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Why are the two forms of glyceraldehyde considered mirror-image isomers?

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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.)

The two forms of glyceraldehyde are considered mirror-image isomers, or enantiomers, because they possess an asymmetrical carbon atom—a central carbon bonded to four different groups—that results in two distinct spatial arrangements that cannot be superimposed on one another.12

These isomers are related to each other as a right hand is to a left hand. In glyceraldehyde, the difference lies in the orientation of the hydroxyl group (—OH) and hydrogen atom attached to the central carbon. As represented by Fischer projection formulas, the two structures are mirror images; they cannot be made identical without breaking and reforming chemical bonds.31

Because of this specific spatial configuration, the two forms exhibit optical activity, rotating the plane of polarized light in opposite directions—one to the right (d- or dextrorotatory) and one to the left (l- or levorotatory).12