Polarization

Discussion

Linear polarization

Light is a transverse electromagnetic wave that can be seen by a typical human. Wherever light goes, the electric and magnetic fields are disturbed perpendicular to the direction of propagation. This propagating disturbance is what makes light a wave. The fact that the electric and magnetic fields are disturbed makes light an electromagnetic wave. The fact that it disturbs these fields at right angles to the direction of propagation makes light a transverse wave. In this section we will explore what it means to be transverse.

Imagine a light wave traveling toward you, on its way to entering your eye. In what direction is the electric field vibrating? (Light is both electric and magnetic, but it is usually the electric field that we are use for describing orientation.) Up and down? Sure. Left and right? Sure, why not. Both alignments are perpendicular to the propagation of the wave.

Most light sources are unpolarized. That is, the electric field is vibrating in many directions — all of which are perpendicular to the direction of propagation. Polarized light is unique in that it vibrates mostly in one direction. Any direction is possible as long as it's perpendicular to the propagation, be it…

How to produce polarized light.

1. reflection from a dielectric surface
   glare
   reflected light is partially polarized at right angles to the plane of incidence
   Brewster angle: reflected light is completely polarized when the reflected ray is perpendicular to the refracted ray (show that the tangent of the Brewster angle equals the index of refraction)
2. scattering
   blue sky, rainbow
   light scattered from small molecules is polarized at right angles to the direction of propagation of the original beam
3. dichroic crystals
   Dichroism: The property of presenting different colors by transmitted light, when viewed in two different directions, the colors being unlike in the direction of unlike or unequal axes.
   absorbs the component of wave polarized in a particular direction
   tourmaline
   quinine iodosulfate in viscous plastic — crystals oriented by extrusion
   giant thin crystals of iodosulfate of quinine
   Herapathite: the sulfate of iodoquinine
   Edwin Land (1909–1991) United States
4. birefringent crystals
   calcite (Iceland spar)
   crystal with a preferential direction of polarized and propagation
   "o" ray and "e" ray (ordinary and extraordinary)

Circular polarization

I will eventually add some detail to this section, but for now enjoy these animations.

Applications

Polarized light carries information. Magnetic fields, chemical interactions, crystal structures, quality variations, and mechanical stresses can all affect the polarization of a beam of light.

spectroscopy, polarimetry, defectoscopy, astronomy, platography, material research, laser applications, light modulation, agricultural production, electric power generation, environmental control devices, molecular biology, biotechnology

polarized sunglasses, photography

glare, scattering (polarized sky)

How am I going to cover this all?

Liquid crystals

Blah.

3d movies.

I may just skip over this for now. It's not a topic I understand well.

Optical activity

When light transmits through certain transparent materials, the material will rotate the plane of polarization. This process is called optical activity or optical rotation and the material is described as optically active or rotatory.

Think of a polarized light wave as an analog clock flying toward you through space, face forward, hands pointing to the 12. Shifting the hands a bit to the right is a small clockwise rotation, shifting them to the left is a counterclockwise rotation. The Latin words for right and left are dexter and laevus, respectively. Optically active materials that rotate the polarization of light clockwise are said to be dextrorotatory, while those that rotate it counterclockwise are said to be levorotatory. (Yes, I know that the Latin word sinister also means left, but that's not the version of left that was chosen in this case.)

The degree (pun absolutely intended) to which an optically active material rotates the plane of polarization a light depends on the material itself (some are more optically active than others) as well as the number of molecules the light wave interacts with (more molecules means more rotation). Typically the total amount of rotation experienced by a wave of light as it travels from one place to another (an extensive quantity of a situation) isn't as interesting as the amount relative to a standard (an intensive quantity of a material).

The specific rotation or rotatory power of an optically active material is the ratio of the rotation angle to the path length and density.

where…

When the material in question is a solution, the definition is modified slightly. The value is relative to concentration instead of density.

Chirality

Your typical human is equipped with two hands — a left hand and a right hand. Each has four fingers, a thumb, and a palm. (In this context, a thumb is not considered a finger.) The palm is in the middle, the fingers are attached to the top, the thumb sticks out one side. I think you know what I'm talking about.

