What is Numerical aperture (NA)?

Numerical aperture (NA) is how good a lens or eye can see tiny details.

Imagine you're looking at a small toy car on the floor from above. If your eyes are close to the car, you can see all the little parts, like the wheels and the windows. But if you’re far away, everything looks blurry and squished together. That’s because your eyes (or a lens) have a limit to how much detail they can catch.

Now think of numerical aperture as how wide your "vision" is, like having a big window instead of a small one. A bigger window means you can see more details, even from far away. In the world of tiny things, like microscopes or fiber optics, a higher NA means a lens can capture more light and show finer details.

Think about a magnifying glass. If it has a high numerical aperture, it’s like having a bigger, clearer window to see the little parts of something tiny, just like how you can spot every detail on your toy car when you’re close!