Oh sure, reader Nora, ask me a hard question on a Monday! I’m barely awake and I’m supposed to talk physics! All right then, but be prepared: if I don’t make sense, you’ll have no one to blame but yourself. People who are actually knowledgeable about physics, please check me.
You amateur photographers out there likely know what polarized light is. If you think about individual rays of light, they move through space in a wave shape, like humps a garden hose when you flick it up and down. Only visible light is like millions of tiny garden hoses, all carrying waves to the retina of your eye. The thing about those little garden hoses, however, is that they’re not all oriented at the same angle. Some of them are oscillating up and down. Others are oscillating side to side, still others are oscillating at every possible angle in between.
Following me so far? Now imagine you only wanted to allow garden hose humps that are oscillating straight up and down into your eye. You might put up some sort of a barrier with vertical slits in it, so that only the up-and-down humps would fit through it. This is essentially what a polarizing filter or polarized sunglasses do. They only let light waves that are oscillating at a certain angle through. The effect is that much of the light is blocked, glare is reduced and it’s easier to see (especially if you’re near water and light waves are being reflected at all sorts of crazy angles).
So now imagine you’re a scientist in a lab. You’re curious about what happens to polarized light — light with rays that are all oscillating at the very same angle — when it passes through different materials or liquids. You’d set up a polarized light source, aim it at whatever transparent or semi-transparent material you were investigating, and measure to see if the light changed in any way when it emerged out the other side. A device that does this very thing is called a polarimeter.
As it happens, when you pass polarized (up-and-down oscillating) light rays through a concentrated solution of sugar (sucrose) and water, they emerge from it tilted — rotated — to the right. However, if you apply acid and heat to the sugar water and break the sucrose into its component parts — glucose and fructose — then the rays emerge from the solution tilted to the left. The optical rotation is inverted, in other words, and that’s where the term “invert sugar” comes from.
How did a technical term from the world of physics come to be employed in the pastry kitchen? I have no idea. I don’t know why anyone would want to know what happens when you pass polarized light through sugar water, either. It’s why I bake.