To answer this question we are going to start on a somewhat strange path: we have all felt the vibration associated with the sound of thunder or a loudspeaker near us in a concert. This vibration is due to the fact that the sound waves travel through the air, making it oscillate as it passes back and forth. The waves of light, to travel, also make the medium through which they move vibrate in a certain way.
Scientists understand that light behaves as if it were made up of particles and waves. Therefore, it is as if a beam of light was formed by small individual balls (photons), which also vibrate as waves when they move. Each photon within a ray of light vibrates in a certain way. If everyone vibrates the same, it’s a laser beam. If each one travels in his own way, we have light like that of the sun or that of a light bulb. In between, there are different levels of organization or disorder among the photons. One of them is polarized light.
Polarized glasses eliminate reflections and sparkles of sunlight on shiny surfaces
We said that each particle of light vibrates the electromagnetic field in a certain direction. It may happen that in a beam of light there is a majority of photons that vibrate in any of these modes and we speak of polarized light, vertical, horizontal, or circular. There are semitransparent materials capable of selecting the photons that pass through them, so that only those that vibrate in a concrete form can pass throu them.
For example, we can pass only the photons that vibrate vertically. A filter will pass 50% of the normal light, since statistically half of the photons will vibrate in that direction. On the other hand, if the incident beam is very polarized, we can turn our filter until we find the right angle, and eliminate most of the light.
What does all this have to do with glasses? Well, let’s get serious … As we said before, most of the light sources that surround us are not polarized. Some glasses with a polarizing filter will simply let 50% of the incident light pass, which is not very helpful. But there are processes that can happen to a beam of light that cause a strong polarization. One of them is reflection: when light is reflected on a surface, the reflected beam is polarized with more or less intensity. For this reason, glasses with a polarized filter are especially good for eliminating reflections and flashes of sunlight on shiny surfaces. Consequently, they are useful in places where these flashes are common: in the snow, in the sea, or on a road where the sun is reflected in the glazed or metallic surfaces of the cars.
With two pairs of glasses and rotating one of them, we go from normal intensity to full opacity
From the practical point of view it is easy to distinguish polarized glasses from ones that are not. If we keep polarized glasses in front of us and, looking through one of its crystals, we rotate them, we will see how the intensity of the light varies in the different objects. We can verify it in an especially easy way, as we said before, if we look towards an object where the light is reflected, or also towards the sky (the blue of the sky is also polarized, in this case because the light that is dispersed in the atmosphere is polarized partially).
With two pairs of glasses we can also observe that, looking through the crystals of the two and rotating one of them, we will go from the normal intensity to full opacity. Why? Because the first lens allows only polarized light to pass through at a certain angle, and the second one can be in the same direction (allowing the same light to pass) or in the exactly perpendicular, in which case absolutely no light passes through. This arrangement is used in fact in polarization studies in the laboratory, and in that context we speak of a polarizing lens (the first) and an analyzer (the second).