A flip, but not a flop…

“Why is it that when you look in the mirror, the left and right directions appear flipped, but not the up and down?”

Stop reading!

Do not read further until you have honestly tried answering that question!

The question was asked at the Physics StackExchange.

As often is the case, using only text is not at all good when it comes to explaining physics [^]; adding figures does help [^]. And then, animations are even better at it than having just “dead” (static) figures. Going further, interactive graphics, which let the user participate in manipulating the presentation of information, of course beats those mere animations. Better than that, if possible, is an actual demonstration in real life, accompanied by an explanation using simple words.

…As far as the above question is concerned, the Physics Girl [^] does a fairly good job [^].

The best mode of teaching-learning, of course, is an actual and immediate interaction with a person, who in turn might use (and allow you to use) any and all of the above options!

And that’s the reason why, regardless of how much technology progresses, the actual person-to-person type of teaching will never go out of business.

A Video I Liked:

A `Thought Leader’ gives a talk that will inspire your thoughts: [^]



10 thoughts on “A flip, but not a flop…

  1. What a bizarre question – only somebody who has not thought this through would ask it! When I look in the mirror, I see a reflection and NOT another person looking back at me. So, to imagine that the right side of the image is the left side of a person looking back at me, whose image is the left-to-right swap of my own, is a meaningless idea. When I look into the (plane) mirror, I see my right side in the right side of the image, my left side in the left side of the image, my top at the top of the image, and my lower part in the lower part of the image. There is no confusion here.

    The confusion of the person who asked that question arises because he has conflated his perception of a relfection of the image of a person with an actual person, thereby imagining that the right side of the image is the left side of the person and the left side of the image is the right side of the person. But that is just dumb. There is NO person there… just a reflection… left side is left, right is right, top is top and bottom is bottom.

    • No, the question itself is not bizarre. The frat boys at MIT would toss it at a freshie too. Check out Feynman here: https://www.youtube.com/watch?v=jYmclzXff7o . (But yes, the Physics Girl explains it better. It’s just that she goes from one aspect of the issue to another aspect a bit too speedily.)



    • Fair enough. Just for laughs, then? Kind of the academic version of builders sending the new apprentice to the harware store to purchase a left-handed screwdriver and a can of chequered paint. The poor lad always knows he’s being hazed but what choice does he have if he wants to eventually get along with the older bullies he is forced to work with?

  2. I guess I should elaborate for those who are still confused.

    The interpretation that leads to flip but no flop is faulty from the start. It is the view that what is in the mirror is actually behind the glass as a copy of what is in front of it but rotated 180 degress about a vertical axis at the reflecting surface. Does a mirror work like that? Of course not! It is arbitrary and is created by the illusion that the apparent (“virtual”?) person “behind” the glass (we will call him, Reflecto) is an exact replica of the actual person in front of the glass (whom we shall refer to as Perfecto), forcing the misinterpretation that, since Perfecto and Reflecto are apparently the same person, Reflecto’s left seen at Perfecto’s right should be the same as Perfecto’s left, and vice-versa. By this interpretation we have the apparent geometric transformation of the 180 degree rotation of the universe about a vertical axis at the mirror surface, but that is NOT how a mirror works! We merely need to recognise that Reflecto is NOT a person on the other side of the glass but a reflection of Perfecto, and likewise all his surroundings are relected and not objects on the other side of the glass. Once we do away with that faulty starting-point nonsense, we do away with the silly idea that the mirror works by rotating the universe 180 degress about a vertical axis at the reflecting surface. We then see that the top of the reflection is at the top of the source, bottom is at bottom, left is at the left and right is at the right. If we interpret Reflecto as being Perfecto on the other side of the glass, why stop at one rotation? Isn’t that just arbitrary? Why not a second rotation about a horizontal at right angles to the reflecting face to perform that missing flop? The two rotations can then sum into a single rotation about a horizontal axis along the mirror surface but now top is bottom and bottom is top…. Pirates of the Carribean, anybody?

    A faulty starting point often leads to an apparent paradox. A correct understanding of the starting point leads to a paradox-free understanding. Just ask Einstein.


  3. 180 degrees… not degress… a consequent typo due to the extreme cold here this morning, the very stiff and numb fingers. They are warmer, now. Wish I lived in the tropics… rainy hot weather is far and away infinitely more endurable than cold, dark, dry weather. I have tried both and hot is better than cold.


    • 🙂 … Envy me; it’s monsoon here in India—the loveliest season of the year (IMO).

