In a nutshell, light travels in the form of waves that basically vibrate in random directions and when this light is reflected off a shiny or transparent surface it is partly or fully polarised. The only exception being when the light is perpendicular to the surface, so when Edwin H. Land invented polarised lenses back in the 1930’s, he found that by using vertically polarised filters, he could block a lot of the glare producing light that was emitted horizontally to the surface.
Today, polarised lenses are made mainly from acrylic, polycarbonate or CR-39 synthetic plastics which include a polarisation filter enclosed in these lenses.
Polarised sunglasses have been designed to ‘absorb’ up to 97% of glare from flat, reflective surfaces such as snow, water and road surfaces. Our iris’ contract in bright light conditions helping to reduce the amount of light that enters our eyes but they are unable to block a certain amount of glare, so polarised sunglasses not only perform the function of fashion accessory but also of an important safety device used by the likes of drivers, sportsmen and yachtsmen. In fact, these kinds of people have been using polarised sunglasses for years but it has only become popular amongst the general public in recent years.
Polarised lenses used in sunglasses are great for reducing glare but they do have some disadvantages including difficulty in reading LCD screens such as ATM bank screens and some car dashboard monitors. It has been shown that images whose light reflects at certain angles are invisible to the eye as seen through polarised sunglasses. Also, certain topographical features may not be visible in snow, for example, which could make it quite dangerous. This is because the polarising filters in the sunglasses block some of the light rays reflecting from the surface.
On the flip side of the coin though, polarised sunglasses are great for people with long-sightedness disorders such as hypermetropia and presbyopia by helping the iris focus easier on nearer objects.
The main difference between a normal pair of sunglasses and the polarised versions are, whereas normal sunglasses decrease the actual intensity of all light entering the eye through the lenses, the polarised lenses although doing the same, would do so in a more selective manner.
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