1

• Scattering and Polarization• Light waves can scatter off

water droplets, dust particles and other molecules in air– Some wavelengths in white

light scatter more than others (Rayleigh scattering)

– This is why the sky is blue

• The polarization of light refers to the plane in which the waveform and electric force field lies

• Most light is unpolarized– The waveforms and electric

force field on a given ray keep jumping around in orientation while always perpendicur to the ray

• Scattered or reflected light is polarized in particular planes– Polaroid sunglasses can block

out polarized light from sky and ground

• 3D movies & other applications of polarizers;

Physics 1230: Light and ColorChapter 13

How do 3D movies use

polaroid filters

(polarizers)?

Moving things with light without touching

http://icamconferences.org/ivan_talk/

Video synchronized with the ppt.-file of the presentation

Reminder: HW #11 due on Thursday

Today: Finish Ch.13 material

Thursday: Chapter 14+…

• Lab Tour (optional): after the class time on Tuesday, December 8, 4:30-6PM.

• Computers & cell phones allowed in class today!!!

What can a light wave do when it encounters matter?

• Be TRANSMITTED – laser aimed at water or glass

• Be REFLECTED– specular reflection of light by a

mirror– diffuse reflection of the light in

this room off all the other students

– reflection is re-radiation of light by the electrons in the reflecting material

• Be ABSORBED– Cyan light shining on a red apple

is absorbed by electrons in the apple

• Something new!!• A light wave shining on

molecules in the air or plastic or other “transparent” materials can be

• SCATTERED– Light ray moves over to the

side in all directions rather than forward, backward or being absorbed.

– Intensity of the scattered light can depend on wavelength

What is Rayleigh scattering?(or why is the sky blue)

• The shorter the wavelength, the more light is scattered– blue is scattered more than red.– this is why the sky is blue and

sunsets are red. (Fig. 13.1)

Think of white light from sun as a mixture of R, G and B

Blue is scattered the most so sky looks blue when we look away from the sun

For same reason sun looks yellow (red + green)

More atmosphere allows next shortest wavelengths (green) to scatter so sunset looks red

Rayleigh scattering

•Intensity inversely proportional to λ4

•Light waves with longer wavelength scatter more

•But why blue and not violet?

Why blue and not violet?

Spectral Spectral response of conesresponse of cones in typical human eyein typical human eye

rela

tive

resp

onse

rela

tive

resp

onse

•Our eye sensitivity to the blue color is much stronger

than that to violet!

•We perceive the deep waters being of blue color

for the same reason!

Earth

Atmosphere

Color of Day-time sun vs. Sunset

Rays from Sun (not scattered)

White or yellow

Red Sun/sky during the Sunset

Why the sky is blue?

• A. Because of scattering of short-wavelength blue light coming from the Sun;

• B. Because of light absorption similar to that in color filters;

• C. A & B• D. Because of dispersion

and total internal reflection;• E. None of the above;

What color of the sky one can see from Jupiter?

• D. Black• A. Red;• B. White;• C. Blue;• E. Yellow;

Without Rayleigh scattering the Sky would be

• A. Red;• B. Green;• C. Blue• D. Black;• E. White;

•Larger particles scatter red as well as blue and hence look white.

•Dust or smoke•Clouds; •Milk;•Colloidal suspension

•There are many other types of scattering…

Light scattering in milk and smoke?

Why the sunset is red?

• A. Because of light absorption similar to that in color filters;

• B. Because of scattering of blue and green light and only red light coming to our eyes;

• C. Because of its polarized nature;

• D. Because of chromatic & spherical aberrations;

• E. None of the above;

Polarized Light due to reflection

Secularly reflected light blocked using a polarizer (Polaroid film)

http://en.wikipedia.org/wiki/File:Mudflats-polariser.jpg

Review: What is the wave of a mono-chromatic (single wavelength) wave along a ray?

