1. PF Contest - Win "Conquering the Physics GRE" book! Click Here to Enter
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Why don't mirrors invert the image vertically?

  1. Jun 15, 2014 #1


    User Avatar
    Gold Member

    I was looking at the hover text of this xkcd comic. It said:

    I tried to answer the question myself for a while and arrived at the conclusion that I'm a brain-damaged nitwit. Then it occurred to me that the best course of action would be to ask people with higher intelligence. So, can any of you explain the reason to me?

    Thank you. All the best. :smile:
  2. jcsd
  3. Jun 15, 2014 #2
    Why do you think they have reversed left-right? If you raise your left hand the person in the mirror raises a hand on the left side. It does not reverse images left-right. If you are projecting your consciousness into the image of the mirror and pretending that you are looking out of the mirror towards the room then you can think of yourself as reversed... But then you could just as easily pretend you are looking into the wall... Its a psychological phenomenon. You are anthropomorphising or projecting consciousness and perspective onto that which has none. This is easy to do because it looks like a person, but of course we know its not. Its an image that reflects without flipping. What is up reflects on the top, what is on the left reflects on the left.
  4. Jun 15, 2014 #3


    User Avatar

    Staff: Mentor

    Suppose you are facing north and standing in front of a mirror. You point a finger to the west and your image points a finger to the west. You point a finger straight up and your image points a finger straight up. You point a finger to the east and your image points a finger to the east. You point a finger down and your image points a finger down. There's no reversal going on so far.

    But when you point a finger to the north.... Your image points a finger to the south. Come to think of it, although you're facing north your image is facing south. And if you rotated the whole affair, mirror and all, so that you were facing west, then your image would be facing east.

    The mirror is reversing the directions that you call "front" and "back"; the left/right thing is just the result of you defining left and right in terms of the direction that you are facing.
  5. Jun 15, 2014 #4
    Fortunately, this question can be answered by one of the greatest :smile: :

    Last edited by a moderator: Sep 25, 2014
  6. Jun 15, 2014 #5


    User Avatar
    Staff Emeritus
    Science Advisor

    Normally when you look at an object in a mirror its left and right sides are swapped with regards to yours, which is why when people shake hands their right hands are diagonal to each other. But a mirror allows you to see that side of an object without having to turn it around and reverse it.

    The reason objects don't appear to be reversed vertically is because our world is inherently built with clear-cut down and up directions, but not left-right directions. That is, your left and my left are different depending on which way we are facing, but up and down are always the same no matter which way you turn. We can turn left and right all day long, but you never flip upside down to turn around.

    Do this: Take a piece of paper with words on it and flip it upside down in front of a mirror. The words no longer appear reversed, they appear upside down! Similarly, if you and I are standing next to each other, facing the same direction, and I do a handstand, our left and right directions are still the same, but our up and down directions are now reversed!
  7. Jun 15, 2014 #6


    User Avatar
    Science Advisor
    Gold Member

    I made this some time ago:
    It might help visualise what other posters were talking about.

    The point of making the diagram was to show that the left-right inversion perception is just as arbitrary as up-down inversion, and the particular preference humans exhibit is likely due to our lateral symmetry. An imaginary creature with an up-down symmetry only would insist that a mirror inverts those two directions, an a creature with no symmetry would probably never even think a mirror could invert anything but the front-back direction.
  8. Jun 15, 2014 #7
    If you are precise, you will find that the mirror does invert vertically in the same matter as it does for left-right. What do we mean to say the mirror inverts left and right? Well, if I rotate the image in the mirror about the z (up) axis and try to line it up with myself, the left and right sides would be switched. Similarly, if I rotate the image about the horizontal axis (the x axis in the above post), I would find that my head and feet are swapped. But people don't think to make that rotation as much as they do the first one.
  9. Jun 15, 2014 #8


    User Avatar
    Gold Member

    Thank you so much for sharing that excellent video. It finally made me understand. I am also grateful to everyone else for providing wonderful insight on the issue.
    Last edited by a moderator: Sep 25, 2014
  10. Jun 15, 2014 #9


    User Avatar

    Staff: Mentor

  11. Jun 15, 2014 #10
    "When you look at words in a mirror, how come they're reversed left to right but not top to bottom? What's special about the horizontal axis?"

    If you take the mirror down from the wall and lay it on the floor. Then place the word on the floor next to the mirror, standing upright. Then sit on the floor opposite the word. And look at the word in the mirror. It will be reversed vertically. But not horizontally. Further, if you stand up and look down, you will see yourself looking upward. With you right hand on the right. Your right, that is to say, your reflections' left.

    So, the word was reversed horizontally in the question posed because the mirror was oriented so as to create that effect. It needn't be. There are alternatives.
    Last edited: Jun 15, 2014
  12. Jun 16, 2014 #11


    User Avatar
    Homework Helper

    Feynman explains it very well. We imagine that the man in the mirror got to where he is, by starting where we are, and then walking around the mirror, and turning around to face us. Interestingly, we don't imagine that he got there by climbing over the mirror, and then facing us head-to-toe. This is because it is much easier for us to notice the difference between our head and our toes, than the difference between our left side and right side. So horizontal gets mixed up, rather than vertical, because the horizontally incorrect imagination is much closer to the true image than the vertically incorrect imagination.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook