How does polarised sunglasses work
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Light reflected at glancing angles off off non-metal surfaces is strongly polarized parallel to the plane of the surface (this is basic physics quantified by Snell’s law and Fresnel coefficients).
The the light that typically causes ‘glare’ in strong sunlight is therefore horizontally polarised.
The general light of a scene is otherwise mostly unpolarized.
Polarising sunglasses pass vertically polarised light, but block the horizontal polarisation of the glare. In this way they make most of the scene 50% darker, but make the glare much much more than 50% darker, making for more comfortable viewing.
By way of experiment, if you were to put your head on its side (or rotate the glasses through 90 degrees) the glare would be passed at near full-strength, while the rest of the scene is attenuated by 50% – making the glare worse!
Similarly, if you look at glancing-angle reflections in windows through polarising glasses, you will see the reflections come and go as you rotate the glasses.
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