Educational Questions & Answers

What are polaroid sunglasses made of?

ANSWER I: Polaroids are thin and large sheets of crystalline polarising material (made artificially) capable of producing plane polarised beams of large cross section.

As early as 1852, it was discovered by researchers that synthetic small needle-shaped crystals of iodosulphate of quinine possess the property of polarising light. These crystals are not stable. A Polaroid sheet is prepared from the suspension of these crystals of nitrocellulose. To impart stability, its thin sheet is mounted between two sheets of glass or celluloid.

It can also be obtained by stretching a thin sheet of polyvinyl alcohol strained with iodine. When such a sheet is subjected to a large strain, the molecules get oriented in the direction of applied strain. If the stretched sheet of polyvinyl alcohol is heated in the presence of a dehydrating agent such as hydrochloric acid, it becomes strongly stable. Each polaroid sheet is enclosed between thin glass plates so as to provide mechanical support.

Polaroids are of two types. H-polaroid and the k-polaroid. The main difference is that k-polaroid are not strained with iodine.

ANSWER II: A light wave vibrating in more than one plane is referred to as unpolarised light. Polarised light waves are those in which the vibrations occur in a single plane. It is possible to transform unpolarised light into a polarised one and this process is called polarisation.

A Polaroid filter is able to polarize light because of the chemical composition of the filter material. The molecules of the filters are all oriented along the same direction so that they all absorb light of the same polarisation.

When light reflects from a horizontal surface at an angle, the reflected light tends to be polarised horizontally. At a specific angle, the light is completely horizontally polarised because any vertically polarised light that hits the surface at this angle is allowed to enter the surface without reflection. Since reflections from horizontal surfaces are mostly horizontally polarised, glare is mostly horizontally polarised.

Polarising sunglasses deliberately block horizontally polarised light to reduce glare. There are several types of lens material. CR-39 is a plastic made from hard resin that meets optical quality standards. Polycarbonate is a synthetic plastic material that has great strength and is very lightweight.

Eyeglasses that darken when exposed to the sun are called photochromatic. Photochromatic lenses rely on a specific chemical reaction to UV radiation.

These lenses have millions of molecules of substances, such as silver chloride or silver halide, embedded in them. The molecules are transparent to visible light in the absence of UV light. But when exposed to UV rays in sunlight, the molecules undergo a chemical process that causes them to change shape. The new molecular structure absorbs portions of the visible light, causing the lenses to darken. The number of the molecules that change shape varies with the intensity of the UV rays.

When we go indoors and out of the UV light, the reverse chemical reaction takes place. The sudden absence of UV radiation causes the molecules to return to their original shape, resulting in the loss of their light absorbing properties.

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Other Questions & Answers

What are polaroid sunglasses made of?