Mirror Reflections

Snow White is getting ready for a date with Prince Charming. Just before the royal coach arrives, she decides to check her appearance. Usually she just asks her mirror for the fairest, but this time she pauses when she sees her reflection and says, "I wonder why the mirror shows my left is on my right, but my head and feet in the proper place?" Alas, her talented mirror, which can be an irritating know-it-all when it comes to kingdom politics, is rendered speechless and has no reply. While she discusses her curfew with the seven dwarfs, let's get an SwRI whizard to answer her question.

"Snow has asked an interesting question, and many have mused over this peculiarity during glances at a mirror. Is there some strange law of optics that can distinguish between top and bottom but not right and left? Is it possible that all the mirrors in the world have been installed the wrong way and should be rotated 180 degrees?

"Actually, the answer doesn't depend on optics or methods of mirror installation so much as the way we perceive things. Try a few experiments: Stand in front of a mirror and look at the reflection of objects on the right side of the room. The objects that are on the right side of the room will appear on the side of the mirror to your right. Wave with you right hand. Your image in the mirror will wave back with the hand on your right. Point to the left -- your image in the mirror will point toward your left.

"Thus the mirror is honestly reflecting whatever is in front of it (and that's exactly what we would expect from a mirror with enough integrity to give honest answers to an angry wicked step-mother). Then what is confusing Snow White? She is interpreting the mirror as a window and expecting the image to appear as her twin standing outside the window. Imagine that Snow did have a twin who walked away from her through a window and then turned to face her. The act of turning would put the twin's right side on Snow's left, but would still leave her head and feet 'in their proper place,' and this is what she is expecting to see.

"So the next time someone complains that politicians use 'smoke and mirrors' to confuse the public, remember that mirrors actually reflect things as they are. After all, Snow White know that she could rely on her mirror to tell her she's still the fairest lady in the kingdom."

Thanks to this month's Whizard, Institute Engineer Dr. Ernest Franke of the Automation and Data Systems Division. A specialist in sensor technology and robotics, Franke's recent activities include data visualization, image processing, and sensor fusion.

The Lighter Side SwRI Home

March 25, 2013