Sun glasses are protective devices that prevent eyes damaging and discomfort by reducing bright light and UV rays (Partridge, Eric The New Partridge Dictionary of Slang and Unconventional English). In the early 20th century, they were also known as sun cheaters (American slang).
When choosing sunglasses, better to invest in wraparound frames, they limit sunlight more effectively; protection of UVA and UVB rays should be 99 to 100%; gray or brown are the best for color recognition. The darkness of your lenses has no effect on UV protection. People with eye disorders such as cataracts, macular degeneration and retinal disease are especially susceptible to UV rays and should take extra precaution.
Ultraviolet radiation is harshest when the sun is highest - from 10 a.m. until 2 p.m. The closer you are to the earth’s equator; the higher altitude, and the more reflective the surface, the stronger it is. Ultraviolet rays pass through clouds, so don’t be fooled into thinking protective eyewear isn’t necessary when the sky is cloudy. Ultraviolet rays also reflect from walls, roads, snow and sand and your eyes can suffer from high energy light waves even in shaded areas
The United States Environmental Protection Agency (EPA) developed the ultraviolet Index, and to protect your eyes from harmful solar radiation, glasses should block up to 100 percent of ultraviolet rays. A pair of $5 glasses may filter out 100% UV rays just as well as a $200 dollar pair. Generally, more expensive lenses have higher quality and provide a sharper, clearer image with less distortion. UV light is in wavelengths UVA (up to 400 nm) and UVB radiation (290 to 320 nm), both can be harmful to your eyes.
One of important features of your shades is protecting your eyes from ultraviolet light from the sun. A pair of sunglasses which do not filter out UV light can actually cause more damage to the eye than no sunglasses at all. Eyes behind shades dilate and more UV light reach the retina, unprotected exposure to UV light can lead to serious eye problems. So when buying sunglasses, look for the percentage marking of UV blocking. Now many glasses block 99-100% of ultraviolet light
Polarized shades are coated with a special film that helps reduce
glare. Polarization reduces glare coming off reflective surfaces like
water or pavement, this does not offer more protection from the sun, but
can make activities like driving or being on the water safer or more
enjoyable. Polarized sunglasses have been popular for years with boaters
and fishermen who need to reduce reflected glare from the water
surrounding them (https://bryancohad.tumblr.com/)