Louis Pasteur's experience:

As a student at L'Ecole Normale in Paris he devoted himself to the property according to which some compounds
cause rotation of the polarized light plane. Only radiations with a magnetic field having an intensity oscillating on one plane (plane of polarization) can go through a Polaroid lens.
If however this polarized light meets, after the lens, an optically active substance, the plane rotates of a certain angle around the propagation axis. The compounds studied by Pasteur included the tartaric acid and its salts (tartrates) prepared by vintners from the cask tartar containing potassium acid tartrate (cream of tartar). The French Biot and the German Mitscherlich had proved that the watery solutions of tartaric acid and tartrates made the light polarization plan rotate clockwise. By producing tartaric acid, some factories isolated another acid too: it had the same composition but its compound was likely to be different because it was optically inactive. It was therefore called differently from the tartaric acid: paratartaric acid or, more commonly, racemic (from Latin 'racemus' - bunch of grapes). In 1844 Mitscherlich wrote that one of the salts deriving from it, the sodium and ammonium racemate, was absolutely identical to the corresponding tartrate, even in its crystalline form.
That article spurred Pasteur: he felt annoyed by the coincidence of so many properties with such a clear-cut difference in the optical activity.While observing the crystals of 19 by-products of the tartaric acid he noticed that they all showed facets that made them asymmetric: something like a series of either all left or all right hands. Being persuaded that there was a link between optic activity and molecular asymmetry, and thinking that the latter should reflect itself in a crystal asymmetry, he was really astonished to find out, in 1848, asymmetric
crystals also in the optically active salt studied by Mitscherlich. At first he thought that his theories were wrong, but he soon realized that those crystals showed two opposite asymmetries, like a series of all right or all left pairs of hands. He patiently separated the right crystals from the left ones: the first had the same optical activity as their corresponding tartrate whereas the second ones made the light polarization plane rotate equally but in the opposite direction.
The racemic acid was composed of tartaric acid molecules and of their mirror images: the two optical activities counterbalanced each other. Pasteur's discovery brought to understand the structure of the organic compounds.