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Physics 1161: Lecture 18, Slide 1
Total Internal Reflection, Brewster’s
Angle, Dispersion, Lenses
• Today’s Lecture will cover textbook
sections 26-5 – 26-
Physics 1161: Lecture 18 Exam
III
Physics 1161: Lecture 18, Slide 2
Total Internal Reflection
normal
θ 2
θ 1
n 2
n 1
Recall Snell’s Law: n 1 sin(θ 1 )= n 2 sin(θ 2 )
(n 1 > n 2 ⇒ θ 2 > θ 1 )
θ 1 = sin-1(n 2 /n 1 ) then θ 2 = 90
θc
Light incident at a larger angle will only have reflection (θi = θr)
θi^ θr
“critical angle”
Physics 1161: Lecture 18, Slide 3
Can the person standing on the edge of the pool be prevented from seeing the light by total internal reflection?
- Yes 2) No
Preflight 18.
Physics 1161: Lecture 18, Slide 4
ACT: Refraction
• As we pour more water into bucket, what
will happen to the number of people who
can see the ball?
1) Increase 2) Same 3) Decrease
Physics 1161: Lecture 18, Slide 5
Fiber Optics
At each contact w/ the glass air interface, if the light hits at greater than the critical angle, it undergoes total internal reflection and stays in the fiber.
We can be certain that ncladding < ninside
ninside
Add “cladding” so outside material doesn’t matter!
ncladding
noutside
Physics 1161: Lecture 18, Slide 6
Brewster’s angle
…when angle between reflected beam and refracted beam is exactly 90 degrees, reflected beam is 100% horizontally polarized!
Reflected light is partially polarized (more horizontal than vertical). But…
horiz. and vert. polarized
θθ θθB θθθθB
90º-θθθθB
horiz. polarized only! n^1 n 2
tan θB =
Physics 1161: Lecture 18, Slide 10
Preflight 18.
Wow look at the variation in index of refraction!
Where is red? Where is blue?
Physics 1161: Lecture 18, Slide 11
Flat Lens (Window)
n 2 n 1
Incident ray is displaced, but its
direction is not changed.
←t→
θθ θθ 1 θθ θθ 1
If θθθθ 1 is not large, and if t is
small, the displacement, d,
will be quite small.
d
Physics 1161: Lecture 18, Slide 12
Converging Lens
All rays parallel to principal axis pass through focal point F. (^) Double Convex
P.A.
Preflight 18.6 F
A beacon in a lighthouse produces a parallel beam of light. The beacon consists of a bulb and a converging lens. Where should the bulb be placed?
nlens > noutside
F
P.A. F
Physics 1161: Lecture 18, Slide 13
- Rays parallel to principal axis pass through focal point.
- Rays through center of lens are not refracted.
- Rays through F emerge parallel to principal axis.
Assumptions:
- monochromatic light incident on a thin lens.
- rays are all “near” the principal axis.
Converging Lens Principal Rays
F
F
Object
P.A.
Image is (in this case): Real or Imaginary Inverted or Upright Enlarged or Reduced
Physics 1161: Lecture 18, Slide 14
ACT: Converging Lens
Which way should you move object so image
is real and diminished?
(1) Closer to lens
(2) Further from lens
(3) Converging lens can’t create real
diminished image.
F
F Object
P.A.
Physics 1161: Lecture 18, Slide 15
Image Object
Object^ Image
Object
Image
3 Cases for Converging Lenses
This could be used as a projector. Small slide on big screen
This is a magnifying glass
This could be used in a camera. Big object on small film
Upright Enlarged Virtual
Inverted Enlarged Real
Inverted Reduced Real
Inside F
Past 2F
Between F & 2F
