Polarization

One cannot study optics without knowing about the finer details of polarization. Polarization of light refers to the particular direction that the tip of the electric field points when propagating in free space or in a medium (this is not to be confused with atomic polarization, which is the sum of the dipole moments). The reason why we consider the orientation of the electric field and disregard the magnetic field comes from their relative orders of magnitude in producing a force. The reason why we care about polarization is because certain elements in your optical setup will induce polarization without you even intending to, which leads to problems with output energy, focus, etc. Thus, if you don’t manage your polarization, your experiments won’t perform as well!

Type of Polarization

Now that we know more about what polarization is and why we care about it, let’s figure out how it can manifest, shall we?

Linear Polarization

Linear polarization is the simplest type of polarization. The direction of polarization can be any of 360 degrees, but most commonly people want to either have vertical or horizontal polarization to make management easier.

Circular Polarization

Circular polarization is exactly what it sounds like: light that moves in a circular motion! This is an ideal form of polarization, but not as common to be naturally occurring. In circular polarization, the intensity across the beam is constant. One of the most important aspects of circular polarization is knowing whether the polarization is left or right-handed; that is, whether is rotating clockwise or counterclockwise. Different textbooks and scientists have different definitions of what direction to measure the rotation from (ie. whether you say it is rotating clockwise while looking at the beam coming towards you, or clockwise from the beams point of view), so just be careful to specify the orientation.

Elliptical

Elliptical polarization is the most common type of polarization, because it can take on any number of elliptical shapes. Elliptical polarization is characterized by three main attributes: azimuth angle, ellipticity, and handedness. I will also be posting a video later on describing in detail how to calculate these values.

The ellipticity of an ellipse is given by:

e=\frac{b}{a}

Polarizers and Wave Plates

The most common methods to control polarization, and indeed some of the most common optical devices in general, are linear polarizers and wave plates. To create linear polarization, you can use a linear polarizer, which gives you the ability to change your polarization to any angle. (I will post a video later on demonstrating the usefulness of a linear polarizer).

To create elliptical or circular polarization, we can use wave plates. The two most common types of wave plates are half-wave plate and quarter-wave plates. A half-wave plate, as you might guess, rotates the incident beam by half a wave, or equivalent to a Pi/2 phase shift.

Optical Activity

A very interesting optical phenomenon that occurs as a result of polarization is optical activity. Optical activity refers to the rotation of the polarization plane as the light passes along the length of a material. For example, given a jar of regular ole corn syrup from the grocery store, you can get a variety of colors out if you place one polarizer on one end of the jar and you rotate another polarizer above it. Below is a link to a video exhibiting this phenomenon:

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