Question: what are dextrorotatory and levorotatory substances?
Answer: The prefix dextro comes from the Latin word dexter. It means "to the right". A dextrorotatory compound is often, but not always, prefixed "(+)-" or "D-". If a compound is dextrorotatory, its mirror image counterpart is levorotatory. That is, it rotates the plane of polarized light counterclockwise (to the left).