chapter 13 polarization

8/12/2019 Chapter 13 Polarization

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Scattering and Polarization

Adding vectors

Polarization by reflection

Polarization by scattering

3D movies

Polarizers and analyzers


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Adding Vectors

We can decomposeany

polarization direction into an x-

component and a y-component

x-component

y-component

X

Y

P

Q

But we dont have to use x and

y. We can use any axes that are

perpendicular to one another

Adding Vectors

Now a polarization that was just

x in the first set of axes has two

components, p and q

x-component

y-component

X

Y

P

Qq-component

p-component


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Concept Question

Which polarization has the largest x-component?

x

y

x

y

x

y

A B C

Electromagnetic Waves

The polarization is defined as the direction of

oscillation of the electric field

Polarized along the z-axis


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Polarizing Filters

Polarization by Reflection

=

Incident ray

Reflected ray

Transmitted

(refracted) ray

AIR

GLASS

Review reflection and refraction:


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Brewsters AngleConsider the special case where the transmitted and

reflected rays are at right angles to one another

=

Incident ray

Reflected ray

Transmitted

(refracted) ray

AIR

GLASS

Brewsters AngleWe can decompose the incident light into two parts: polarized in

the plane of the figure, and perpendicular to the figure. We will

consider the part in the plane of the figure

Incident ray

Reflected ray

Transmitted

(refracted) ray

AIR

GLASS

Electric field

oscillation


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Brewsters AngleThe oscillating charges in the glass are what produce the reflected

ray. However, they cannot radiate along their direction of travel.

Thus there CANNOT be a reflected ray in this special situation.

Incident ray

Reflected ray

Transmitted

(refracted) ray

AIR

GLASS

Electric field

oscillation

Brewsters Angle

This special angle of incidence is called Brewsters

Angle

For light incident at this angle, there is not reflected

ray polarized in the plane of the figure.

Thus the reflected lightis polarized perpendicular

to the plane of the figure


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Brewster Windows

This effect is also used when you have polarized lightand you dont want any of it to be reflected

Laser light is typically polarized, so this can be used

inside a laser to make sure we arent losing energy to

surface reflections

Brewster

window

Polarizing Sunglasses and Glare

Sunglasses lenses with a polarizer in them can

eliminate specular surface reflections


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Polarizing Sunglasses and Glare

The light coming from everywhere else in the scenewill be unpolarized

Half of it will be transmitted by the glasses

Concept Question

Which polarization should sunglasses reject to

eliminate glare from water puddles and other shiny

surfaces? Assume the surface is horizontal, as

shown in the diagram.

A. Sideways

B. Up and down

The reflections from the horizontal puddle will be polarized horizontally. So to eliminate

glare from this surface, we want to reject the horizontal polarization, thus rejecting the

surface reflections


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Polarizing Lenses: Example

Without Polarizing lenses

With Polarizing lenses

Polarizing Camera Filters


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Polarization by Scattering

Consider the following

analogy:

If we wiggle the rope up and

down, a wave is generated

in the cross rope

If we wiggle the rope

sideways, there is no wavegenerated in the cross rope


If we wiggle the rope at an

angle, the resulting wave in

the cross rope will only be

up and down

The resulting wave will belinearly polarized, even

though the incident wave

was not

Sideways component

Does nothing

Up and down component

Creates new wave in rope

X

Y

Incident wave


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Light from the sun is unpolarized. It hits particles inthe atmosphere, causing them to oscillate

The oscillating particles then radiate light

X

Y

Z


There will be no oscillation in the z direction

The wave traveling in the y-direction will thus be

polarized in the x-direction

X

Y

Z


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Concept Question

Along which axis will the wave traveling in the x-direction by polarized? (views E3 and E4, circled)

A. X

B. Y

C. ZX

YZ


Recall the we see the blue in the sky because of

light scattered by the atmosphere

The light coming from points in the sky 90from

the sun will be linearly polarized

When the sun is overhead, the

sky near the horizon will be mostpolarized


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When the sun is at the horizon,

the point of maximum

polarization will be straight up in

the sky.

The part of the sky directly

opposite the sun will be

unpolarized, as will areas of the

sky very near the sun

Other areas of the sky will bepartially polarized

Camera Filters and the Sky


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3D Movie Projection and Viewing

3D movies are actually stereograms, which createthe illusion of depth by showing your eyes slightly

different images

This was originally done using glasses with red

and cyan lenses and pictures that looked like this:


Modern 3D movies are projected using different

polarizations instead of different colors.

Without glasses, the images look like this:


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The two overlapping images are actually projectedthrough a polarizing filter, alternating in rapid

succession

The glasses separate the polarized light and each eye

sees something different, creating the illusion of

depth

The system actually uses circularlypolarized light, so

the glasses wont work like your polarized sunglasses

Polarizers and Analyzers

Polarizing filters can also be used to detect the

presence of polarized light

When used in this way, it is called an analyzer

How is this useful?


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By changing the relative

orientation of the two

polarizing filters, the

intensity of the light

that makes it all the way

through can be adjusted



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This is useful for adjusting the intensity of lightwithout changing the parameters of the beam

An aperture reduces the size of the beam

Dimming a light bulb often changes the color

temperature

Filters are impossible to adjust continuously

This is very common way to control laser beam

intensity

Next Time

We will do a review, I still havent figured out the

exact format

I will put together a study guide for the material

covered since the last exam

I will send out emails with this information, so

dont delete them!

chapter 13 polarization

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