Are all sunglasses the same?
Extensive work has been done in the Laser/Optical Radiation Program at the US Army Center for Health Promotion and Preventative Medicine. Serious concerns were raised about the protective benefits of many sunglasses. Their research goes as far as raising the question: is wearing certain kinds of sunglasses more damaging than not wearing sunglasses at all? This seems incredible, but there is a real issue to be considered.
On a sunny day, the naked human eye is troubled by glare. To avoid the discomfort of the glare, our natural reaction is to avert the face, and to squint the brow and the eye. These instinctive protective mechanisms reduce the amount of UV and visible light entering the eye. Our pupils (the opening through which light enters the eye) also constrict, which again reduces the amount of light entering the eye. This is a normal aversion reflex to bright light.
Now consider what happens if we wear normal sunglasses on a bright day. Because the direct visible light entering our eyes is decreased, our natural protective mechanisms do not kick in. Our pupils remain dilated (that is, more open), we don’t avert our face from the light and we don’t squint.
In other words, our eyes remain far more open and exposed to the light.
Although the normal sunglasses are blocking some of the direct light that would otherwise make us uncomfortable, most sunglasses do not block sufficient light from the sides, top and bottom. This means that light (including damaging UV light) gets into our eyes from these peripheral areas. What’s more, because most sunglasses do not have special coatings on the backs of the lenses, the light that enters from the sides, top and bottom can be reflected straight into the eye.
So normal sunglasses can create a situation where our eyes’ natural protective mechanisms are switched off or decreased. Paradoxically, the eye in it's relaxed state allows more light into it!