Polarising filters are not the only way to polarise light. When unpolarised light is reflected from a non-metallic surface, such as water, glass or a wet road, the reflected light is partially plane polarised. More of its oscillations are parallel to the reflecting surface, so light reflected from a horizontal surface is mostly polarised in the horizontal plane.The light that is refracted into the material is also partially polarised, with more of its oscillations in the plane at right angles to those in the reflected light.
At one particular angle of incidence, called the Brewster angle, the reflected light is completely plane polarised. This happens when the reflected ray and the refracted ray are at 90° (perpendicular) to each other. For water the Brewster angle is about 53°.Polaroid sunglassesGlare is bright light reflected from water, snow or a road. Because this reflected light is mostly horizontally polarised, Polaroid sunglasses are made with a vertical transmission axis. They absorb most of the horizontally polarised glare while still letting through about half of the unpolarised light from other objects. An angler wearing them can see fish below the water surface more clearly.If the wearer tilts their head through 90°, the transmission axis becomes horizontal and the glare is transmitted again.Photographers use a rotatable polarising filter on the camera lens in the same way, to cut out reflections from water or shop windows.You can test whether reflected light is polarised by viewing it through a single polarising filter and rotating the filter: the reflection changes from brightest to dimmest every 90°.
At one particular angle of incidence, called the Brewster angle, the reflected light is completely plane polarised. This happens when the reflected ray and the refracted ray are at 90° (perpendicular) to each other. For water the Brewster angle is about 53°.Polaroid sunglassesGlare is bright light reflected from water, snow or a road. Because this reflected light is mostly horizontally polarised, Polaroid sunglasses are made with a vertical transmission axis. They absorb most of the horizontally polarised glare while still letting through about half of the unpolarised light from other objects. An angler wearing them can see fish below the water surface more clearly.If the wearer tilts their head through 90°, the transmission axis becomes horizontal and the glare is transmitted again.Photographers use a rotatable polarising filter on the camera lens in the same way, to cut out reflections from water or shop windows.You can test whether reflected light is polarised by viewing it through a single polarising filter and rotating the filter: the reflection changes from brightest to dimmest every 90°.