Polarised Sunglasses
What is polarisation? Where does it come from? How are polarised sunglasses made? Who is asking all these questions? Lets tackle these and more one at a time so you can look pretty clever at key moments.
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What is Polarisation?
Light is an electromagnet wave with an alternating electric and magnetic field. The fields stay perpendicular to each other but can be at any angle with respect to the direction of travel. Sunlight is unpolarised, meaning that its rays have their fields rotated to any angle with no preference. Polarised and unpolarised light is equally bright.
Where Does Polarised Light Come From?
Any non-metallic, reflective surface actively creates polarised light and water is good at it! When light hits the surface of the water, some of it is refracted and some of it is reflected.
The reflected light is not the same as the incident light. Water atoms absorb and re-transmits light, and in so doing promotes a polarisation parallel its surface.
Polarised & Unpolarised Sunglasses
Unpolarised sunglasses reduce the overall brightness of the image by blocking a percentage of the light. The lenses contain a cloud of tiny opaque particles at a scale too small to see with the naked eye, which shades the whole image equally.
Polarising sunglasses work slightly differently – they preferentially reject more of the polarised than non-polarised light. Since reflective surfaces create polarised light, polarised sunglasses selectively reduce the brightest reflections without pushing the rest of the image into darkness.
How Polarising Lenses are Made
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The lenses of polarised sunglasses are made by stretching transparent PVA (Polyvinyl Acetate) plastic in one direction to align the long chain molecules. The sheet is then dipped in Iodine where atoms bond with the PVA chains. The Iodine treated molecules are further stained to increase the filtering percentage to the desired amount. The resulting polarising plastic filter is sandwiched between clear materials for protection.
Notes
- Maximum polarisation of reflected light occurs when the sun is at about 37 degrees to the surface of the water from your point of view.
- Brewster’s Angle
- Is the angle at which incident light with a particular polarisation fully passes into the water with no reflection. For all other angles, a fraction of the incident light is reflected from the surface.
- Varies depending on the refractive index of the two media.
- Can be used to control the amount of reflected and refracted light in optical devices like lasers, cameras and prisms.
References
Disclaimer
Care has been taken to keep the information in this article as accurate as possible but errors are possible, so be aware of the full disclaimer here.