mediumwhat am-i

I sit on a bridge. Some people will look through me while others wonder what I hide. What am I?
Common Wrong Answers
“A troll”
A troll is a mythical creature often associated with bridges in folklore, but it doesn't fit the clue about people looking through or wondering what it hides.
“A window”
While a window is something you can look through, it is typically part of a building and not something that sits on a bridge.
“A telescope”
A telescope is used for looking through, but it is not something that sits on a bridge and doesn't match the riddle's context.