Babinet's principle

Have you ever played with shadows? You know, when the sun is shining and you block the light with something and a dark area appears behind it? Well, Babinet's principle is kind of like that, but with light!

Babinet's principle says that if you have an object that blocks light in one way, and you have another object that lets light through in the same way, the two objects will create the same pattern of light and dark areas behind them!

Let's imagine you have a round hole in a piece of cardboard. If you shine a light through it, you'll get a round spot of light. But if you put a circle made of opaque material over the hole, you'll block the light and get a dark spot instead. So what would happen if you put a round glass disk over the hole instead?

Babinet's principle tells us that you'll get the same pattern of light and dark whether you use the opaque circle or the glass disk, because they each block the light in the same way.

Basically, Babinet's principle says that if you have two objects that let light through and block light in the same way, they'll create the same pattern of light and dark behind them. So if you're ever playing with shadows or looking at how light behaves, Babinet's principle can help you understand what's going on!