1. AP Physics 2
Chapter 24
Section 4

2. Plane Polarized Light
Light in which the oscillations are confined to a single plane that includes the line of propagation
ex – shake a rope up and down –
vertically polarized
-- shake a rope side to side –
horizontally polarized

3. Plane Polarized Light
Light produced by a single vibrating electron is polarized
- if electron vibrates vertically it produces vertically polarized light
Light from a common light source isn’t polarized
- electrons that produce the light are vibrating in random directions

4. Plane Polarized Light
Ordinary light can become plane polarized through interactions with matter
1. scattering
2. reflection
3. refraction
4. selective absorption

5. Selective Absorption
In 1928 a college student named Edwin Land developed a filter that could polarize light
Used micro-crystals of quinine iodosulfate embedded in a thin plastic sheet
This filter used selective absorption to polarize light

6. Selective Absorption
Dichroism – property of crystals in which one polarized component of incident light is absorbed and the other is transmitted
dichoric crystal
Filter Land discovered is now known as a polaroid filter

7. Polarizing Filters
Polarizer – filter that is used to polarize light that is initially unpolarized
- if vertical transmission axis, the light that passes through comes out vertically polarized
Analyzer – subsequent filters that are used to adjust the orientation and/or intensity of the polarized light
Often represent unpolarized light like
When unpolarized light passes through a polarizer, only vibrations that match up with the transmission axis will pass through or vibrations that have a component that match up

8. Polarizing Filters
When unpolarized light passes through a polarizer the intensity of the light drops by 50%
Law of Malus - I = Io cos2 Θ
where I = final intensity
Io = initial intensity
Θ = angle between filters

9. Sample Problem
An unpolarized laser beam of intensity 1000 W/m2 is incident on a Polaroid sheet with a vertical transmission axis. The light passing through this sheet strikes a second Polaroid sheet with a transmission axis at an angle of 30 degrees from the vertical. Find the polarization and the intensity of the light emerging from the second sheet.
500 W/m2
375 W/m2

10. Polarization by Reflection
When sunlight reflects off the surface of calm water it is partially polarized
At certain angles, all the light hitting a reflecting surface is polarized
Polaroid sunglasses reduce glare because they have a vertical transmission axis and absorb all the horizontally polarized light
Polarization by reflection doesn’t occur for metals
The light that is reflected is polarized parallel to the reflecting surface

11. Polarization by Refraction
In certain materials (called double-refracting or birefringement materials) the speed of light is not the same in all directions
Unpolarized light splits into 2 plane polarized rays traveling with different speed and different angle of refractions
Rays are polarized in perpendicular directions

12. Polarization by Scattering
When light strikes an atom it causes the atom’s electrons to vibrate
These vibrating electrons emit light with the same frequency but scattered in various directions
Light scattered perpendicular to the incident beam are polarized