The concept of sunglasses seems quite simple, a couple of darkened lenses in a frame to block out the sunlight. However, there is more to sunglasses these days and good modern sunglasses work in four major ways.

• Provide protection from UV (ultraviolet) rays. UV rays are emitted by sunlight and can damage the retina and cornea in the eye. A good pair of sunglasses can completely block out all UV rays. Beware of cheap sunglasses however as these can do more damage than good. Cheap sunglasses block some light, allowing your iris to expand but do not block UV rays and therefore increase your exposure to UV rays.
• Provide protection from glare. Glare is caused when sunlight reflects off certain surfaces such as snow or water. These reflections can cause distractions or even obscure areas of vision completely. Good sunglasses can remove 100% of glare. This is done by something known as polarization.
• Provide protection from intense lights. Intense light can be very dangerous to the eye. When too much light is getting into your eyes, the iris starts to close but this can only close so far. If there is still too much light then you begin to squint in an attempt to block out some more. If after all this, still too much light is entering the eye, then retina starts being damaged and can lead to permanent eyesight problems. A good pair of sunglasses can reduce the amount of light entering the eyes by as much as 97%, vastly reducing the possibility of eye damage.
• Block specific light frequencies. There are particular frequencies of light that can blur your vision or increase viewing contrast. By choosing the right color for your sunglasses you can tailor them to specific requirements. e.g. Fishing, skiing.

In general, sunglasses allow some light to be transmitted and some light is reflected away. This occurs at various stages - as the light hits the outside of the lens, the interior of the lens, and the back of the lens. Sunglasses are classified by tint, light transmittance and UV absorption.