SciTech

How Things Work: sunglasses

Credit: Kelsey Scott/Operations Manager Credit: Kelsey Scott/Operations Manager

While Pittsburgh weather might have you feeling otherwise, spring is officially upon us. And even though that currently means a high of 45˚F with flurries, soon the mercury will rise and the skies will clear. One necessary component of spring and summer will probably be sunglasses.

Besides being a fashion accessory, sunglasses provide necessary health benefits. Howstuffworks.com explains that a high-quality pair of sunglasses can provide protection from harmful ultraviolet (UV) rays, protect your irises from overexposure to intense light and glare, and block out certain frequencies of light. And even though all sunglasses — whether they’re Ray-Bans or a $10 pair from Walmart — are made of some sort of tinted plastic or glass, you might be sacrificing some of the aforementioned benefits for price or style. But how do these accessories manage to protect your eyes from the sun?

First, some background on the phenomenon of light. The different colors of visible light — the ones we see in a rainbow — each have a unique wavelength and frequency. Light from the sun is white light, which is a combination of all colors. Other forms of light, such as UV radiation, have frequencies so high that our eyes cannot see them.

There are three different categories for the light we do see: direct, reflected, and ambient. As you might expect, direct light is light that comes directly from the source to your eyes. You would encounter direct light if you stared at the sun or a streetlight (not recommended). Reflected light, typically called glare, is light that has bounced off a reflective surface: Water, sand, and snow can form particularly harsh reflected light. Ambient light is light that has no clear source, an example of which is the glow that surrounds a city at night.

The ideal pair of sunglasses uses multiple technologies to shield against harmful light. The most familiar and obvious technology is the tint of the lens. Each tint — gray, green, yellow, brown, and even purple — has its own different function. While a gray tint is more all-purpose, yellow tinted glasses specifically filter out the blue wavelength to give its wearer a clearer, crisper image. The lenses become tinted by a process that includes smearing molecules that can absorb light wavelengths onto clear polycarbonate (plastic) lenses.

Another form of technology that sunglasses typically use is an ultraviolet coating. UV light is grouped into two categories based on its wavelength: UVA and UVB. Your cornea can typically absorb all of this light, but they can sometimes penetrate to the lens of your eye. This prolonged exposure can eventually form cataracts or even cause eye cancer. Sunglasses protect your eyes by absorbing the UV wavelengths, which is done by applying a UV coating to the lenses.

One last, popular piece of technology in sunglasses is the photochromic lens, which are able to change color when exposed to UV light. Many well-known sunglasses brands, like Transitions lenses, are made of plastic lenses that are coated in a film of organic molecules. According to The New York Times, these organic particles undergo a chemical reaction when they interact with UV rays and change shape so that the lens becomes tinted. Lenses that are made of glass typically have crystals that can also alter their orientation when exposed to UV light.

As you can see, a proper pair of sunglasses utilizes a lot of different technologies to protect your eyes. So the next time you’re shopping for a new pair of shades, look closely at the label to see what protection it offers. Now that you’re aware of the importance of eye protection, you have no excuse to forgo function for fashion.