“Objects in mirror are closer than they appear.” It's a warning you see in basically every car, but why can't passenger-side mirrors display objects accurately? Well, it's actually a careful design choice made with safety in mind.
The way we see things is dependent on how light reflects off objects around us. An object's color, texture, shape, and other characteristics influence the direction and intensity of light that bounces off them. If the objects are reflected off an intermediate object, like a mirror, our perception of the original object may be distorted.
The shape of the mirror also makes a difference in our perception. In the U.S., passenger-side mirrors are convex (curved slightly outward), whereas driver-side mirrors are flat. A convex mirror placed on the passenger side reduces the driver's blind spots on that side of the vehicle by presenting a wider field of view, but it also makes other cars appear farther away due to a slight distortion caused by the shape. The flatter mirror on the driver’s side produces a more accurate depiction of what’s behind the car with a more narrow field of view, since light bounces off in the same direction that it hits the mirror and doesn't distort the reflection of the object.
When the two mirrors' reflections are combined in the driver's point of view, drivers have the ability to both see wider areas on the passenger side while keeping their eyes (mainly) on the road. The flat-convex combo has been the U.S. standard for years, though the U.S. Department of Transportation is looking into the safety benefits of two convex mirrors, which European cars usually sport.
For now, always remember to check your mirrors frequently, and look over your shoulder before you change lanes. (Don't forget your turn signal!)