Open me, and you can't see me without a mirror. Close me and you can't see me at all. What am I?

Common Wrong Answers
“Window”
A window can be opened and closed, but it does not require a mirror to see through it. You can see through a window without needing to reflect.
“Book”
A book can be opened and closed, but it doesn't relate to visibility in the same way. You can see the pages or text without a mirror.
“Door”
A door can also be opened and closed, but it does not pertain to visibility or reflection. You do not need a mirror to see through a door.
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