These lenses are used in sunglasses to reduce horizontal glare from reflective surfaces, e.g. the surface of a lake, or the bonnet of a car, much like a venetian blind controls sunlight through a window. They accomplish this through a process called "polarisation".
What is Glare?
When light bounces off of a surface, its light waves travel in all directions: some light travels in horizontal waves, while others travel in vertical waves. When light hits a surface, typically light waves are absorbed and/or reflected in a random manner. However, if light hits a reflective surface (such as water, snow, even cars or buildings) at just the right angle, some of the light becomes "polarized" or 'polarization'. This means that vertical light waves are absorbed while horizontal light waves bounce off the surface, this light can become polarized, resulting in glare that can interfere with your vision by striking the viewer's eyes intensely. Only polarized lenses can remove this glare.
Polarised lenses are laminated with tiny vertical stripes that only allow vertically angled light to enter the wearer's eyes. Glare is eliminated because the horizontal light waves cannot bypass the vertical filter.
Sunglasses and camera lenses are often polarised to reduce glare from surfaces, such as light reflecting off a lake or the bonnet of a car.
How Do Polarised Lenses Work?
When light bounces off of a surface, its waves tend to be strongest usually horizontally, vertically, or diagonally. This is called polarization.
Polarised lenses contain millions of parallel rows of tiny iodine crystals or dichroic dyes (so small they can’t be seen with the naked eye).
Like venetian blinds, the horizontal rows of iodine crystals contained within the polarised lens block out horizontal polarized light waves, letting only (non-polarised) vertical light waves reach the eye polarisation blocks as much as 50% of the light passing through a lens. The person looking through the lens can still see clearly in most cases, but it reduces the brightness and glare of light.