Snell’s Law, Total Internal

Reflection, Brewster’s Angle, Dispersion, Lenses

Physics 102: Lecture 18

Phys 102 recent lectures

• Lecture 14 – EM waves

• Lecture 15 – Polarization

• Lecture 20 & 21 – Interference & diffraction

• Lecture 16 – Reflection

• Lecture 17 – Spherical mirrors & refraction

• Lecture 18 – Refraction & lenses (today!)

• Lecture 19 – Lenses & your eye

Light as a wave

Light as a ray

Snell’s Law: A Quick Review

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

44

Total Internal Reflection

normal

2

1

n2

n1

Recall Snell’s Law: n1 sin(1)= n2 sin(2)

(n1 > n2 2 > 1 )

1 = sin-1(n2/n1) then 2 = 90

c

Light incident at a larger angle will only have reflection (i = r)

ir

06

“critical angle”

For water/air:n1=1.33, n2=11 = sin-1(n2/n1) = 48.80

Fiber Optics

13

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

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 57% 43%

“There are millions of light ’rays’ coming from the light. Some of the rays will be totally reflected back into the water,but most of them will not.”

8

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

11

ACT: Refraction

11

• 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

Brewster’s angle

15

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

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.

19

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

When glare is around B, it’s mostly horiz. polarized!

21

26%

9%

66%

0%

Polarizing sunglasses (when worn by someone standing up) work by absorbing light polarized in which direction?

• horizontal• vertical

53%

47%

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

23

Skier sees blue coming up from the bottom (1), and red coming down from the top (2) of the rainbow.

Rainbow: Preflight 18.5

25

Wow look at the

variation in index of

refraction!

Which is red?

Which is blue?

Blue light is deflected more!

LIKE SO! In second rainbow pattern is reversed

25

Flat Lens (Window)

n1n2

Incident ray is displaced, but its direction is not changed.

t

1

1

If 1 is not large, and if t is small, the displacement, d, will be quite small.

d

27

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.

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: real, inverted and enlarged (in this case).

35

Image

Converging LensAll rays parallel to principal axis pass through focal point F. Double Convex

30

P.A.

F

nlens > noutside

F

• At F

• Inside F

• Outside F

P.A.

F

F62%

12%

25%

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?

40

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

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.

40

F

F

Object

P.A.

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.45

Image

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

Demo

50

F

F

Object

P.A.

See You Wednesday