCAT practice test 9
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Light is an EM wave
Distance
-The electric field propagates through space (so
does magnetic field) like a wave on a string
traveling wave
-Just like string, pieces of the field arent
propagating, they move up and down at the right
time like bits of a string (or people in a
stadium wave). It is really just the disturbance
that propagates.
-Weird thing about fields is we need no string.
The disturbance travels through a vacuum.
-Scientists used to think all space was filled
with a medium they called Ether Michelson and
Morley disproved its existence in 1887 Einstein
figured out why this makes sense in 1905- special
relativity
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EM Waves
Max 1
Max 2
x
distance
, wavelength (spatial period)
If we sit at the x at wait until the field goes
max again, thats one period, T
Frequency (which we perceive as color for visible
EM waves)
Velocity is just the distance between maxes,
divided by the time it takes for max 2 to arrive
at x
In a vacuum for all EM waves,
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Graphical display of traveling waves
Plane Wave
Side view
crest
valley
Arial view
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Graphical display of traveling waves
Spherical wave (point source)
Arial View
Like dropping a stone in a pond
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EM Spectrum
Cell phone 1 GHz or 109 Hz
Visible 400-700 nm
Bluer means higher-frequency, shorter wavelength
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What are the funny arrows?
Arrows represent field lines, their direction
indicates polarization of the wave (more on that
later) length indicates strength
q
Positive test charge in the path of an EM wave
Mass on Spring
Does this motion/system remind you of anything
from PH 101?
SHM
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But Where did we get this wave to begin with?
EM waves like light are created by accelerating
charge
nucleus
Electron, -q
m
To create the wave like the one above, we need
the electron to move up and down in SHM. Could
have been created by pulling on the electron in
an atom (like to another orbital), and letting go.
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EM waves interacting with matter
nucleus
Electron, -q
m
Driven mass-on-a-spring is a simple but fantastic
model for EM wave hitting molecules of
non-conductors like glass, pure water, plastic
Electric field drives electron up and down at the
frequency of electric field. If you use blue
light, electron goes up and down at blue
frequency.
If the atom is driven at its resonant frequency
it absorbs lots of energy (Ex. Tacoma narrows
bridge child on a swing).
Video Demo glider track resonance
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Absorption
At resonance most of an EM wave (or any wave) is
absorbed by the system energy is given up as
heat
Your microwave oven makes great use out of this.
Water molecules have strong resonances at
microwave frequencies. The electrons shake like
hell, and the energy is converted to heat.
Thats why your food gets hot (it is made mostly
of water) and your plate doesnt.
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Index of refraction
Index of refraction is a measure of the delay in
response of the electron to the driving EM wave
nucleus
Electron, -q
m
The electron is not going to instantaneously move
up and down in response to the field. Nothing is
instantaneous in real life.
The extra-time it takes to move the mass of the
electron effectively slows down the speed of the
EM wave.
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Index of refraction
Some people think of this delay in the response
as the light being absorbed by the atom (not lost
as heat) and then re-emitted
nucleus
Electron, -q
Takes time to move the mass
A chemist might draw the process like this
Virtual Excited State
Jablonksi Energy Diagram
ground
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Index of refraction
We quantify the index of refraction, n, as the
ratio of speed the light in a vacuum to its
slower speed in the material
Always larger than 1.0 since nothing is faster
than light (more on that later with relativity)
Examples
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In a dense material, I have more electron mass to
move up and down
Therefore in a denser material, what happens to
the index of refraction?

• The index is lower since the velocity is faster
• The index is lower since the velocity is slower
• The index is higher since the velocity is faster
• The index is higher since the velocity is slower

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Dense material
-If the material is dense, the bound electrons
have more inertia
-More inertia means harder to get moving
-Hard to get moving means more delay, slower
velocity
-Slower velocity means larger index,
Example, glass
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Refraction
Refraction bending of a wavefront at a boundary
between two media, due to the change in velocity
Normal Line
Incoming wave
Air,
Water,
Slower velocity, higher index
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Which way does it bend?
-From fast to slow medium (high index), towards
normal
-From slow to fast (lower index), away from the
normal
Firm Ground
Muddy Mess
Slower medium, higher index
Helpful Analogy wheel on an axel
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Wavelength change
Frequency of wave does not change across the
materials
If in order for frequency to
stay same
Helpful Analogy Marching band
Firm Ground
Muddy Mess
Slower medium, higher index
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Homework on WebAssign
Ch22 problem 7 Ch23 problems 27,
36 Corresponding Reading 22-3, 11-14 23-4,
23-5, 23-6