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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
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Page 1: Lect18 handout

Physics 102: Lecture 18, Slide 1

Snell’s Law, Total Internal

Reflection, Brewster’s Angle, Dispersion, Lenses

Physics 102: Lecture 18

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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

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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

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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

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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

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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

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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

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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

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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.

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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

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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

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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!

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Physics 102: Lecture 18, Slide 13

LIKE SO! In second rainbow pattern is reversed

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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.

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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”

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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.

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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?

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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

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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.

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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

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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.