Optical flat

Okay kiddo, imagine you have a see-through toy car that you can put things on top of. Now, let's say you're trying to see if the top of your toy car is completely flat.

You could try to eyeball it, but that's not very accurate. Another way is to use an optical flat - which is like a special kind of see-through piece of glass that's so flat it's like a mirror.

When you put the optical flat on top of your toy car, it makes a seal with the surface of the car. Then, you shine a light through the optical flat and look at the reflection. If the reflection is perfectly clear and undistorted, then the surface you're testing is also flat.

Optical flats are used by scientists and engineers to check if surfaces are flat enough to be used for things like making mirrors or lenses. It's kind of like a flatness measuring tool!