optical activity
Shine a special kind of light — light whose waves all vibrate in a single plane — through a tube of certain liquids, and something strange happens: the plane of vibration emerges rotated, as if the molecules had gently twisted the light on its way through. Substances that do this are called optically active, and the discovery was an early, beautiful clue that molecules have a three-dimensional shape.
Optical activity is the ability of a substance to rotate the plane of plane-polarized light. The cause is chirality: a chiral molecule interacts ever so slightly differently with the two circular components of polarized light, and because its mirror image is a different molecule, the net effect is a measurable twist. A chiral substance present as a single enantiomer is optically active; its mirror-image enantiomer rotates the light by exactly the same amount in the opposite direction. Achiral molecules, and any 50:50 mixture of two enantiomers, produce no net rotation at all.
Historically, optical activity was how chirality was first detected — Pasteur famously sorted mirror-image tartrate crystals by hand and showed their solutions rotated light oppositely. Today the polarimeter still gives a quick, non-destructive readout of whether a sample is a pure enantiomer, a mixture, or achiral, and the magnitude of rotation reports on enantiomeric purity. It is the experimental fingerprint of handedness.
A solution of pure (R)-(+)-glucose in a polarimeter rotates polarized light clockwise. A solution of its enantiomer would rotate it counterclockwise by exactly the same angle; a 50:50 mixture of the two would show zero rotation.
A single enantiomer twists the light; a racemic mixture cancels out to zero.
A subtlety: a sample showing zero rotation is not necessarily achiral. It could be a racemic mixture (equal enantiomers cancelling) or, occasionally, a chiral compound whose rotation is too small to detect at the chosen wavelength. Zero rotation alone does not prove a molecule is achiral.