How Come: car side-mirror distortion

Why are objects in a car's side mirror closer than they seem to be? -- asks Lee Cohen of East Meadow reader

Objects in a rearview mirror are indeed closer than they appear. But thankfully, unlike in Jurassic Park, being chased by a 20-foot-tall, 7-ton T. rex is not an everyday driving occurrence. Usually, the most menacing object appearing in a rearview mirror is an angry, tailgating driver.

Some fast mirror facts: Any smooth, reflective surface can be a mirror, from a quiet pond to a shiny chrome toaster. Our rocky moon lights up the night sky because it reflects nearly 14 percent of the sunlight that strikes it back toward the Earth. But a mirror, such as a shiny butter knife or spoon, reflects nearly all of the incoming light, absorbing just a tiny fraction.

To reflect back a recognizable image, a surface must be virtually bump-free. For the least distortion, a mirror must be very flat, its surface varying up or down by no more than 1/25,000 of an inch.

A flat bathroom or dressing room mirror is usually made of glass, coated on one side with a thin layer of shiny metal, usually aluminum. But mirrors come in other varieties than flat planes. Mirrors that curve inward, like the bowl of a spoon, are called "concave."

A concave mirror makes objects look bigger -- both wider and shorter -- because of how light bounces off its curving surface. Want to look at your pores or teeth? Grab a makeup or shaving mirror. These concave mirrors provide you with a handy enlarged view of your face.

Other curving mirrors bulge toward you, like the spoon's rounded back. These "convex" mirrors make objects look smaller overall, while appearing taller and skinnier. If you've ever seen a distorted, supermodel version of yourself in a fun-house mirror, you've looked into the tricky mirror's convex section.

Why all the distortion? Look at yourself in a curved mirror and the light that reaches your eyes is coming from odd angles, rather than in neat, parallel lines. Imagine looking into the back of a spoon. Light reflected from the top bounces up, while light from the bottom reflects down. So your face, from forehead to chin, is elongated and skinny.

Which brings us back to cars. Rearview mirrors on the sides of cars are convex to shrink reflected images. These bulging mirrors allow the driver to see more on either side of the car, providing a wider field of vision -- which helps prevent accidents. At the same time, the mirrors make objects in the rear look smaller, and more distant.

So that T. rex? It's actually right behind you.