Operation of linear polarizing film
Linear polarizing film is essentially a grid polarizer applied to the active layer of the film. The polarizer consists of thin lines (stripes) with equally small gaps between them. The dimensions of these elements are comparable to the wavelengths of light. The direction of the lines determines the plane of polarization.
If two pieces of polarizing film are placed so that their polarization planes align, light passes through both films. If one is rotated 90 degrees (crossing the polarization planes), a dark area appears where no light passes. This happens because light polarized by the first film can easily pass through the second only if it is oriented the same way. When the polarization planes are crossed, the polarized light from the first film is completely blocked.
This film illustrates the most important property of polarizing film — transmitting a plane electromagnetic wave oriented according to the film’s polarization plane.
Video demonstrating the behavior of a polarizer in LCD screens.
Computer and laptop monitors are built, among other things, with polarizing films. A second polarizer placed in front of the screen and aligned with the polarization axis is completely invisible.
As you can see, the laptop screen has its polarization axis set “diagonally.”
Rotating the polarizer by 90 degrees creates a black area through which no light passes.
The property of blocking polarized light by a polarizer set at a 90-degree angle has been used to eliminate light reflections. Light reflected at a shallow angle from flat surfaces undergoes spontaneous polarization, so placing a polarizer in its path allows us to “filter out” the polarized component.
Video demonstrating the effect of a polarizing film on eliminating reflections on a mobile phone screen.
As you can see, rotating the film by 90 degrees completely removes the reflection, making the phone screen readable. The video shows the film being rotated around the center of the sample, first at some distance from the camera lens, and later moved as close as possible to the lens. The best results are achieved when the film is positioned as close to the lens as possible. If we rotate the film so that its polarization axis is at 90 degrees to the polarization of the reflected light (crossed polarizers), the screen becomes clearly visible. Conversely, when the polarization axes are aligned, the reflection on the phone screen prevents information from being read.
Video demonstrating the action of a polarizing film by eliminating reflections on a flat surface.
As in the previous video, light reflects at a small angle from a flat surface creating a strong polarized glare. Although this surface reflects light weakly (film on a book), a glare appears that makes it impossible to read the title.
The video shows the film being rotated around the center of the sample in front of the camera lens, between two extreme positions:
- the film is aligned parallel to the polarization plane of the light – all polarized light passes through
- the film is aligned perpendicular to the polarization plane of the light – all polarized light is blocked
Setting the polarizing film so that the polarization planes are at a 90-degree angle to each other practically removes the glare completely, allowing useful information to be read. When there is no glare, the polarization planes are crossed. As before, the best effect is achieved when the polarizer is as close to the camera lens as possible.