What are the optical isomers called that are not mirror images of each other?

Explain: Butan-$2$-ol is chiral (asymmetric) and optically active.

The two enantiomers of secondary butyl chloride differ from each other in which one of the following properties?

Which among the following compounds shows optical activity?

An aqueous solution containing compounds $A$ and $B$ shows optical activity. $A$ and $B$ are stereoisomers. Which of the following possibilities cannot be correct?

In a mixture, two enantiomers are found to be present in $85 \%$ and $15 \%$ respectively. The enantiomeric excess $(ee)$ is (in $\%$)