Optical Activity and Mirror-Symmetry

TL;DR

This paper clarifies that non-chiral molecules can exhibit optical activity, which is often misunderstood, by explaining the mirror-symmetry cancellation mechanism at the molecular level.

Contribution

It provides a conceptual explanation of optical activity in non-chiral molecules using mirror-symmetry principles without complex formulas.

Findings

Non-chiral molecules have optical activity at the molecular level.

Optical activity cancels out in liquids due to mirror-image molecules.

Aligned non-chiral molecules can exhibit optical activity.

Abstract

A misconception that non-chiral molecules have no optical activity at all is widespread. However, at molecular level even non-chiral molecules have optical activity. Optical activity of a non-chiral molecule is canceled out by that of another molecule in its mirror image in normal liquids. We describe the canceling mechanism by using mirror-symmetry of physical laws without resorting to detailed formulas. The description will be helpful for overcoming the misconception. Optical activity can be understood from the opposite viewpoint by the description. Aligned non-chiral molecules have optical activity.