9.8 Polarization

Polarization is a property of transverse waves, such as light, that describes the orientation of their oscillations. Light is an electromagnetic wave consisting of oscillating electric and magnetic fields that are perpendicular to each other and to the direction of wave propagation. Polarization specifically refers to the direction of the electric field's oscillation.

• Unpolarized Light: In ordinary light (from the sun or a light bulb), the electric fields of the light waves vibrate in all possible directions perpendicular to the direction of travel.
• Polarized Light: In polarized light, the electric field oscillations are confined to a single plane.
• Polarization: The process of transforming unpolarized light into polarized light.

1. Polarization by a Polaroid Filter

A Polaroid filter is a material that polarizes light. It is made of long-chain molecules aligned in a specific direction, creating a "polarizing axis."

• Function: The filter absorbs light waves whose electric fields are perpendicular to its polarizing axis and transmits waves whose electric fields are parallel to it.

• Effect on Intensity: When unpolarized light passes through a single Polaroid filter (the polarizer), its intensity is reduced by half.

• Malus's Law: If already polarized light of intensity $I_0$ passes through a second Polaroid filter (the analyzer), the intensity of the transmitted light ($I$) depends on the angle ($\theta$) between the polarizing axes of the two filters.

$I=I_0{\cos }^2\theta$

• When the axes are parallel ($\theta =0^{\circ }$), all light passes through ($I=I_0$).
• When the axes are perpendicular ($\theta =90^{\circ }$), no light passes through ($I=0$).

2. Polarization by Reflection

Unpolarized light can also become partially or fully polarized when it reflects off a nonmetallic surface, such as water, glass, or a road.

• Mechanism: The reflected light is predominantly polarized with its electric field vibrating parallel to the surface. This reflected, polarized light is what we perceive as glare.
• Brewster's Law: For a particular angle of incidence called the polarizing angle $i_p$, the reflected light is completely plane polarized. It is given by $n=\tan i_p$.

• Application (Polarized Sunglasses): Polarized sunglasses have filters with a vertical polarizing axis. Since glare from horizontal surfaces (like water or roads) is horizontally polarized, the sunglasses block this glare, reducing eye strain and improving visibility.

Applications of Polarization

1. Photography

Photographers use polarizing filters to reduce glare from surfaces like water and glass. By rotating the filter, they can also darken a blue sky, making clouds stand out more dramatically, as light from the sky is also partially polarized.

2. Stress Analysis

In engineering, transparent plastic models of mechanical parts are placed between two polarizing filters. When the model is put under stress, it becomes birefringent and alters the polarization of the light passing through it. This creates colorful patterns that reveal the areas of high stress, helping to identify potential points of failure.

3. 3D Movies

Modern 3D movies often use circularly polarized light. Two projectors display two different images, one with left-circularly polarized light and the other with right-circularly polarized light. The 3D glasses have corresponding polarizing filters for each eye, so each eye sees only one of the two images. The brain then combines these two images to create the perception of three-dimensional depth.

4. Optical Activity

Certain substances like sugar crystals or tartaric acid have the ability to rotate the plane of polarization of light passing through them. This phenomenon is known as Optical Activity.

Possible Questions and Answers

Q: Can sound waves be polarized?

A: No. Polarization is a property exclusive to transverse waves, where the oscillations are perpendicular to the direction of travel. Sound waves are longitudinal waves, meaning their oscillations (compressions and rarefactions) are in the same direction as their travel. They do not have a perpendicular orientation to restrict, so they cannot be polarized.

Q: What is the difference between a polarizer and an analyzer?

A: They are the same type of filter. The term polarizer is used for the first filter that polarizes unpolarized light. The term analyzer is used for a second filter that is placed after the first one to examine (or analyze) the state of the polarized light.

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