Louis Pasteur, Biological Physicist

Polarization was a new topic in physics at that time. <a href="https://en.wikipedia.org/wiki/%C3%89tienne-Louis_Malus" rel="noopener ugc nofollow" target="_blank">Étienne-Louis Malus</a>, a fellow Frenchman, discovered the <a href="https://en.wikipedia.org/wiki/Polarizer#Malus'_law_and_other_properties" rel="noopener ugc nofollow" target="_blank">Law of Malus</a>, governing how much light passes through two polarizers, in 1808, just 14 years before Pasteur’s birth. Pasteur’s friend and mentor <a href="https://en.wikipedia.org/wiki/Jean-Baptiste_Biot" rel="noopener ugc nofollow" target="_blank">Jean-Baptiste Biot</a> first showed that polarized light could be <a href="https://en.wikipedia.org/wiki/Optical_rotation" rel="noopener ugc nofollow" target="_blank">rotated</a> when passed through certain crystals. Pasteur’s contribution was to prove that crystals formed from <a href="https://en.wikipedia.org/wiki/Tartaric_acid" rel="noopener ugc nofollow" target="_blank">tartaric acid</a> could rotate polarized light either clockwise or counterclockwise, depending on the <a href="https://en.wikipedia.org/wiki/Chirality" rel="noopener ugc nofollow" target="_blank">chirality</a> of the crystal (this acid is asymmetric, having two forms that are mirror images of each other, like the left hand and the right hand). In a famous experiment, he inspected the structure of each crystal under a microscope and determined if it was left or right handed. When he then separated the two types of crystals he could obtain rotation in either direction, although a mixture of the two crystals did not rotate light. This discovery, made in 1848, at first appears to arise from physics and chemistry alone, but its relation to biology is that most biological molecules exist in only one version. Handedness matters in biology. Debré writes <a href="https://bradroth.medium.com/louis-pasteur-biological-physicist-534bdc6d18f3">Website</a>