Polarized Glasses: How Do They Work?

Sometimes, our brain’s capacity to treat light results in too much information. In some cases, less light is preferable for a more efficient information treatment. Therefore, in an effort to lower the amount of information entering our brain, we instinctively squint our eyes or wear sunglasses, for example. Today, we can even completely eliminate certain stray light information by wearing polarized glasses, which in turn greatly enhance our sight comfort.

What is polarization?
What is parasitic light information?

Polarization

Natural sunlight is depolarized which means that the light rays that reach us don not have a specific orientation. Going back to our example of the waving skipping rope, we can represent the light waves as follows:

We see here four (4) light waves of the same amplitude and frequency, going in the same direction, but without any privileged orientation, just as if we were to wave the rope in different orientations (up to down, left to right or diagonally).

This is called an unpolarized light wave. “Unpolarized” is the term used to mean “without any privileged orientation.” If we position ourselves in the axis of these light waves, we see them point in different directions.

The above right drawing shows four (4) waves among the infinite number of waves reaching us from all possible orientations. Therefore, an unpolarized sunray is represented like this:

All these waves form an unpolarized light ray.

Parasitic Light Information

Parasitic light information can be defined as radiance reflecting on an object that doesn’t give any information such as colour, texture or shape about it. This light, which is entirely reflected by the surface, like a mirror, parasites the useful information about the objects.

When this light is reflected on a horizontal surface (such as water, roads or snow on ski slopes), it becomes naturally polarized within a privileged axis: the horizontal axis.

As a result, most of the information is reflected along this axis, causing our glare.

The polarized glasses are always polarized in the vertical direction (↨), because it is the parasitic reflections coming from horizontal surfaces which are the most disturbing in the vast majority of the cases (surface of the water in a boat or surface of the road in a car).

If you are an amateur photographer, you already know polarized filters mounted on cameras are rotative. The reason is that it is imperative that the filter angle be adjusted to keep the polarization in the desired axis (↨) in order to take pictures in different positions.

This effect of natural reflection of light on the windows gives a certain intimacy at the residential level but poses problems at the commercial level. Polarized filters are also installed on certain store windows to avoid this type of reflection and thus allow passers-by to clearly see the displays inside the stores at any time of the day.

Science is everywhere!

You may also have noticed that some screens appear black when viewed with polarized glasses. This is because these screens emit polarized light that is exactly 90° from the polarization angle of your glasses. A mechanism occurs that is identical to that of polarized light reflected from the surface of the water, but this time the effect is undesirable.