      Overall, I always thought that a “cold” weather (as in half of the USA) is easier to deal with than a “hot” weather (as in India). If it is a cold weather, you can always wear warm clothings. However, none has at least so far developed a technology whereby simple, easily wearable clothes giving an air-conditioning or cooling effects, can be worn. The worst weather is, IMO, the hot and humid one, as in the late May in Mumbai/coastal regions in India (assuming that you have air-conditioners available in the hot and dry regions).



    • Very rarely use air-con. Sucks the life out of the air, makes it too dry. I spent a very short time in a place with 35 degree C temps at consistently over 93% RH (as well as raining now and then) and loved it. Rather sweat than shiver any day. Trouble with putting on clothes for the cold is that when you have enough clothing to keep warm while sitting still at a desk in a cold place, you are so overloaded with clothing that you simply cannot move. It is horribly uncomfortable and the inevitability of chill blains on my toes due to sudden exposure to warm shower water only leads to greater discomfort. This morning I was shaking, not just shivering. You can’t do anything, you can’t concentrate, you can’t sit still, read, type, write, nothing but shiver. Frankly, it is horrible. Once I had enough clothing on I could not move, so I went for a brisk walk to warm me up a litle and take off a couple layers. It works but wears off all too soon. Most annoying. Cold, stiff fingers, wet nose, dry eyes, very dry and hardened skin (contracted capilaries reduce moisture supply to the skin quite dramatically), sore toes, aching feet and an itch on my back that I simply cannot scratch through a ton of clothing… nah, for me hot is better by far! Just not the very dry heat. That is just nasty.

  4. Person-to-person teaching is absolutely central but it only works if the teacher actually understands the subject matter on a deep level or the student has a great capacity for insight beyond the teacher’s explanation which covers any lack of sufficiently deep understanding on the teacher’s behalf. I was fortunate to have at least two good teachers in my university days. The rest was really up to me, for the most part. A combination of a teacher with deep understanding and good communication skills and a keen and diligent student with a capacity for insight is the best combination. I never have believed that true genius is born. Like a sprng of water, it needs a source to fill it and make it flow.

    • Thanks for all your comments.

      IMO, a true genius is neither only born nor only made. As to the making part of it: a genius is self-made, primarily. However, that does not mean that he does not need guidance (sometimes running into years or even decades) for his talents to fully blossom in the best possible manner. As to the self-made part, apart from free-will, I also believe in re-birth, and thus believe that the statements: “a genius is self-made” and “a genius is born” do not contradict each other. In the primary sense of the terms, he is always self-made. However, according to the re-birth ideas, the making of the self itself can span across life-times. A child-prodigy cannot be explained otherwise.



    • “A combination of a teacher with deep understanding and good communication skills and a keen and diligent student with a capacity for insight is the best combination.”

      Yup, you said it.

      Personally, I had a very great maths teacher, but only in the XI and XII standards, and only in a private coaching class. His name was Mr. Wani. (That was the only time I ever took any coaching. The second time I tried coaching was for engg. graphics in I sem of I year, but dropped going to it mid-way.)

      Looking in retrospect, I was lucky, very lucky, to have Mr. Wani as a teacher at just the critical juncture of my learning life. He gave me (us) some very lasting insights about the true nature of maths, esp. calculus, and calculus is so basic to all of college/university physics and engineering. No, he was not the most perfect imaginable. For instance, he did not explicitly state that the infinitesimal is a concept of method. (If he were to state that, many of us students were smart enough that we could have easily got it.) Yet, he did present some informal arguments, even if these were not in the syllabus, to bring out just how it indeed was that. And he always taught with a great sense of humour—and a great sense of condensing down a complex chain of reasoning into a neat nugget. In any case, Mr. Wani was better than many of the most celebrated teachers available today on YouTube. Without the solid foundation he provided, I simply could not have come to appreciate Morris Kline’s books, and as a consequence, come to formulate new insights about the diffusion equation—a part of my PhD research.

      Leaving Mr. Wani’s singular exception aside, most every one of my maths teacher was either hopeless or beyond hopeless. Without an exception, they were excessively pedantic, and emphasized a mixture of rote-learning, mindless memorizing, and advocated that to be seen smart was better than being smart, and that the route to achieve the former was through solving hundreds of sums for endless hours involving abstract symbolic manipulations without any meaning ever attached either to the symbols or to the manipulations.

      The scene was better, far better, when it came to physics and engineering. I had several good teachers in these subjects right from the school-time through the UG and PG engineering.

      Thanks for your thoughtful comments, though!


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