Physics, Secondary ed level, Tutorial

Just a minute: how do polarized sunglasses work?

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According to our best understanding of the observable Universe, it is filled with an omnipresent electromagnetic field. Certain perturbations of that field correspond to what we call (electromagnetic) radiation, the visible part of which we call light or light particles, or photons. These disturbances can have specific but differing frequencies, which, when visible, we may perceive as red, yellow, green, blue or violet. Every photon has a frequency: some value is going up and down over time. It turns out that discovering what it is that is changing over time, is the key to unlocking the answer to how polarized sunglasses work.

Traditionally, it’s useful to mathematically separate the electromagnetic field into two components: the magnetic and the electric subfield. For a slightly more in-depth discussion of this topic, have a look at Why, exactly, do glass and liquids refract light? Geometrically, these subfields are orientated perpendicularly to one another. Have a look at Figure 1.

Figure 1. A cartoon of the electromagnetic field. It consists of five parts. Their descriptions are given in the main text.
Figure 1. A cartoon of the electromagnetic field.

Part a. A three-dimensional electromagnetic field (EM-field) pervades the observable Universe. Here, it is depicted as a finite block but that’s just a cartoony metaphor. In reality, it has the shape of the Universe, and it’s seemingly infinite, or, to be more precise, it’s everywhere you can possibly look.

Part b. As said before, it turns out to be very useful to mathematically separate the EM-field into two components: the magnetic and electric subfields, depicted here as two planes orientated perpendicularly.

Part c. When a photon passes through space, this is where the EM-field is disturbed. It is a local change of electric and magnetic values back-and-forth over time. Very important to note: there is nothing in space going up and down or left or right, it is just a cartoon depicting changing values of the respective subfields. The only thing that is actually spanning through space is the trajectory of the photon, depicted by an orange arrow.

Part d. For our polarized sunglasses, only the electric subfield is relevant, so, we’ve left out the magnetic arrows, just the electric arrows are shown.

Part e. Of course, no light beam consists of merely one photon. In reality, a bundle of billions of photons are whizzing through space. The orientation of their EM-components will be at all sorts of angles.

Inside the glasses

Figure 2. A cartoon of atoms in the polarized filter of sunglasses. On average, they are lined up in a chain in a way as to allow the electron cloud to mainly move up and down, not left and right. The material is said to be vertically polarized.
Figure 2. A cartoon of atoms in the polarized filter of sunglasses. On average, they are lined up in a chain in way as to allow the electron cloud to mainly move up and down, not left and right. The material is said to be vertically polarized.

The atoms of polarized sunglasses are lined up in a chain of atoms in such a way that the most wiggle room they have is in the vertical direction as depicted by Figure 2. Incoming photons transfer their energy, through the EM-field, to the wiggling electron cloud, which starts wiggling even more but only in the vertical direction. The latter will activate the EM-field with a vertically orientated electric subfield component, thereby propagating the vertically polarized parts of the incoming light beam.

Photons with a horizontal polarization, i.e. with a horizontally orientated electric subfield, wiggle the long chains of the sunglasses’ atoms in the horizontal direction. Their energy gets distributed over billions of atoms, horizontally, and is merely dissipated as heat: too low for light propagation. So, basically, these type of photons disappear and the sunglasses heat up a little bit.

Lastly, photons with an electric subfield perturbation at an angle in between the horizontal and vertical direction will sometimes pass through, sometimes will dissipate.

Why are polarized sunglasses vertically polarized?

When light hits a surface, the outgoing or reflected light mostly consists of photons with the same electric orientation as that of the reflecting surface. So, roads and water mainly reflect horizontally polarized light. When you’re navigating a vehicle, you would definitely want to prevent blindness from reflections off of the road or water.

Pilots

Computer screens also emit polarized light. If you hold polarized sunglasses in front of it and turn them, at some point, a computer screen’s light will be blocked.

A pair of polarized sunglasses are held in front of a computer screen. They are rotated at an angle of 90 degrees. The image of the computer screen disappears as the sunglasses turn black, blocking every light coming off of the screen.

This is also why pilots don’t wear polarized sunglasses: a slight turn of the head would make it impossible to quickly and reliably read vital information off of their instruments. So, despite what expensive brands would like you to believe, a set of polarized sunglasses called something like ‘aviator sunglasses’ are useless in real-life aviation.

To check if your sunglasses have genuine polarized filters, tilt them in front of a working computer screen. This way you’ll know which pair to leave at home before flying an aircraft.

Cinema

Watching a 3D film in the cinema requires a different type of polarized glasses. So, despite what many people may have told you, it’s not that one glass has been vertically polarized and the other horizontally. True, your left eye needs to receive slightly different images from your right eye but this is achieved in a far more ingenious way.

As you would want the audience to be able to watch the film despite their (sometimes involuntary) head movements, both the film projector and the 3D glasses are cleverly exploiting circular polarization. Otherwise, the moment you would lovingly tilt your head towards your company’s shoulder, a simplistic left-right combination of horizontal and vertical polarization would render any film star on the big silver screen into a vague and flat character.

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