Shoot like a Pro

What is polarization?

Written by Bernard Dery

If you want a better understanding of how to use a polarizing filters, here’s some almost non-technical information about polarization.

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.

Oscillation

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.”

pf-lightdirection

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:

elements

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.

PF lines

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

About the author

Bernard Dery

Bernard was born and still lives in Quebec City, Canada. A doctorate candidate in Physics, Bernard works full-time as an optical designer at EXFO, a world leader in telecom tests and measurements tools. His professional interests go from fiber optics to atmospheric data collection and lasers.

His first contact with photography was a Vivitar fixed lens camera that he had purchased at age eight. His interest developed slowly, and for many years Bernard limited himself to family and travel pictures.

Bernard discovered the world of digital photography in February 2004, when he purchased his first digital camera. Developing an interest for composition, he experimented with many subjects: landscapes, nature, cities and architecture.

Bernard
soon began to invest more time and energy into his new found hobby, finding new and beautiful subjects to capture everyday. His challenge is to capture the mood and emotion carried by a scene as it presents itself to him.

A trip to Italy in
September 2004 fueled his growing interest with numerous magical photography opportunities.

Bernard and his wife like to spend free time near a lake at his family's summer house, where he finds new subjects to capture every day. His interests are varied and include hiking, camping, swimming, boating and reading, as well as most winter
sports.

Bernard, known as bdery on forums, uses a Pentax K20D and W80, Canon S2 and a Nikon SQ as a backup.

Thumbnails are of photos by Bernard Dery - used with permission
Copyright Bernard Dery All rights reserved

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