phy1161 18 handout internal reflection...
TRANSCRIPT
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Total Internal Reflection, Dispersion,
Polarization
• section 26-8
Physics 1161: Lecture 18Total Internal Reflection
normal
θ2
θ1
n2
n1
Recall Snell’s Law: n1 sin(θ1)= n2 sin(θ2)
(n1 > n2 ⇒ θ2 > θ1 )
θ1 = sin-1(n2/n1) then θ2 = 90
θc
θi
θr
“critical angle”
Can the person standing on the edge of the pool be prevented from seeing the light by
total internal reflection ?
1) Yes 2) No
Preflight 18.1
10
58% 42% 8
Indices of Refraction
What is the critical angle of a
diamond-air boundary?1 2
1
sinc
n
nθ − =
Internal Reflection in Diamond
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The diagrams show incident rays approaching a
boundary with a second medium. The relative indices
of refraction of the two media are indicated. In which
diagram will total internal reflection occur, providing
the angle of incidence exceeds the critical angle?
1 2 3 4
0% 0%0%0%
1. Diagram A
2. Diagram B
3. Diagram C
4. None of these
Fiber Optics
Dispersion
prism
White light
Blue light gets deflected
more
nblue > nred
The index of refraction n depends on color!
In glass: nblue = 1.53 nred = 1.52
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Skier sees blue coming up from the
bottom (1), and red coming down from
the top (2) of the rainbow.
Rainbow: Preflight 18.5
Which is red?
Which is blue?
Blue light is deflected more!
Alexander’s Dark Band
• Sky is light inside primary
rainbow
• Dark between primary
and secondary bows
• Light beyond the
secondary rainbow
• Dark region between is
called Alexander’s Dark
Band
22o Halo
• A halo is a ring of light surrounding the sun or moon.
• Most halos appear as bright white rings but in some instances, the dispersion of light as it passes through ice crystals found in upper level cirrus clouds can cause a halo to have color.
Randomly oriented hexagonal ice crystals
Diameter less than 20.5 micrometers
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Sundogs
• Sundogs or parhelia
on right and left of
sun
Flat faces horizontally oriented
Hexagonal ice crystals
Diameter greater than 30 micrometers
Polarization
• Transverse waves have a polarization
– (Direction of oscillation of E field for light)
• Types of Polarization
– Linear (Direction of E is constant)
– Circular (Direction of E rotates with time)**
– Unpolarized (Direction of E changes randomly)
x
y
Linear Polarizers
• Linear Polarizers absorb all electric fields
perpendicular to their transmission axis.
Molecular View (link)
http://www.colorado.edu/physics/2000/index.pl?Page=polarization/blocking_light.html
Unpolarized Light on
Linear Polarizer
• Most light comes from electrons accelerating in random
directions and is unpolarized.
• Averaging over all directions: Stransmitted= ½ Sincident
Always true for unpolarized light!
Polarizing sunglasses are often considered to be better than tinted glasses because
they…
Preflight 18.3, 18.4
• block more light
• block more glare
• are safer for your eyes
• are cheaper
21
27%
62%
9%
1%
Polarizing sunglasses (when worn by someone standing up) work by absorbing light
polarized in which direction?
• horizontal
• vertical
60%
40%
Brewster’s angleReflected light is partially polarized (more horizontal than vertical). But…
horiz. and vert.
polarized
θB θB
90º-θB
90º
horiz.
polarized
only!n1
n2