Speed of light in empty space is c = 186,000 miles/sec = 3 x 108 meters/sec

RayWaveform

Amplitude(maximum height)The intensity of the light is proportional to the amplitude squared. Large amplitude means bright light. Low amplitude means dim lightW

avel

engt

h

Review: What is light "made of?"

• Light consists of electric (and magnetic) fields moving through space at the speed of light

The waveform can be replaced by a set of arrows

perpendicular to the ray whose tips lie along the waveform

Crossed polarizers: block propagation of electromagnetic waves

Waves on a rope

Think about the waves on a rope:

For light: Polarizers (Polaroid films)

Same story:

What is polarized light?

x

Looking at ray "head-on" see green arrows up & down

y

z

x

y

z

• Light is polarized if the waveform and electric force field arrows remains in the same plane

– The (green) electric force arrows must always be perpendicular to the ray

• This is a light ray traveling in the z-direction and polarized in the y-direction

• Here is a light ray traveling in the same direction but polarized in the x-direction

• We will visualize the polarization in the x-y plane, looking at rays head-on

– The green force arrows point up and down or left and right, stacked up behind one-another.

– Here is the convention for visualizing vertical and horizontal polarization

y

x

In which direction is this light ray polarized?

• A. Vertical;• B. Horizontal;• C. Along the light

propagation direction (yellow arrow);

• D. None of the above;

y

x

What is unpolarized light?

• For unpolarized light the plane of polarization keeps jumping around– But the electric field arrows

remain perpendicular to the ray (direction of travel of the wave)

– We visualize this in the x-y plane (looking into the ray) as shown at right

• The many crossed double sided arrows are the symbol for unpolarized light

• See Physics 2000

x

y

z

electric force arrows jump around while remaining perpen-dicular to the ray

wave travels in z-direction

http://www.colorado.edu/physics/2000/index.pl

When unpolarized light reflects off a horizontal surface (such as water or beach) near a special angle, the

reflected light is polarized in the horizontal direction– The special angle of incidence is

where the refracted ray and reflected ray are perpendicular to each other

– This is called Brewster's angle– To understand, imagine the electric

force arrows of the incident unpolarized light to be decomposed into two perpendicular polariza-tions

• the first polarization is horizontal (force arrows are parallel to the flat reflecting horizontal surface and perpendicular to the ray)

• in the 2nd (Fig. 13.5), the arrows are perpendicular to both the ray and the horizontal force arrows

– The second polarization cannot be sustained in the reflected ray because the force arrows would be parallel to that ray (impossible for a light ray)

– Hence, only the horizontal polarization survives in the reflected ray

Polaroid sunglasses (and camera filters) take advantage of this effect to block bright reflected light from the

snow, beach, road, water, sky, etc.

• When you wear polaroid sunglasses they block light polarized in the plane of your two eyes– When your head is vertical this is the

horizontal plane– Since reflected light from the ground

or scattered light from the sun are mostly polarized in the horizontal plane such light is mostly blocked

• Demo with polaroid filters– The first filter blocks one polarization

and lets through the other (perpendicular) one

– The second blocks more and more of the remaining polarized light as its axis is rotated so as to permit only the polarization blocked by the first

– When the axes of the two polaroid filters at at right angles to each other no light comes through

• Physics 2000 polarization applets• The polaroid filter works because it

has long molecules lined up parallel to each other like hairs

– Components of light with polarization parallel to the molecules are absorbed and components of light polarized perpendicular to the molecules are absorbed and don't go through

• Other demos with crossed polarizers

– Third polarizer at angle in between lets light through again

– Stressed plastic between polarizers shows colors which reveal the stress patterns

Some material from Chapter #8

How do 3D movies use

polaroid filters?

A different approach:

• Use color filters to make the left and right eyes perceiving slightly differen images

• http://www.3dmovies.com/

Clicker Question:

In 3D movies:

A.Left eye sees the same as the right eye;

B. Left eye sees the inversed image compared to that seen by the right eye;

C.The images seen by the two eyes are slightly different and taken from different angles.