How do polarized sunglasses work?

posted Dec 14, 2014, 9:14 PM by Patrick Poole   [ updated Dec 14, 2014, 9:15 PM ]
Nice hat (and sunglasses)
Recently we talked about some cold weather physics, so this week I’m going to get your mind off of blizzards and onto beaches. You can probably guess that sunglasses make things look darker by preventing some sunlight from reaching your eyes, but what about polarizing sunglasses—are they really better for your eyes? (It turns out yes!) And what makes some screens look weird when you look at them through polarized sunglasses?

Most materials can become polarized when there is an electric field present—this means their positive and negative pieces spread out from each other just a bit

First let me explain where the word polarize comes from. The atoms that make up matter always have a positively charged nucleus with negatively charged electrons surrounding it. It’s the attraction from these oppositely charged particles that hold atoms as a whole together (although there are some other interesting forces at play that we’ll discuss in a future post).

Atoms have positive and negative parts

Usually the electrons of an atom are roughly evenly distributed about the nucleus, but that can change when an external electric field is turned on. Since the positive nucleus and negative electrons have different charge, they respond in different ways to this external electric field. The field pushes on the nucleus and pulls on the electrons, and the net result is just a little bit of imbalance in their positions. If you turn the electric field off the electrons will move back to being centered on the nucleus, but while the field is on there are held apart just a bit.

When the electrons aren’t quite centered on the nucleus we say the atom is polarized, meaning that it has a positive and negative electric pole (sort of like the North and South poles of a magnet). Other positive things will tend to move away from a positive pole and towards a negative pole. So if you have your external electric field near a block of material, all of the atoms in that material will tend to shift and be polarized in this way.

Electric fields polarize atoms
Light is composed of traveling and oscillating electric and magnetic fields, and we call the direction of the electric field the polarization direction

We’ve talked before about how light is sometimes best described as a particle and sometimes as a wave. In the wave picture, light is an oscillating, traveling electric and magnetic field. You can still think of little waves traveling along, with brighter light sources giving off more of these waves.

It turns out that this oscillating electric field will always be perpendicular to the magnetic field oscillation, and both of those are perpendicular to the direction the light is traveling. In this way light really is a three dimensional effect: you need one direction for the travel and one direction each for the electric and magnetic fields to oscillate.

We just mentioned how external electric fields will polarize an atom by shifting the negative electrons a little bit with respect to the positive nucleus. When a light wave goes through a material, the oscillating electric field does exactly the same thing—the only difference is that now the electric field is oscillating, too, so the amount of polarization of the atom will also change as the light wave goes past it. Since it’s the electric field of the light that will do the polarizing to an atom, we call the direction that the electric field points the polarization direction of the light wave.

Typically a light source will emit all polarizations (electric field directions) of light, which means it will cause atoms that it travels through to oscillate in many directions

Your everyday light bulb emits unpolarized light—that is, all of the light waves it’s sending out have electric fields that are oscillating in different directions. As that light goes through something like a piece of glass, or even air, it causes atoms in that material to oscillate. In this case one atom will oscillate in a totally different direction than its neighbor because they each got hit with different polarizations of light from the bulb.

Unpolarized light from bulb

It’s possible to have all of the waves emitted by a source of light have their electric fields in one direction. In this case we say the light is polarized, and we usually state what direction the oscillations happen in—horizontal or vertical with respect to the floor, for example. One of the properties of laser light that makes it useful is that the light can be polarized to a very high degree.

Polarized coatings on sunglasses have molecules that are inhibited from oscillating except in one direction

It turns out it’s possible to make a material where the atoms can oscillate in one direction much more easily than they can in other directions. As a result, light coming through the material won’t be able to travel through if its electric field is lined up with one of these inhibited directions. This is exactly how polarized coatings are made: the constituent atoms are only allowed to oscillate along one direction. So light waves that come in with exactly the wrong polarization will get cut out entirely, and light that comes in at an angle will only partially get through. The end result is at least half of the light gets cut down by the polarizing filter, and it could be more if you happen to be looking at light that is mostly polarized in that disallowed direction.

Polarizing filter

Bonus physics—Cool effects from polarized sunglasses

Polarized sunglasses are important because they cut down even more on the amount of light entering your eyes, including harmful ultraviolet rays from the sun. It’s important to protect your eyes with sunglasses, or even better polarized sunglasses, for the same reasons it’s important to protect your skin with sunscreen.

Polarized screen

Other technology takes advantage of light polarization, too. For example, a polarized coating is sometimes applied to car windshields, and if you’re wearing your own polarized sunglasses you’ll see a slight colored tint on those cars. Also, you’ll find that rotating your smart phone sideways will prevent you from seeing the light from the screen. That’s because the filters that make some displays use polarized light, too, but that’s another post.

Also, it turns out that the glare coming from water at the beach or a pool is typically polarized in a certain direction, even though the sunlight that made it initially wasn’t polarized at all! This is another interesting polarization topic we’ll discuss in a future post.