Question: Why do mirrors reverse my reflection left-to-right but not top-to-bottom? — JJ, Nome, AK
Answer: They don’t JJ. Mirrors reverse you front-to-back. The law of reflection says the angle of incidence of a light ray is equal to its angle of reflection. The graphic shows how light rays behave when they encounter a plane mirror. As the observer, you see your left shoulder on the left side, and right shoulder on the right side. Your eye-brain system assumes the source of a light ray is in the direction the ray comes from (dashed lines in graphic). There’s no real light coming from inside the mirror, so your reflection is a virtual image. This is as opposed to a real image, where the light rays are actually coming from where the image appears to be.
Your brain interprets this as a left-to-right reversal because the eye-brain system assumes the “copy” of you (the image) should be “you facing you” but that’s not what you see. If the mirror truly flipped you left-to-right then the green and red shoulder markers would be interchanged. That’s not the way mirrors work — at least for plane mirrors. So why don’t mirrors that appear to flip you left-to-right also appear to flip you top-to-bottom?
Imagine a plane mirror attached to the ceiling above you. If you were to look up at your reflection it would indeed be flipped top-to-bottom compared to your physical self. This is the same “front-to-back” flip shown in the graphic. Only this time the reflection recreates you upside-down with the floor “above” your head.
Ambulances rely on this front-to-back flip to identify themselves in your rear view mirror. The letters that say “AMBULANCE” are reversed front-to-back. Here’s a great image showing how that works:
Note that both the actual word and its mirror image can be read properly. Also note that on the real letter “E” you can read the word FRONT, and on the mirror image letter “E” you can read the word BACK. This clearly shows that a mirror image is flipped front-to-back.
And that’s why you see the front of an ambulance labeled like this:
Look at that photo reflected from a mirror and you’ll see how it works. Granted, if you flip that image left-to-right it also becomes readable, but that’s not what your rear view mirror is physically doing. Imagine that same lettering applied to the windshield of the ambulance. From inside the cab it would be just as readable as in someone’s rear view mirror.
There are optical configurations using two mirrors that can truly flip left-to-right, creating an image of you from the mirror’s point of view, as you would see yourself. Curved lenses and curved mirrors can also perform this trick. But your standard wall-mounted mirror is always going to do a straightforward front-to-back flip.
Next week in Sky Lights ⇒ Rotating Detonation Rocket Engines