Consider the mantis shrimp, if you dare. These predatory crustaceans come in two types: the smashers with a pair of legs adapted as clubs, and the spearers who have evolved barbed tips to their appendages. What they both have in common are incredible eyes, with up to 16 types of photoreceptor cell (humans bumble around in relative darkness with only three) some of which can detect circularly polarised light, which they might possibly use to signal to one another. This circular polarisation means that while the light waves continue to move in a straight line, their electromagnetic fields rotate perpendicularly (at a right angle) to this direction. Photographers will know this from the circular polarising filters that can be screwed onto a camera lens to increase colour saturation and contrast in a scene at the cost of a little brightness.
Think of chirality, therefore, as handedness, or the direction of a helix. It may twist to the left, or to the right. Think of your hands, or look at them if you have them nearby. Put them palm to palm and their shapes match, but put one on top of the other and they’re clearly different. Chiral structures are everywhere – in the shells of snails, in the shapes of pasta, in the screws holding your desk together. In the world of molecules, chirality is important: a collection of atoms that twists one way may unblock your stuffy nose, but become methamphetamine if it spirals the other. Please don’t write in to tell us that crystal meth also unblocks your nose.
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