Reflection, Refraction, and Dispersion
Math of 3D waves….
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I remember a lot of this stuff from physics, but I think we did
most of it really qualitatively and without a ton of math, I
remembered those equations in the book, but is there more
math to go along with optics this time around?
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Nope. Seemed pretty easy to follow. I didn't know that the
diameter of the hole actually affected the light. I thought that
was cool. I want to play with that.
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We won’t use as much math as you will in Physics 441/442 or 471, but
we will probably use more than you did in high school.
We’ll do several demos with this, but you will have to wait a couple of
weeks.
Oooh! In the corner of one page it said to ask about why red-eye
happens in photos, and I actually know that one! It's because
the large amount of light bounces off the back of your eye
because it can only absorb so much light at once. It happens
with animals too, but different colors show up because they
have extra layers in their eyes that humans do not.
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Cool!
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A beam of light passes through a hole of
diameter d in a metal plate. Under what
condition are we allowed to ignore the
diffraction or “spreading” of the light?
Answer: when lambda << d
Ray Approximation
lambda = d
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lambda = d/4
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lambda = d/10
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What is the physical reason for diffraction? I
understand what happens, just not why.
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Basically the wave interferes with itself. This is on
the big questions we will try to answer in this unit
and the next.
Why does the diameter of the hole matter?
I've always wondered why light moves slower when
it's not in a vacuum.
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Light is an electromagnetic wave, i.e., an
oscillating electric field.
A “waving” electric field causes the electrons in
the matter to wiggle.
The wiggling electrons generate a new oscillating
electric field.
The actual observed wave is the superposition, i.e.,
sum, of these two waves.
The superposition of these two waves have the
same frequency, but different wavelengths!
Different
wavelengths
have different
speeds!
Dispersion!
Blue
Green
Red
Which color of light travels fastest in glass?
a.
b.
c.
d.
Red
Green
Blue
All the same
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Light ray at boundary
fast light
(smaller n)
slow light
(larger n)
“Normal” line.
The normal line is
the line that is
perpendicular to the
surface. This is
always unique.
fast light
(smaller n)
slow light
(larger n)
Advertisement for Physics 442/471
top
bottom
I send white light into a prism as shown below
(n>1). Will the red part of the “rainbow” be on
the top or the bottom of the outgoing fan of light?
a. Top
b. Bottom
Discuss with your neighbor:
In the movies, you sometimes see an actor looking
in a mirror and you can see a front view of
his/her face in the mirror. During the filming of
such a scene, what does the actor see in the
mirror?

Answer: The camera!
“Time-reversal symmetry”
fast light
(smaller n)
Reflection occurs off of any
boundary, not just “mirrors”
slow light
(larger n)
When will you have no reflection?
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If near perpendicular (1D Problem)
Look familiar?
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For arbitrary angle (not needed for HW/exam)
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Reflection/Refraction using water-soluble oil
“Blackboard Optics”