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 interested in.) 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. The electric field is vibrating in many directions; all 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.
- reflection from a dielectric surface
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)
blue sky, rainbow
light scattered from small molecules is polarized at right angles to the direction of propagation of the original beam
- 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
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
- birefringent crystals
calcite (Iceland spar)
crystal with a preferential direction of polarized and propagation
"o" ray and "e" ray (ordinary and extraordinary)
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)
Chirality is the property of some objects that makes them distinguishable from their mirror images. Objects that exhibit chirality are said to be chiral. Human hands are the most easily accessible examples of chiral objects, which is why chirality is also often described as handedness. Chirality is just a painfully clever scientific word derived from the Greek word for hand — χερι (kheri).
A typical hand consists of four fingers, a thumb, and a palm. (In this context, a thumb is not considered a finger.) Using the two hands of one person, it is only ever possible to get two of these parts to point in the same direction at the same time.
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. 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).
Optical rotation is the ability that all chiral molecules have to rotate plane polarized light. Think of a polarized light wave as a hand on an analog clock pointing to the 12. Shifting that hand a bit to the right rotates it clockwise, shifting it to the left rotates it counterclockwise. The Latin words for right and left are dexter and laevus, respectively. Chiral molecules that rotate the polarization clockwise are said to be dextrorotatory, while those that rotate it counterclockwise are said to be levorotatory.
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 (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.
|name(s)||classification||chirality||optical activity||[α]* [°/dm]|
|glucose (dextrose, blood sugar)||monosaccharide||right-handed||dextrorotatory||+52.5|
|fructose (levulose, fruit sugar)||monosaccharide||right-handed||levorotatory||−88.5|
|disaccharide of glucose and fructose||right-handed||dextrorotatory||+66.4|
|invert sugar||an equal mixture of glucose and fructose||right-handed||levorotatory||−19.7|
|disaccharide of glucose and galactose||right-handed||dextrorotatory||+52.4|
|disaccharide of two glucose units||right-handed||dextrorotatory||+138.5|
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.
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.