The two hands of your typical human are similar, but not quite the same. One is the mirror image of the other which makes it possible to tell them apart. A right hand will not fit comfortably into a glove designed for the left hand. Hands are handed. They have this property called handedness, also known as…

Chirality is the property of some objects that makes them distinguishable from their mirror image. Objects that exhibit chirality are said to be chiral, objects that don't are said to be achiral. The term is derived from the Greek word for hand — χερι (kheri). All chiral molecules are optically active (but not all optically active molecules are chiral).

Chirality is an interesting word since it describes the lack of something. A chiral object lacks reflection symmetry. It cannot be superimposed on its mirror image. It can never be brought into complete alignment with its own reflection. The left and right hands of a person can never be positioned so that all three parts (fingers, thumb, palm) point in the same direction. Try it yourself and see.

Certain molecules, especially those containing carbon, are chiral. This is important for the fields of organic chemistry (the word organic refers to carbon in this context) and molecular biology (life as we know it is carbon-based).

Determining whether a particular compound is right- or left-handed is determined by a particularly complicated set of rules that I don't understand (and don't care to understand at this moment), but being able to do so is especially important in organic chemistry (so if you're an organic chemist you better figure it out). Something possibly useful to know for physics students is that all naturally occurring sugars are right-handed and all naturally occurring amino acids are left-handed (except glycine, which is not chiral).

Sugars

All sugars produced by living things are right-handed molecules, but they may rotate the polarization of light in either direction. Glucose is the most abundant simple sugar (a monosaccharide) and is the primary source of energy for all living things. Its name comes from the Greek word for sweet, γλυκος (glykos). Because it rotates plane polarized light clockwise it is also known as dextrose. Fructose is another simple sugar. Its name comes from the Latin word for fruit, fructus. Because it rotates plane polarized light counterclockwise it is also known as levulose.

Chemically bonding glucose and fructose produces sucrose — the stuff that most people today would call sugar (or maybe table sugar). Its name comes from the French word for sugar, sucre. The disaccharide sucrose is dextrorotatory but a mixture of the monosaccharides glucose and fructose is levorotatory. "Invert sugar" is made by heating a solution of sucrose and water. The two halves of the disaccharide separate (hydrolyze) and the rotation caused by the fructose dominates. The polarization of the solution has been "inverted" but the sugars themselves have not had their chirality inverted. Doing this would require the inversion of the molecule in three separate places, which is an extremely tricky thing to do.

Enantiomers

Organic compounds that exist in both left and right handed forms are called stereoisomers. Those that are perfect mirror images of one another are called enantiomers. They demonstrate equal amounts, but opposite directions of optical rotation. In all other respects, their physical and chemical properties are identical. Their physiological actions may differ, because enzymes and other biological receptors can readily discriminate between many enantiomeric pairs. The other isomers may be indigestible or even toxic. Some are just interesting.

Asparagine is an amino acid — one of 22 that make up all naturally ocurring proteins. It was the first to be discovered (1806).

Aspartame

Monosodium glutamate

Carvone is a member of a family of chemicals called terpenoids. Carvone has two enantiomers: a right-handed form which is found in the seed oils of caraway, dill, and anise; and a left-handed form which is found in spearmint oil. The difference in the two flavors is evidence that odor receptors have activation sites that are chiral. Your nose can smell the handedness of some molecules.

The words stereoisomer and enantiomer are built up from Greek roots. This was the style among the educated in the 19th century when these words were invented. (All the cool kids were doing it.)

stereoisomers

Working backwards, the -mer suffix comes from the Greek word μέρος (meros), which literally means "part" but in the context of this discussion would be better thought of as "molecule". The iso- prefix comes from the Greek word ίσος (isos) for "equal", so iso-mers are "equal-parts" or "equal-shares" (like shares in a business). Strictly speaking, isomers aren't molecules that are "equal" to each other. If they were, we wouldn't need a word for them. It's more like they're "similar". Lastly, the stereo- prefix comes from the Greek word στερεός (stereos), which literally means "solid" but in this context would be better thought of as "three-dimensional". Make sense?

enantiomers

This one is easier. The prefix enantio- comes from the Greek word for opposite — έναντιος (enantios). In the context of chirality this would be interpreted as "opposite hand" as a noun or "oppositely-handed" as an adjective. We already connected the suffix -mer to the word "molecule". Now mash the two together and see what you get…

Photoelastic stress

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