Reaching for our sunglasses on a bright, sunny day is second nature; we all know that spending time in the Sun puts us at risk of eye damage and no one enjoys a squinting-induced headache. Perhaps you take them for granted, but there’s more to your sunnies than shaded lenses.

Aside from lending mere mortals an air of film star mystique, sunglasses’ premier function is to block the Sun’s harmful ultraviolet (UV) rays. There are two types of UV rays: UVA (which cause skin cancer and premature ageing) and UVB (responsible for sunburn). Both types have higher frequencies than the visible light our eyes can perceive. They damage our eyes the same way they damage our skin, except that even in the shade reflected rays pose a threat to our eyes.

Sunglass lenses are made from glass, plastic or polycarbonate, with a special UV-absorbing coating. A good pair blocks more than 99 per cent of UV radiation from reaching your eyes. Tints and mirror coatings relieve you from squinting, by absorbing or reflecting intense, dazzling light in the visible part of the spectrum (the light we can actually see).
The highest-grade sunglasses also incorporate a polarising film to combat glare from reflective horizontal surfaces like water, sand and snow. Light waves vibrate just like sound waves do. There’s a mish-mash of horizontal and vertical components to these vibrations, but when light waves strike a uniform horizontal surface they are reflected with a strong, horizontal polarisation. The glare we experience is the jam of light waves all vibrating in the exact same plane. Sunglasses fitted with a polarisation film eliminate this kind of glare by only enabling vertically polarised light to pass through.

Selecting your perfect pair

UV damage is cumulative, meaning it’s never too early – or too late – to start protecting your eyes from the Sun. The most important thing is to choose sunglasses that offer 100 per cent UVA and UVB protection, just as you would with sunscreen. The larger the frames you pick out, and the more they hug your face, the less stray light will reach your eyes from around the edges.

Most people mistakenly believe that the darkness of the lenses is what protects their eyes. In fact, some clear transparent lenses can offer full UV protection, but those with a tint will cut out some portion of the light in the visible part of the spectrum too. Different tints offer various advantages – for example, ambers give sharp definition while greens reduce glare and increase contrast. Finally, if you intend to spend time on the water, beach or ski slopes – invest a bit more and up your protection level with polarising lenses.