Driver? Passenger? Never been in a car in your life? You probably still know the line: "Objects in mirror are closer than they appear." It appears in so many contexts it's nearly lost all meaning. Were it not for its continued presence on every right side-view mirror in North America, it may have been consigned to confusing-proverb status long ago.
In fact, the statement is a safety warning (or disclaimer, if you want to be cynical about it) intended to decrease driver misperceptions of the space to the immediate right of the car. It means exactly what it says: When a driver sees a car in the passenger-side mirror, that car is closer than the reflection would indicate.
Good thing to know, for sure. The passenger mirror distorts the driver's perception of an object's distance. But it's a safety trade-off, because the same mirror, and for the same reason, enhances the driver's perception of something arguably more essential to avoiding accidents.
It starts with the science of light and mirrors and creating images -- in this case, how a particular kind of mirror can distort size, and therefore distance.