In order to better understand how to use a polarizing filter, we will first take a look at polarization itself. Since a formal explanation requires a good understanding of optical science, we will present a simplified but generally accurate explanation.
Polarization is part of the basic properties of light. It relates to the fact that light is made up of waves. When you’re looking at a light source (the Sun, a light bulb, or a reflective surface for instance) those wave are coming straight towards you. And these waves, just like any others, oscillate.
Vertical oscillation of a wave, as seen from the side. The arrow represents the movement of the wave as seen from the front.
But since there are a multitude of waves emitted by any light source, these oscillations will generally be oriented in multiple directions. This is called “unpolarized light.”
Multiple polarisation orientations, as seen when looking at a light source
As you can see, any of those waves can be represented by a vertical and a horizontal element:
Those vertical and horizontal elements are what interest us, because they’re related to polarization. When light reflects on a non-metallic surface, such as glass, water, or even foliage, only one element (say, the vertical one) will be reflected. For instance, referring to a previous figure, the blue wave would be totally reflected, the red wave wouldn’t be reflected at all, and the black waves would be partially reflected (only their vertical components would be reflected). After the reflection, light will oscillate in one direction only (that’s called “linearly polarized light”).
How do polarizers work?
A polarizer works by transmitting only light oscillating in one orientation (vertical or horizontal). Light oscillating in the other orientation is absorbed or reflected by the filter. Light having both horizontal and vertical components will be partially transmitted.
As you can guess, with most light sources that means about half the incoming light will be transmitted through the polarizer. However, if the light is polarized linearly (for instance after reflecting on glass), using a polarizer will let either all the light, some light, or no light through depending on its orientation.
The lines represent a polarizing filter, and the blue arrow incoming light (linearly polarized). First image: no light passes through. Second image: some light passes through. Third image: all light passes through.
Illustrations and text by Bernard Dery