What is Rayleigh's criterion?

Rayleigh’s criterion is a rule that helps us tell when two things are close enough to look like one, or far enough apart to see them as two separate things.

Imagine you're looking at the headlights of a car coming toward you at night. When the car is far away, the two lights seem like one big light. But as it gets closer, they start to look like two lights. That’s when Rayleigh’s criterion says: “Hey, now I can tell them apart!”

Think of your eyes, or a microscope or a telescope, like looking through a tunnel. If two things are too close together, they get squished into one blurry spot through the tunnel. But if they're far enough apart, the tunnel lets you see them as two separate spots.

This idea is used by scientists to figure out how good their tools are at seeing tiny details, like in a bug’s eye or on a faraway planet!

So Rayleigh’s criterion isn’t magic, it's just a clever way of saying: “I can tell these two things apart now!”