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Physics 102: Lecture 18, Slide 1
Snell’s Law, Total Internal
Reflection, Brewster’s Angle, Dispersion, Lenses
Physics 102: Lecture 18
Physics 102: Lecture 18, Slide 2
Summary of today’s lecture
• Examples of refraction– 1) Total internal reflection– 2) Brewster’s angle– 3) Dispersion (rainbows)– 4) Lenses
Physics 102: Lecture 18, Slide 3
Demo: Snell’s Law
n2
n1 > n2
When light travels from one medium to another the speed changes v=c/n, but the frequency is constant. So the light bends:
n1 sin(1)= n2 sin(2)
1
2
r
incidentreflected
refracted
n1 > n2 2 > 1
Light bent away from normal as it goes in medium with lower n
Physics 102: Lecture 18, Slide 4
1) Total Internal Reflection
normal
n2
n1 > n2
Snell’s Law: n1 sin(1)= n2 sin(2)
(n1 > n2 2 > 1 )
1 = sin-1(n2/n1) then 2 = 90
Light incident at a larger angle will only have reflection (i = r)
“critical angle”
For water/air:n1=1.33, n2=11 = sin-1(n2/n1) = 48.80
1
2
i > c
r c
Physics 102: Lecture 18, Slide 5
Fiber Optics
Telecommunications
Arthoscopy
Laser surgery
Total Internal Reflection only works if noutside < ninside
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.
ninside
noutside
Physics 102: Lecture 18, Slide 6
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
Physics 102: Lecture 18, Slide 7
ACT: Refraction• As we pour more water into bucket, what
will happen to the number of people who can see the ball?
1) Increase2) Same 3) Decrease
Physics 102: Lecture 18, Slide 8
2) Brewster’s angle
When angle between reflected beam and refracted beam is exactly 90 degrees, reflected beam is 100% horizontally polarized !
Reflected light is usually unpolarized (mixture of horizontally and vertically polarized). But…
tanB
n2
n1
n1 sin B = n2 sin (90-B)
n1 sin B = n2 cos (B)
horiz. and vert.
polarized
B B
90º – B
90º
horiz. polarized
only!n1
n2
Physics 102: Lecture 18, Slide 9
ACT: Brewster’s Angle
When a polarizer is placed between the light source and the surface with transmission axis aligned as shown, the intensity of the reflected light:
(1) Increases (2) Unchanged (3) Decreases
T.A.
Physics 102: Lecture 18, Slide 10
Polarizing sunglasses are often considered to be better than tinted glasses because they…
Preflight 18.3, 18.4
• block more light• are safer for your eyes• block more glare• are cheaper
Polarizing sunglasses (when worn by someone standing up) work by absorbing light polarized in which direction?
• horizontal• vertical
Physics 102: Lecture 18, Slide 11
3) Dispersion
Prism Blue light gets deflected more
nblue > nred
The index of refraction n depends on color!
In glass: nblue = 1.53 nred = 1.52
White light
blue < red
red
i
blue
Physics 102: Lecture 18, Slide 12
Skier sees blue coming up from the bottom (1), and red coming down from the top (2) of the rainbow.
Rainbow: Preflight 18.5
Wow look at the
variation in index of
refraction!
Which is red?
Which is blue?
Blue light is deflected more!
Physics 102: Lecture 18, Slide 13
LIKE SO! In second rainbow pattern is reversed
Physics 102: Lecture 18, Slide 14
4) Lenses
Focal point determined by geometry and Snell’s Law: n1 sin(1) = n2 sin(2)
Converging lens:– Rays parallel to P.A. converge on focal point
Diverging lens:– Rays parallel to P.A. diverge as if emerging from focal point behind lens
Larger n2/n1 = more bending, shorter focal length.Smaller n2/n1 = less bending, longer focal length.n1 = n2 => No Bending, f = infinity
F
“Plano-convex”
“Plano-concave”
P.A.
F P.A.
Physics 102: Lecture 18, Slide 15
Converging & Diverging Lenses
Converging lens:– Rays parallel to P.A. converge on focal point
Diverging lens:– Rays parallel to P.A. diverge as if emerging from focal point behind lens
“Plano-convex”
“Plano-concave”
Converging = fat in the middle
Diverging = thin in the middle “Double concave”
“Double convex”
= =
= =
“Convex-concave”
“Concave-convex”
Physics 102: Lecture 18, Slide 16
1) Rays parallel to principal axis pass through focal point.2) Rays through center of lens are not refracted.
3) Rays through F emerge parallel to principal axis.
Converging Lens Principal Rays
F
F
Object
P.A.
Image is: real, inverted and enlarged (in this case).
Image
Key assumptions: • monochromatic light incident on a thin lens.
• rays are all “near” the principal axis.
Physics 102: Lecture 18, Slide 17
Converging LensAll rays parallel to principal axis pass through focal point F. Double Convex
P.A.
nlens > noutside
F
• At F
• Inside F
• Outside F
P.A.
F
Preflight 18.6A 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?
Physics 102: Lecture 18, Slide 18
3 Cases for Converging Lenses
Object
Image
This could be used in a camera. Big object on small film
InvertedReducedReal
Past 2F
ImageObject
This could be used as a projector. Small slide on big screen
InvertedEnlargedReal
BetweenF & 2F
Image Object
This is a magnifying glass
UprightEnlargedVirtual
Inside F
Physics 102: Lecture 18, Slide 19
ACT: Converging LensWhich 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 102: Lecture 18, Slide 20
1) Rays parallel to principal axis pass through focal point.2) Rays through center of lens are not refracted.
3) Rays toward F emerge parallel to principal axis.
Diverging Lens Principal Rays
F
F
Object
P.A.
Only 1 case for diverging lens:Image is always virtual, upright, and reduced.
Image
Physics 102: Lecture 18, Slide 21
Which way should you move object so image is real?
1) Closer to lens
2) Further from lens
3) Diverging lens can’t create real image.
ACT: Diverging Lenses
DemoF
F
Object
P.A.
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