Which type of isomer emits no optical activity?

Racemic isomers are mixtures that contain equal amounts of two enantiomers, which are stereoisomers that are non-superimposable mirror images of one another. Because they consist of both enantiomers in equal proportions, the optical activities of each compound cancel each other out. As a result, a racemic mixture does not exhibit any net optical activity when observed under polarized light, making it an ideal example of a type of isomer that emits no optical activity.

In contrast, configurational isomers, which include both enantiomers and diastereomers, typically possess optical activity because they do not balance out each other's effects in mixed forms. Conformational isomers can change their spatial arrangement through rotations around single bonds, and specific conformers may or may not be optically active depending on their symmetry and structural features. Geometric isomers, which arise from restricted rotation around double bonds (cis/trans isomerism), can also have differing optical activities, but these characteristics do not inherently lead to a lack of optical activity.

Thus, the absence of optical activity in a racemic mixture is a defining feature, reinforcing the correct identification of this type of isomer.