Main Difference – Polarized vs. Unpolarized Light
Depending on the way that the electric fields in light waves oscillate, we classify waves into polarized and unpolarized light. The main difference between polarized and unpolarized light is that polarized light has electric fields oscillating in one direction, whereas unpolarized light has electric fields oscillating in all directions.
What is Polarized Light
Light is an electromagnetic wave. It consists of an electric and a magnetic field oscillating at right angles to each other. The wave itself propagates at a direction perpendicular to oscillations in both the electric and magnetic fields. This is shown in the diagram below: the oscillations of the electric and magnetic fields are shown by black arrows while the gray arrow shows the direction in which the wave is traveling.
Oscillations in an electromagnetic wave.
In polarized light, oscillations take place in a single direction. Typically, when we consider the direction of oscillations, we consider the direction of oscillation of the electric field (if the electric field is oscillating in a single direction, the magnetic field is also oscillating in a single direction, because they ae always at right angles to each other).
What is Unpolarized Light
Sunlight, or light emitted by a filament lamp, are unpolarized. This means that the oscillations of light waves are not all in a single direction. Light is given by these objects as a result of random processes taking place in atoms of those objects. As a result, electromagnetic waves coming from these objects have oscillations in random directions.
Unpolarized light can be made into polarized light by passing the light through a polarization filter. Polarization filters are made of long chains of organic molecules, arranged in parallel to each other. When light passes through a polarization filter, the filter absorbs components of electric fields in the light that are parallel to the direction in which the organic molecules are arranged. Therefore, the light coming out of a polarization filter would have its electric field is oscillating along one direction. In other words, the light that comes out would be polarized.
How a polarizing filter works: unpolarized light incident on a polarizing filter has oscillations along all directions. Once this light passes through the filter, the light that comes out has oscillations only along one direction.
Polaroid sunglasses make use of polarizing filters. The light coming from glares off the roads or from a water surface is polarized. Since polaroid sunglasses have a polarizing filter, when you look at a glare through these sunglasses, a significant portion of the light coming from the “glare” is unable to travel through the sunglasses and reach your eyes.
Surface of a lake seen without a polarizing filter (left) and with a polarizing filter (right). The polarizing filter cuts off a significant portion of the glare.
In one type of polarization called circular polarization, the direction of oscillation of light at any given position is constantly changing. 3D glasses used in films make use of this type of polarization. Here, the picture produced on the screen is composed of two images, one giving off light whose direction of polarization changes in the clockwise sense, while the direction of polarization of the other image changes in the anticlockwise sense. Each side of the 3D glasses has a polarization filter that lets in only one of these types of light. In this way, each eye receives a separate image and the brain interprets these images into 3D form.
The video below has a good description, along with several demonstrations with polarized light.
Difference Between Polarized and Unpolarized Light
Direction of Oscillations
In polarized light, at a given time, the oscillations can take place along one direction only.
In unpolarized light, oscillations can take place in any direction at a given time.
Phase Difference between x– and y– components
For polarized light, the x– and y– components of the electric field has a constant phase difference between them.
In unpolarized light, the phase difference between the x– and y– components of the electric field changes unpredictably.
Sources
Light produced by natural sources are always polarized.
Unpolarized light can be produced when unpolarized light undergoes reflection, scattering or travels through a material that can cause polarization.
Image Courtesy:
“Electromagnetic wave” by User:LennyWikidata (Own work) [CC BY-SA 3.0], via Wikimedia Commons
“a wire-grid polarizer” by Bob Mellish (Own work) [CC BY-SA 3.0], via Wikimedia Commons
“Reflection of cloud removed on water surface using polarizer filter” by Amithshs (Own work) [Public Domain], via Wikimedia Commons
