Optical Isomerism

Optical isomers are a genuinely unusual case: two compounds that are IDENTICAL in every physical and chemical property EXCEPT one -- the direction in which each rotates the plane of plane-polarised light passed through it. This phenomenon is optical isomerism, and any compound capable of rotating polarised light this way (glucose is the standard example) is said to be optically active, the property itself called optical activity. An isomer that rotates the plane of polarisation to the right (clockwise, viewed facing the oncoming light) is dextrorotatory -- from the Latin dexter, 'right' -- and is denoted with a 'd' prefix or a (+) sign; one that rotates it to the left (anticlockwise) is laevorotatory -- from laevus, 'left' -- denoted 'l' or (-).

Enantiomerism, specifically, describes the case where an optically active substance exists in two (or more) isomeric forms that are otherwise physically and chemically identical but rotate plane-polarised light by an EQUAL angle in OPPOSITE directions; such a pair is called enantiomers, and the phenomenon enantiomerism. Structurally, enantiomers are always non-superimposable mirror images of one another -- if you could reflect one enantiomer in a mirror, the reflection is the OTHER enantiomer, and no amount of rotating either molecule in space will ever make the two coincide. …