Sunglasses or sun glasses are a visual aid, variously termed spectacles or glasses, which feature lenses that are colored or darkened to prevent strong light from reaching the eyes. Many people find direct sunlight too bright to be comfortable, especially when reading from paper in direct sunlight. In outdoor activities like riding, skiing and flying, the eye can receive more light than usual. It has been recommended to wear these kind of glasses whenever outside to protect the eyes from ultraviolet radiation, which can lead to the development of a cataract.

Uses: 
Visual clarity and comfort: They can improve visual comfort and visual clarity by protecting the eye from glare. Various types of disposable sunglasses are dispensed to patients after receiving mydriatic eye drops during eye examinations. 
 
Protection:
Ultraviolet radiation (UV) can cause short-term and long-term ocular problems such as photokeratitis, snow blindness, cataracts, pterygium, and various eye cancers. Medical experts often advise the public on the importance of wearing sunglasses to protect the eyes from UV. In the European Union, a CE mark identifies glasses fulfilling quality regulations. In the preparation for solar eclipses, health authorities often warn against looking at the sun through sunglasses alone. There is no demonstrated correlation between high prices and increased UV protection.
A 1995 study reported that "Expensive brands and polarizing sunglasses do not guarantee optimal UVA protection." The Australian Competition and Consumer Commission, has also reported that "Consumers cannot rely on price as an indicator of quality". More recently, High energy visible light (HEV) has been implicated as a cause of age-related macular degeneration, and some manufacturers design to block it.  Sunglasses may be especially important for children, as their ocular lenses are thought to transmit far more HEV light than adults (lenses "yellow" with age). Some sunglasses also pass ANSI Z87.1 requirements for basic impact and high impact protection. These are voluntary standards, so not all comply, nor are manufacturers required to comply.  

Sunglass standards:
1 - The Australian Standard is AS 1067. The five sunglass ratings under this standard are based on the amount of light they absorb, 0 to 4, with “0” providing some protection from UV radiation and sunglare, and “4” a high level of protection.
2 - The US standard is ANSI Z80.3-1972. According to the ANSI Z80.3-2001 standard, the compliable lens should have a UVB (280 to 315nm) transmittance of no more than one per cent and a UVA (315 to 380nm) transmittance of no more than 0.5 times of the visual light transmittance
3 - The European standard is EN 1836:2005. The four ratings are 0 for insufficient UV protection, 1 for sufficient UV protection, 2 for good UV protection and 3 for full UV protection.    

Water sunglasses:
Water sunglasses, also known as surfing sunglasses, surf goggles and water eyewear consist of eyewear specially adapted to be used in turbulent water, such as the surf. Features normally available included shatter proof & impact resistant lenses and a strap or other fixing to keep glasses in place during sporting activities, buoyancy to stop them from sinking, should they be displaced from the wearer nose cushioned vent or other method to eliminate fogging.  Many sports utilize these sunglasses including surfing, windsurfing, kiteboarding, wakeboarding, kayaking, jet skiing, bodyboarding, and water skiing.    

Lens:
The color of the lens can vary by style, fashion, and purpose, but for general use, green, grey, yellow, or brown is recommended to avoid or minimize color distortion which would be dangerous when, for instance, driving a car.
 Gray lenses are considered neutral because they do not enhance contrast or distort colors.
 Brown and green lenses cause some minimal color distortion, but have contrast-enhancing properties.
 Red lenses are good for medium and lower light conditions because they are good at enhancing contrast but causes color distortion.
 Orange and yellow lenses have the best contrast enhancement at depth perception but cause color distortion. Yellow lenses are commonly used by golfers and shooters for its contrast enhancement and depth perception properties.
 Blue and purple lenses offer no real benefits and are mainly cosmetic.
 Clear lenses are used typically to protect the eyes from impact, debris, dust, or chemicals.
Some with interchangeable lens have optional clear lenses to protect the eyes during low light or night time activities.

Polarized lenses (made from Polaroid or a similar material) to reduce glare caused by light reflected from polarizing surfaces such as water (see Brewster's angle for how this works) as well as by polarized diffuse sky radiation (skylight). This can be especially useful when fishing, as the ability to see beneath the surface of the water is crucial.

Mirror coating can also be applied to the lens. This mirrored coating reflects some of the light when it hits the lens before it is transmitted through the lens making it useful in bright conditions. These mirrored coatings can be made of any color by the manufacturer for styling and fashion purposes. Glasses of this type, are sometimes called mirrorshades. A mirror does not get hot in the sunlight and prevents scattering in the lens bulk. Their popularity with police officers in the United States has earned them the nickname "cop shades". The two most popular styles for these are dual lenses set in metal frames (which are often confused with Aviators), and "Wraparound" (a single, smooth, semi-circular lens that covers both eyes and much of the same area of the face covered by protective goggles, combined with a minimal plastic frame and single piece of plastic serving as a nosepick), are quite popular in the world of extreme sports.     
 With the introduction of office computing, ergonomists can recommend mildly tinted glasses for display operators to increase contrast.
 
Plastic lenses are typically made from acrylic, polycarbonate, or CR-39. These lenses are lighter than glass lenses, and offer more resistance to shattering than glass. Polycarbonate lenses are the lightest, and are also almost shatterproof, making them good for impact protection. CR-39 lenses are the most common plastic lenses, due to their low weight, high scratch resistance, low transparency for ultraviolet and infrared radiation, and other advantageous properties. 

Glass lenses have the best optical clarity and scratch resistance, but are heavier than plastic lenses. They can also shatter or break on impact.  

Aviators are glasses with an oversized teardrop-shaped lens and thin metal frames. This design first appeared for issue to U.S. military aviators. Their popularity with pilots, military and law enforcement personnel in the United States has never wavered. In addition to pilots, this style gained popularity with young people in the late 1960's and continues to be very popular today.   

Glacier glasses are made with round lenses and leather blinders that protect the eyes by blocking the sun's rays around the edges of the lenses. Because they provide extra protection from bright sun and light reflected by snow and ice, they are often used when traveling across glaciers or snowfields.