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How does a racemic mixture become optically inactive?

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Two diagrams show the three-dimensional structure of a methane molecule: panel A uses a ball-and-stick model with a central carbon atom connected to four hydrogen atoms in a tetrahedral arrangement, while panel B illustrates the bond angle of 109.5 degrees and bond length of 1.09 angstroms between carbon and hydrogen atoms. — isomerism Tetrahedral geometry of methane: (A) stick-and-ball model and (B) diagram showing bond angles and distances. (Plain bonds represent bonds in the plane of the image; wedge and dashed bonds represent those directed toward and away from the viewer, respectively.)

A racemic mixture is optically inactive because it contains equal quantities of two enantiomers—molecules that are non-superimposable mirror images of each other.12

Each individual enantiomer is capable of rotating the plane of polarized light by a characteristic angle. However, because the two enantiomers are present in equal amounts, the rotatory effect of one component is exactly equal and opposite to the effect of the other. Consequently, these effects cancel each other out, resulting in a mixture that shows no net rotation of light and is therefore optically inactive.132

The transformation of an optically active substance into such an inactive racemic mixture is known as racemization. This process typically occurs through a reversible transformation where the optically active substance is converted into an unstable, symmetric intermediate that lacks optical activity; the subsequent return to a stable state is equally likely to produce either enantiomer, leading to the formation of the racemic mixture.1