How come your reflection in the bowl of a spoon is upside down? asks reader Matthew Wiegert.
An ordinary mirror is made by coating one surface of a piece of glass with a thin layer of metal, like silver or aluminum. It’s this shiny metal that reflects your image. So a shiny, scratch-free metal spoon, such as one made from stainless steel or polished silver, makes a good mirror, too.
When you stand in front of an ordinary flat mirror, light zooming through the room bounces off your body and hits the mirror. The mirror obediently reflects the light back to you, in neat, parallel rays. So you see an image of yourself. But think about it, and you’ll realize there’s something strange about the image. Since a flat mirror reflects exactly what is put in front of it, the mirror image reverses your right and left sides.
Something even stranger happens with the reflection from a spoon. Your image may be distorted in a number of ways, from size to shape. And you may even find that you’ve been flipped upside down. That’s because the bowl of a spoon is curved rather than flat, like a funhouse mirror, or the rear-view mirror mounted on the side of your car.
Mirrors that curve away from the viewer, like the inside bowl of a spoon, are called “concave.” Depending on an object’s distance from the mirror, a concave mirror can make objects look bigger, wider, and shorter. A shaving or makeup mirror is slightly concave, showing you an enlarged image of your face when you’re close to the surface.
Mirrors that bulge toward you, like the back of a spoon, are called “convex.” They make objects look smaller, taller, and skinnier. Rearview mirrors on cars are convex. Because of the way they shrink images, convex mirrors allow the driver to see more of what’s on either side of the car.
Why all the distortion? Look at yourself from in front of in a curved mirror, and the light that reaches your eyes is coming from many angles. Curved mirrors reflect light at odd angles. Look at the bulging back of a spoon, and light from the top bounces up, while light from the bottom reflects down. This means that light from the top of your head will bounce up, while light from the bottom of your face will bounce down. The result: Your face is right-side up, but it looks longer and skinnier.
But the caved-in side of the spoon is another story. Light bounces upward from the bottom of your face as it hits the bowl’s curved bottom. And light from the top of your face reflects downward from the curved top. The result is an inverted image: You, upside down.
By varying the distance of the spoon, you can play with its special effects. From far way, your face will be small and upside down. Get closer to the spoon, and you’ll be upside-down, but normal- sized. A bit closer, and your inverted face will look larger. Finally, come very close to the reflective surface, and your face will appear enormous–but right side up!