Bright sunlight can flash off water, glass or metal. The resulting glare will make it hard to see anything clearly. The solution: Put on a pair of sunglasses with polarized lenses.

Regular, tinted sunglasses contain a coating that reduces sunlight’s intense brightness. It’s like putting thin curtains on a window. But to cut the glare, their lenses need more than a tint. They need what’s more like window blinds — something that lets some light through while blocking the glare.

“Polarized lenses reduce eye strain and improve visibility and contrast,” explains physicist Liliane Ventura. She works at the University of São Paulo, in Brazil. There, she studies sunglasses and other vision aids.

Special filters in polarized lenses reduce “the disruptive effects of glare,” she says. That makes “outdoor activities like driving, fishing and skiing safer.”

But an even bigger problem for our vision can be the sun’s ultraviolet (UV) light. Chronic exposure to these rays can lead to permanent eye damage — such as cataracts and even cancer.

Sunglasses without UV filters may fail to protect our eyes against this damaging light, Ventura’s team has found. Knowing that not all sunglasses are the same, we can look at how these different filters work.

What is polarization?

Light travels as a jumble of waves. “Think of sunlight as a bunch of waves vibrating in all possible directions,” Ventura says. “That’s what we call unpolarized light.” When sunlight hits an especially reflective surface, though — such as water or the hood of a car — much of the light will reflect in just one direction. Usually, it’s horizontal.

The randomly focused light is known as polarization. Polarized light vibrates in just one direction.

Most reflections from horizontal surfaces polarize light. We experience this as glare. However, reflections are not the only way to polarize light. Certain filters can polarize light, too. And these polarizers explain the glare-diminishing effects of some sunglasses.

“Very clever guys in the past thought, ‘Why don’t we intercept the pathway of polarized light [through the lenses of sunglasses]?’” explains José Toschi. An optical engineer, he directs research at Suntech Supplies in São Paulo, Brazil. There, he helps design eyewear, including sunglasses.

This approach applies filters to the lenses. These filters contain molecules lined up next to each other. They create a pattern that resembles a picket fence. “Together, these molecular lines work [to block out glare],” Toschi says.

Here’s how it works. Imagine passing a rope through an opening between two pickets in a fence. Tie one end to a tree. On the other side of the fence, move the free end of the rope up and down. The vertical waves that this creates in the rope can slide through the opening of the fence. Wave the rope from left to right, however, and the waves will become horizontal — and won’t pass through the fence. The direction of the waves no longer aligns with the openings between the pickets.

This is exactly what happens with polarized sunglasses. The filter’s tiny molecular lines act like the pickets in the fence. When light waves try to pass through the glasses’ lines, only the vertical waves make it through. All horizontal waves — the glare — get blocked out.

With that glare now removed, colors appear more vivid and everything looks clearer. The same will not happen with sunglasses that are merely tinted. Their lenses simply reduce how much overall light reaches your eyes.

How to tell if sunglasses are protective

If you’re uncertain whether your tinted sunglasses are polarized, take this simple test.

Look at a screen — such as the one on a smart phone — while wearing the glasses. As you tilt your head or the phone, the screen’s color might change or even darken. If it does, congratulations: You’re wearing polarized lenses.

This works because the lens’ filter is “blocking some of the light,” explains Ventura. And how much it blocks will change as you tilt the phone’s screen.

However, Toschi notes, polarizing lenses alone aren’t enough to protect our eyes. The sun’s UV rays can lead to more than short-term discomfort.

For instance, one study in rabbits found that repeated exposure of their eyes’ cornea — the clear, outermost layer — to UV rays reduces the amount of antioxidant enzymes. The paper cautions, this “very probably contributes to the damage of the cornea (and possibly also deeper parts of the eye)” due to highly reactive oxidants.

Protective lenses should also filter out UV light. And that’s not one wavelength but a spectrum of them. They fall into two bands of light known as UV-A and UV-B.

“[Good] lenses should be able to block the whole spectrum,” Toschi says. “So, when one buys sunglasses, one should demand the UV400 certificate from the vendor.” It confirms that these glasses contain a coating that blocks the entire UV-A and UV-B spectrum — wavelengths that span from 280 to 400 nanometers.

What’s coming next?

Sunglasses have been used for centuries. But as new technology emerges, so do new designs.

For instance, not all light passes through sunglasses’ lenses on its way to your eyes. Some can come around the sides, Ventura notes. Some glasses are now being designed to prevent sunlight from entering through frame edges.

“But the research that excites me the most on polarized lenses is nanocoatings,” Ventura says. “These use ultra-thin layers that block glare, UV, fog and reflections all at once.”

Some of these new lenses are even being sold as “smart.” They can adjust their polarization, for instance, so you get less glare when you’re outdoors but more light when you’re indoors.