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The Rainbow Effect With Polarized Lenses Explained

October 18, 2021

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Why do we sometimes see rainbows when wearing polarized sunglasses? It’s not magic. Let’s look at the simple science that creates a rainbow effect with polarized lenses.

If you have a pair of polarized sunglasses you've probably seen a rainbow or two show up in your field of vision. Usually when looking at reflective surfaces such as car windshields. Maybe you've even questioned if you're really seeing what you think you're seeing. You are. The rainbows are real.

Whether or not there's rain involved, the same principle applies to all rainbows: we're looking at light that bends as it moves through a medium. In the sky, that medium is water in the air, separating light into different frequencies, refraction and reflection that our eyes perceive as the colors of a rainbow. The same thing can happen when that medium is transparent such as glass or plastic. But not all transparent materials refract and reflect light the same.

You're Looking at Stress-Induced Birefringence

What's that? It's commonly found with automotive glass that's tempered to guard against shattering, a process that heats and cools the glass on its surface faster than its center, causing the glass to expand and contract unevenly. That's the stress part. The birefringence part is the resulting property that refracts light at different angles. The more stress a transparent material undergoes during production, the more it’s likely to produce a rainbow effect when you're looking at it while wearing polarized sunglasses.

Why don't you see these rainbows when you're wearing non-polarized glasses? Because without polarized glasses all light waves at all angles are absorbed by your eyes. With polarized glasses, however, you can see the refraction because the polarized lenses filter out the horizontal light waves and let in the vertical ones. This filter allows the eyes to see the light refraction and reflection as rainbows.

How Do Polarized Sunglasses Work?

If you want to truly understand the rainbow effect, you need to know how polarized sunglasses work. And to understand that we first answer another question: what is vision? Vision is what happens when light that's reflected from objects hits the human eye, light that is often scattered by the uneven surfaces of objects, such as cars, trees, or rocks—much of what's around us is uneven.

But the eyes can, and often are, hit by unscattered light as well, reflected by flat smooth surfaces such as snow, and water. This unscattered light, which we commonly call glare, is much brighter than scattered light and typically takes a horizontal path—light that reflects off of flat surfaces such as a highway, lake, or snow-covered land is polarized horizontally. To combat this, polarized lenses put up vertical roadblocks against the incoming horizontal glare.

Coated with specialized polymers, polarized lenses have a thin layer of molecules that assemble in rows through a naturally occurring process. These rows are aligned vertically on the lenses during the manufacturing process of glasses. We can almost think of it as putting up a microscopic picket fence between the eyes and light sources—and so lenses that are polarized serve as a vertical filter against horizontal light. In essence, vertical waves of light make it through the slats of the picket fence, horizontal waves are deflected. When horizontal light is deflected, glare is reduced.

To test how they work you can take two pairs of polarized glasses, turn one to a 90-degree angle and look through them both; you should see nearly no light. It's like putting one section of a picket fence on its side in front of another section that’s right side up to create overlapping horizontal and vertical barriers.

Where Do Polarized Lenses Really Come in Handy?

Outdoors. If you spend a lot of time outdoors, be it for work or leisure time, polarized lenses are a particularly good way to go. Because outside is where you'll encounter the high-glare environments that polarized lenses were made for. This glare is especially pronounced if you're around snow or water, both of which reflect horizontal light at a high level. Anyone who has spent any time by the ocean or on a ski slope can tell you that from firsthand experience. In the very same way we protect our skin from exposure to the sun, polarized lenses protect eyes from that very same light source when we’re outside.

So, embrace the rainbow effect! Your eyes aren’t playing tricks on you when you wear polarized sunglasses, you’re just getting a glimpse at one of the wonders of science.

Written by William McCleary for Knockaround.

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