Why polarised sunglasses reduce glare
Polarised sunglasses contain a filter that blocks light with a particular orientation of its electric field. Glare reflected from horizontal surfaces such as water is often strongly horizontally polarised. The lenses are arranged to reduce that component, making the reflected glare less intense.
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Filtering orientation as well as brightness
Light's electric field oscillates across its direction of travel. A polarising filter transmits one orientation more strongly than the perpendicular orientation.
Reflections from a horizontal surface such as water can contain a strong horizontally polarised component. Sunglass filters are typically oriented to reduce that component, cutting much of the distracting glare.
Polarisation and darkness are different properties. A dark lens reduces incoming light, while a polarising filter selectively reduces light according to its orientation.
- Water reflections may become less glaring.
- Some screens look dim or change appearance when viewed through the lenses.
Polarisation also does not by itself establish ultraviolet protection. That is a separate property of the lenses, rather than an automatic consequence of filtering visible glare.