Recently a video has appeared in my Facebook feed in which someone expresses amazement at how mirrors work. Here is an equivalent video:
I say “Equivalent” because the one I keep seeing involves a tennis ball, not an apple, but I couldn’t find that one on YouTube. The idea is exactly the same though.
40% or more of all commenters on Facebook honestly do not get it. They claim it’s an elaborate hoax, or just express shock that light bounces off mirrors in a manner whereby the angle of incidence is equal to the angle of reflection.
I mean, I can understand science tricks involving rare earth magnets or mercury or something people don’t have a lot of hands on experience with, but everyone has a goddamned MIRROR, and we look into them every single goddamned day, so how can so many people not get this? What the fuck?
Exactly – the only thing most of those people use a mirror for is staring head-on at their own faces. Sure, they also have one for seeing to the rear of their car (if they bother, have you seen how people drive) which you’d think would clue them in that depending on what angle you are looking from different things are visible and you get blind spots… but they probably think a car mirror “works different” than a wall mirror or that it’s because of distance or speed.
I learned that what you see in the mirror is different depending on what angle are you looking from when I was in, what, 3rd or 4th grade, then went to the mirror myself and my junior scientist self was delighted to confirm that… and to figure out “of course, because the light bouncing off the apple is not only going in the direction of my eyes”. This last is the critical one. Did not get the formal explanation until Grade 10 physics but it made total sense, with the concept of the Virtual Image. Your brain thinks it’s seeing an image 6 inches “behind” the far side of the mirror because that’s where the focus lines would converge if you traced them straight.
People get their mind blown because they are thinking of the mirror as a camera, that captures the view in one direction (or one angular field) specifically and reproduces only what’s in THAT “forward” view.
There’s a lot of stupidity going around. Or at least a lack of noticing and a lack of curiosity.
One of the catch phrases in my marriage to my late husband was, “But how do it know?” It comes from a shaggy dog/country boy story told by (I think) Andy Griffith. Talking about how an insulated chest will keep hot things hot and cold things cold. “But how do it KNOW?”
Yes, to be fair, I can see where the effect shown in the video might seem a bit “counterintuitive,” at least for some folks, at least at first. But if someone requests an explanation by asking “How does the mirror know?” the person who tries to explain it might be in for a long afternoon.
FWIW, I’m pretty strong with words, and I’m not bad with numbers, but my spatial intelligence is rough. What may seem intuitive or obvious to the OP is really hard for me to wrap my head around, even with diagrams; I accept that it works, but struggle to comprehend it intuitively.
I would ask folks who are mystified by this visual phenomenon, what is it they would expect to see in that space in the mirror? Just emptiness? That might cause them to think slightly more deeply about it.
I don’t much like that diagram, it makes it look as if the eye is looking through the mirror at an actual object. I think it could be more confusing than helpful.
Bolding mine: I think it’s precisely because we think we’re looking into a mirror, but we really aren’t.
When we look “into” a mirror, it creates the illusion that what we’re seeing is in or behind the surface of the mirror. So we have two apples, the real one and the one in the mirror, only they can’t “see” each other because of the piece of paper between them. So how does the apple in the mirror know what the real apple looks like, so it can reflect it correctly?
“How does the mirror know?” is fine as a joke. And the demonstration can make people think about how mirrors work. I have no problem with it as long as no one thinks the mirror actually “knows” something.
It’s similar to people thinking mirrors switch left vs right. There’s no switch. When you see the image of yourself in a mirror, your right hand is still on the right and your left hand is still on the left.
But our brains want to interpret the image as another person, and our bilateral symmetry causes us conflate the image’s left hand with another person’s right hand.
Imagine that instead of one mirrors, there were two mirrors. One mirror is in the center, the object is held against it with a piece of paper. Really, it can be any size - let’s say it’s the same size as the paper so it’s covered 100%. Now, 3 feet off to the right, there’s another small mirror. Now you stand to the right of that mirror so that you’re viewing it like 10 degrees off from the plane of the mirrors, and you look towards the object behind the other mirror. What do you see?
You see the object in the second mirror, not the first one. The second mirror “knows” what’s there. It may help conceptualize what’s happening. In the example in the OP, it’s all just one big mirror, but it’s the part of the mirror off to the side that “knows” what’s there, not the mirror behind the paper.
By the end of the first sentence, I was holding my hands up to model what you were saying. By the time you got to the third mirror, my visualization was shot.
The black is the wall, the gray are the two mirrors, the yellow is the sheet of paper, and the orange is the object. The blue is tracing the light beam/visual information as it makes its way from the object, bounces off the mirror, and hits the eye of the observer. My angles are a little bit off since I just hand-drew it in paint, but in the real world the angle of incidence is the same as the angle of refraction.
This is basically the same diagram as above (although I think the green line makes it confusing for people) but the reason I’m taking the two mirrors approach to the explanation is that the key to understanding this is that the part of the mirror that’s obscured by the paper is not the part that “knows” something is behind the paper, but rather it’s the part of the mirror off to the side that’s bouncing the light. By seperating these two parts into two separate mirrors, it short circuits what people think of as “the mirror” and makes them understand that it’s two separate areas of the mirror we’re talking about. (Or two separate mirrors if one prefers).
I was wondering if looking at video was throwing people off so I went in the bathroom to try it with a book and a Dixie cup. I think what was happening was more obvious since I had to lean over the sink and get my head close to the mirror to get the angle right. But if you have no clue what’s happening that may not help.
