PHYS 252 / 202 1

PHYS 252 & PHYS 202

• Polarization• Scattering• Absorption

PHYS 252 / 202 2

Polarization

The polarization of the light wave is determined by the oscillation axis of the electric field.

polarization type description

linearly polarized axis is in one direction

elliptically polarized axis sweeps out ellipse over time

circularly polarized axis sweeps out circle over time

unpolarized axis changes direction randomly

PHYS 252 / 202 3

Polarization

crest

trough

linearly polarized

PHYS 252 / 202 4

Polarization

Unpolarized Linearly Polarized

PHYS 252 / 202 5

Polarization

Linear Polarization of Light by Dichroic Polarizer (Polaroid)

transmission axis

E

incident electric field

The energy from this perpendicular component is absorbed.

This component gets through.

PHYS 252 / 202 6

Polarization

Effect of a Single Polaroid on Linearly Polarized Light

E E E E

all light passes through

less light passes through

even less light passes through

no light passes through

PHYS 252 / 202 7

Polarization

Two Polaroids

I = Io cos2

The Law of Malus

PHYS 252 / 202 8

Polarization

Linear Polarization of Light by Reflection

polarization angle

PHYS 252 / 202 9

Polarization

Linear Polarization of Light by Reflection

n1 n2 p

air off water 1 1.33 53.1

air off glass 1 1.5 56.3

Examples

tanp = n2 / n1

Polarization Angle

All of the reflected light is linearly polarized parallel to the reflecting surface when the incident angle equals the polarization angle.

PHYS 252 / 202 10

Polarization

Birefringent Material

Birefrigence

• material exhibits two values of refractive index for each frequency of light

• the value depends on the orientation of the electric field with the optic axis of the material

Calciteoptic axis

PHYS 252 / 202 11

Polarization

Linear Polarization of Light by Birefringent Crystal

extraordinary ray

ordinary ray

PHYS 252 / 202 12

Polarization

Rotation of Polarization Axis by Birefringent Material

Amount of rotation depends on

• thickness of material

• wavelength of light

• values of refractive index

PHYS 252 / 202 13

Scattering

Atoms, molecules, and small particles scatter light by absorbing it and re-emitting it in random directions.

Amount of scattering depends on

• size of scatterers

• wavelength of light

PHYS 252 / 202 14

Scattering

• shorter wavelengths scatter much more than longer wavelengths

Rayleigh Scattering

size of scatterers is small compared to wavelength of light

• amount of scattered light 4

1

Example: Scattering of sunlight by oxygen & nitrogen molecules in Earth’s atmosphere

Daytime: sky looks blue, Sun looks yellow/white

Sunrise/Sunset: Sun looks orange/red

PHYS 252 / 202 15

Scattering

sunset

D ~ 820 km

Rayleigh Scattering

D ~ 50 km

daytime

6380 km

www.bigfoto.com/themes/nature/sky/ www.bigfoto.com/themes/nature/sky/

PHYS 252 / 202 16

Scattering

Linear Polarization by Scattering

PHYS 252 / 202 17

Absorption

Atoms and molecules will absorb light energy of certain frequencies (and wavelengths) much better than others.

A plot of the amount of absorption versus frequency (or wavelength) is called a material’s absorption spectrum.

Some example absorption spectra appear on the next slides.

PHYS 252 / 202 18

Absorption

Absorption Spectra of Some Organic Materials

1.bacteriochlorophyll 2.chlorophyll A 3.chlorophyll B 4.phycoerythroblin 5.beta carotene

A careful look at the chlorophyll spectra will explain why most plants are green!

PHYS 252 / 202 19

Absorption

What color is this material?

Absorption spectrum of red garnet.