Published: 26 November, 1998

Mirror's tilt determines what you see in it

Why is it that when I look at one side of the spoon I see my reflection right side up, and when I turn the spoon over I see my reflection upside down?

To make it easier to picture, think of an enormous spoon, about as big as your head, not counting the handle. You can understand how a curved mirror behaves by thinking of it as built up out of lots of little flat mirrors. Suppose the enormous spoon is a wooden one, made to reflect by gluing a lot of little flat mirrors to both its surfaces, like mosaic tiles on the inside and outside of a dome.

Now imagine holding the spoon vertically some distance from your face, and looking directly into the bowl part of the spoon, with the middle of the bowl at the level of your eyes. As you lower your eyes toward the lower part of the bowl, the little mirrors that you see will tilt upwards and you will see in them the reflection of the upper part of your face. But as you raise your eyes toward the upper part of the bowl, the little mirrors that you see will tilt downwards and you will see in them the reflection of the lower part of your face. In other words you see yourself upside down.

But now turn the spoon so you're looking at the outside of its bowl. As you lower your eyes the little mirrors that you see now tilt downwards and you see a reflection of the lower part of your face. And as you raise your eyes the mirrors that you see tilt upwards and you see a reflection of the upper part. So reflected from the outside of the bowl, you look right-side up.