Dear Matthew AnswerMan Alice:
I took out a spoon for my morning cereal. While looking at the spoon, the mirror image of my face was upside down. When I flipped the spoon over, my face was right-side up. What gives? I tried the same thing with a fork and got the same results.
-- Joe Saucedo, Encinitas
The elves have been staring into silverware and giggling all day. Teaspoons make them look more like pinheads than ever. This little diversion is one of those questions we pulled from our bin marked "Slept Through Physics." When it's explained first time around to a room full of teenagers, nobody hears a thing. But when the teens reach full adultitude, it's suddenly a burning issue. We'll keep this short so we don't trigger any residual snooze reflex. First, imagine the big ole Joe Saucedo cereal spoon (in-curved, concave side facing you) as if it were part of the rim of a bicycle wheel. The spokes of the wheel come out of the rim perpendicular to the point where they're seated. Because the rim is in-curved, all the spokes converge in the center.
Something similar happens to the light rays forming the image of the big old Joe Saucedo face. When they hit the shiny bowl of the spoon, each one bounces off at an angle that makes them converge until they meet at a central point (the focal point of the spoon, like the axle of the bike). The focal point is between you and the spoon. Unlike the bike spokes, the reflected light can keep moving in a straight line through the focal point, so by the time it reaches your big ole Joe Saucedo eyes, what was on top is now on the bottom and vice versa. Armed with this info, can I trust you to figure out why the out-curved (convex) back of the spoon gives you an upright image? Here's another factlet for you to test while your Wheaties go soggy. If your close-up vision were good enough and you could get your eye between the spoon and its focal point, you'd see a giant, upright Joe Saucedo eye. And Ma Alice says stop playing with your food. You'll be late for school.