AP Physics 2 Chapter 24 Section 4

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

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

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

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

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

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

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

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

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

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

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