There are several reasons, but some of the most important are:
- Sunglasses should provide protection from ultraviolet rays in sunlight. Ultraviolet (UV) light damages the cornea and the retina. Good sunglasses can eliminate most harmful UV rays.
- Sunglasses must provide protection from intense light. When the eye receives too much light, it naturally constricts the iris and makes the pupil opening smaller to restrict the amount of light that enters your eye. Once it has constricted the iris as far as it can, the next step is squinting. If there is still too much light, as there can be when sunlight is reflecting off of snow, the result is damage to the retina. Good sunglasses can block light entering the eyes by as much as 97 percent to avoid damage.
- Sunglasses provide protection from glare. Certain surfaces, such as water, can reflect a great deal of light, and the bright spots can be distracting or can hide objects. Good sunglasses can completely eliminate this kind of glare using polarization.