Day 162

Polarization

Unpolarized light: all directions of the electric field vector are probable and perpendicular to the propagation direction.

Linearly (plane) polarized light: electric field vectors vibrate in the same direction at all times

Polarization occurs by three methods:

  1. Selective absorption
  2. Reflection
  3. Scattering

Polarization by Selective Absorption (Polaroid sunglasses)

Polarized material generally transmits all light with the electric field vector parallel to the transmission axis; all light with electric field direction perpendicular to the transmission axis will be absorbed.

I = I0 cos2theta

I = Final intensity of light

I0 = intensity of light on analyzer

 = angle between transmission axes of polarizer and analyzer

Look at this animation to see how a Polarizing Filter works. It only allows so much time, so if it times out and you want to do more, just refresh the page.

How a Polarizing Filter Works

Polarization by Reflection

When Angle i = 0o or 90o, light is unpolarized

When angle is between 0 and 90, the reflected beam is partially polarized

Complete polarization occurs when the angle is at the polarizing angle,  p this occurs when the angle between the reflected and refracted beam is 90 degrees; the reflected beam is completely polarized, the refracted beam is partially polarized.

n2/n1 = sin p /cos p or n2/n1 = tan p where n2 = reflecting medium, n1 = transmitting medium (Known as Brewsters Law)

Click below to see an example of Brewster’s Law.

Brewster’s Law

Polarization by Scattering

This occurs when there is absorption and re-radiation of a light by a system of particles (for example, a gas) this leads to scattering.

Click here to see HyperPhysics – Scattering. When the new page opens, scroll down to find the information on Polarization by Scattering.

Some substances are optically active which means they are capable of rotating the plane of polarization of transmitted light.

For example, Karo syrup is optically active when light shines through Karo syrup and a piece of polarizing film is placed over the sample (in a beaker) and rotated, all the colors of the rainbow will be seen!

Click here to read more about what makes a material Optically Active.

Go to the Physics Classroom and read more about Polarization. Test your understanding of the material by taking the self-quiz at the bottom of the page.

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