Stereoisomer his contrasts with structural isomers, which share the same molecular formula, but the bond connections or their order differs. By definition, molecules that are stereoisomers of each other represent the same structural isomer.

An optically active substance is one which can rotate the plane of polarisation of plane polarised light. if you shine a beam of polarised monochromatic light (light of only a single frequency – in other words a single colour) through a solution of an optically active substance, when the light emerges, its plane of polarisation is found to have rotated.

When comparing Fischer projections, it is important to recognize that the horizontal bonds point out of the page, and vertical bonds point into the page. If a substituent is held in place, and the other three substituents are rotated, the new structure is the same as the original structure. Holding the phenyl substituent of 4 in place and rotating the other three clockwise gives 1, so these structures are the same.

These molecules have the same molecular formula (C₇H₁₄), making them isomers. However, the molecules differ in their spatial orientations due to a double bond. Therefore, the molecules can best be described as geometric isomers.

Axial and equatorial are terms that describe the orientation of a single group with respect to a chair and can therefore be ruled out. The bromine and chlorine are both oriented downward, on the same side of the chair. By definition, they are cis to one another.