Q.The number of stereoisomers of 1,2-dihydroxycyclopentane

Optical isomers are identical in every physical and chemical property EXCEPT one -- the direction each rotates the plane of plane-polarised light. Any compound capable of this (glucose is the standard example) is optically active. An isomer rotating the plane clockwise (right) is dextrorotatory ('d' or (+)); one rotating it anticlockwise (left) is laevorotatory ('l' or (-)).

Enantiomerism describes the specific case of an optically active substance existing in two isomeric forms, otherwise physically and chemically identical, that rotate plane-polarised light by an EQUAL angle in OPPOSITE directions -- such a pair are enantiomers. Structurally, enantiomers are always non-superimposable mirror images of one another: reflect one enantiomer in a mirror and the reflection IS the other enantiomer, and no rotation in space will ever make the two coincide.

The structural condition making enantiomerism possible is an asymmetric (chiral) carbon -- a carbon whose four bonds are satisfied by four genuinely DIFFERENT substituents, marked $C^*$. A molecule containing such a carbon, and not superimposable on its own mirror image, is a chiral (asymmetric) molecule, and the property is chirality. …