Polarized glasses are great for eliminating glare, which can help us see hazards and judge textures on the road or trail. But as anyone who has tried them knows, they can also do some odd stuff, like create rainbows around screens, or black spots on car windshields. Jeff Mergy and Anne Nichols of Revant optics helped explain how polarized glasses work, and why they can sometimes make us wonder if we're seeing things.
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Most of the light we see is diffused from a light source like the sun or a lamp, and either absorbed or reflected by the objects around us, which we then see as a color (or a shade, if you’re colorblind). But light reflected from a smooth surface (like ice, wet pavement, or the hood of a car) will travel in a unified direction, which looks like “glare”. This type of light is also referred to as “polarized.”
To explain how polarized lenses deal with glare, Mergy had me imagine a picket fence. If you try to jam a board through the fence horizontally, it will be blocked by the vertical slats. But rotate it vertically, and you’ll be able to pass it through.
Polarized glasses work in a similar fashion, but instead of a fence you have a chemical treatment on the lens which contains molecules that group linearly—and instead of a board, you have horizontally polarized light bouncing off, say, a wet puddle in the road. The molecules in the chemical coating are positioned vertically to absorb that horizontally polarized light, blocking glare and letting you see everything else.
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Mergy suggests a simple test at home to demonstrate the concept. “If you place one pair of polarized sunglasses at 90 degrees to another and look through the lenses you should see almost no light, as one pair is blocking out light in one direction, and the other only allows light to pass in the direction which is being blocked.” Interestingly, this potential blinding hazard is part of the reason that car windshields are not polarized.
So what about those dark stripes that sometimes appear on car windshields? Nichols explains that windshields undergo a tempering process which heats and cools the glass rapidly to prevent it from shattering in the event of an impact. During tempering, the glass tends to cool and contract faster on the surface than in the middle, causing a phenomenon called “stressed birefringence”. “This birefringence causes polarization on a horizontal axis and, combined with the vertical polarization in your glasses, will block out light in some areas” says Nichols.
What about phones? Mergy says that LED displays (which you’ll find on most phones and tablets these days) use polarized filters, which keep the screen viewable from most angles. The polarization over your screen can actually compete with polarization in your sunglasses, especially if you rotate your screen so that the directions of polarizations on both surfaces cancel out all light (think of two sets of window blinds set at opposing angles). Mergy says that modern products are designed with polarized sunglasses in mind, so they shouldn’t go fully black even when rotated. However, not all sunglasses (or screens) are created equally, and imperfect alignment of polarization may cause screens to be less visible with certain glasses.
The rainbows you see with polarized glasses (or without) are a result of light being refracted, or bent as it passes through different mediums. When you see a rainbow in the sky after a storm, you're seeing sunlight passing through water in the air, which behaves a like a prism to separate white light into different frequencies. When you see a rainbow through your polarized glasses (like you might on a phone screen or a windshield) it's likely because another material is refracting the light, like a polymer coating on glass, or a tempered glass screen protector on your phone.
If you find the rainbows and dark spots too distracting, you can always keep a pair of non-polarized sunglasses for the times when you expect to be encounter a lot of screens or tempered glass. But otherwise, just enjoy the benefits of decreased glare and feel better knowing a little more about how it works!
