Polaroid
Encyclopedia
Polaroid is the trademark for a type of synthetic plastic
sheet which is used to polarize light, made by Polaroid Corporation
.
, consists of many microscopic crystal
s of iodoquinine
sulfate (herapathite
) embedded in a transparent nitrocellulose
polymer
film. The needle-like crystals are aligned during manufacture of the film by stretching or by applying electric or magnetic fields. With the crystals aligned, the sheet is dichroic
: it tends to absorb light which is polarised parallel to the direction of the crystal alignment, but transmits light which is polarised perpendicular to it.
The resultant electric field of an electromagnetic wave (such as light) determines its polarisation. If the wave interacts with a line of crystals as in a sheet of polaroid, any varying electric field in the direction parallel to the line of the crystals will cause a current to flow along this line. The electrons moving in this current will collide with other particles and re-emit the light backwards and forwards. This will cancel the incident wave causing little or no transmission through the sheet.
The component of the electric field perpendicular to the line of crystals however can cause only small movements in the electrons as they can't move very much from side to side. This means there will be little change in the perpendicular component of the field leading to transmission of the part of the light wave polarized perpendicular to the crystals only, hence allowing the material to be used as a light polariser
.
This material, known as J-sheet, was later replaced by the improved H-sheet Polaroid, invented in 1938 by Land. H-sheet is a polyvinyl alcohol
(PVA) polymer impregnated with iodine
. During manufacture, the PVA polymer chains are stretched such that they form an array of aligned, linear molecules in the material. The iodine dopant attaches to the PVA molecules and makes them conducting along the length of the chains. Light polarised parallel to the chains is absorbed, and light polarised perpendicular to the chains is transmitted.
Another type of Polaroid is the K-sheet polariser, which consists of aligned polyvinylene chains. This polariser material is particularly resistant to humidity and heat.
s, optical microscope
s and sunglasses
. Since Polaroid sheet is dichroic, it will absorb impinging light of one plane of polarisation, so sunglasses will reduce the partially-polarised light reflected from level surfaces such as windows and sheets of water, for example. They are also used to examine for chain orientation in transparent plastic products made from polystyrene
or polycarbonate
.
The intensity of light passing through a Polaroid polariser is described by Malus' law.
Plastic
A plastic material is any of a wide range of synthetic or semi-synthetic organic solids used in the manufacture of industrial products. Plastics are typically polymers of high molecular mass, and may contain other substances to improve performance and/or reduce production costs...
sheet which is used to polarize light, made by Polaroid Corporation
Polaroid Corporation
Polaroid Corporation is an American-based international consumer electronics and eyewear company, originally founded in 1937 by Edwin H. Land. It is most famous for its instant film cameras, which reached the market in 1948, and continued to be the company's flagship product line until the February...
.
Patent
The original material, patented in 1929 and further developed in 1932 by Edwin H. LandEdwin H. Land
Edwin Herbert Land was an American scientist and inventor, best known as the co-founder of the Polaroid Corporation. Among other things, he invented inexpensive filters for polarizing light, a practical system of in-camera instant photography, and his retinex theory of color vision...
, consists of many microscopic crystal
Crystal
A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...
s of iodoquinine
Quinine
Quinine is a natural white crystalline alkaloid having antipyretic , antimalarial, analgesic , anti-inflammatory properties and a bitter taste. It is a stereoisomer of quinidine which, unlike quinine, is an anti-arrhythmic...
sulfate (herapathite
Herapathite
Herapathite, or iodoquinine sulfate, is a chemical compound whose crystals are dichroic and thus can be used for polarizing light.According to Edwin H. Land, it was discovered in 1852 by William Bird Herapath, a doctor in Bristol. One of his pupils found that adding iodine to the urine of a dog...
) embedded in a transparent nitrocellulose
Nitrocellulose
Nitrocellulose is a highly flammable compound formed by nitrating cellulose through exposure to nitric acid or another powerful nitrating agent. When used as a propellant or low-order explosive, it is also known as guncotton...
polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
film. The needle-like crystals are aligned during manufacture of the film by stretching or by applying electric or magnetic fields. With the crystals aligned, the sheet is dichroic
Dichroism
Dichroism has two related but distinct meanings in optics. A dichroic material is either one which causes visible light to be split up into distinct beams of different wavelengths , or one in which light rays having different polarizations are absorbed by different amounts.The original meaning of...
: it tends to absorb light which is polarised parallel to the direction of the crystal alignment, but transmits light which is polarised perpendicular to it.
The resultant electric field of an electromagnetic wave (such as light) determines its polarisation. If the wave interacts with a line of crystals as in a sheet of polaroid, any varying electric field in the direction parallel to the line of the crystals will cause a current to flow along this line. The electrons moving in this current will collide with other particles and re-emit the light backwards and forwards. This will cancel the incident wave causing little or no transmission through the sheet.
The component of the electric field perpendicular to the line of crystals however can cause only small movements in the electrons as they can't move very much from side to side. This means there will be little change in the perpendicular component of the field leading to transmission of the part of the light wave polarized perpendicular to the crystals only, hence allowing the material to be used as a light polariser
Polarizer
A polarizer is an optical filter that passes light of a specific polarization and blocks waves of other polarizations. It can convert a beam of light of undefined or mixed polarization into a beam with well-defined polarization. The common types of polarizers are linear polarizers and circular...
.
Polyvinyl alcohol
Polyvinyl alcohol is a water-soluble synthetic polymer .-Properties:...
(PVA) polymer impregnated with iodine
Iodine
Iodine is a chemical element with the symbol I and atomic number 53. The name is pronounced , , or . The name is from the , meaning violet or purple, due to the color of elemental iodine vapor....
. During manufacture, the PVA polymer chains are stretched such that they form an array of aligned, linear molecules in the material. The iodine dopant attaches to the PVA molecules and makes them conducting along the length of the chains. Light polarised parallel to the chains is absorbed, and light polarised perpendicular to the chains is transmitted.
Another type of Polaroid is the K-sheet polariser, which consists of aligned polyvinylene chains. This polariser material is particularly resistant to humidity and heat.
Applications
Polaroid sheets are used in liquid crystal displayLiquid crystal display
A liquid crystal display is a flat panel display, electronic visual display, or video display that uses the light modulating properties of liquid crystals . LCs do not emit light directly....
s, optical microscope
Microscope
A microscope is an instrument used to see objects that are too small for the naked eye. The science of investigating small objects using such an instrument is called microscopy...
s and sunglasses
Sunglasses
Sunglasses or sun glasses are a form of protective eyewear designed primarily to prevent bright sunlight and high-energy visible light from damaging or discomforting the eyes. They can sometimes also function as a visual aid, as variously termed spectacles or glasses exist, featuring lenses that...
. Since Polaroid sheet is dichroic, it will absorb impinging light of one plane of polarisation, so sunglasses will reduce the partially-polarised light reflected from level surfaces such as windows and sheets of water, for example. They are also used to examine for chain orientation in transparent plastic products made from polystyrene
Polystyrene
Polystyrene ) also known as Thermocole, abbreviated following ISO Standard PS, is an aromatic polymer made from the monomer styrene, a liquid hydrocarbon that is manufactured from petroleum by the chemical industry...
or polycarbonate
Polycarbonate
PolycarbonatePhysical PropertiesDensity 1.20–1.22 g/cm3Abbe number 34.0Refractive index 1.584–1.586FlammabilityV0-V2Limiting oxygen index25–27%Water absorption – Equilibrium0.16–0.35%Water absorption – over 24 hours0.1%...
.
The intensity of light passing through a Polaroid polariser is described by Malus' law.