Polarised lenses are used in sunglasses to reduce glare from reflective surfaces such as road surfaces, the hood of a car or surface of a lake.
This is done using a process called polarisation, much like a venetian blind controls sunlight through a window.
Sunlight itself is not polarised - light from the sun will either be absorbed or reflected horizontally, diagonally or vertically.
Horizontal reflectivity causes problems for fishermen and drivers etc. The sun's rays bouncing off a horizontal surface such as the surface of the water, the road or a car's bonnet, will strike the viewer's eyes at a similar angle. This light will obscure the useful light that the observer would want to see.
Polarised lenses have a laminated surface containing vertical stripes. These stripes only allow vertically-polarised light to enter the wearer's eyes. Glare is eliminated because the horizontally-polarised light waves cannot bypass the polarised filter.
Polarised lenses will not provide complete protection from glare. If the wearer tilts his head past 45 degrees or so, some of the horizontal light can enter and cause some bright spots.
Some wearers who use polarised lenses in snow may find that they lose some contrast between snow and shadows.
Fishermen and drivers seem to derive the most benefit from polarised lenses, because they deal with mostly horizontal surfaces. A fisherman wearing polarised sunglasses can see below the surface of the water to search for fish and drivers are no longer affected by a constant reflection of light from their car bonnets or the road.
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