Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

A mirror flips left and right, but not up and down

physics.stackexchange.com

Post reply on HN
Print view View original

21–30 of 84 posts

Re: A mirror flips left and right, but not up and down

#21

Post by khafra »

InclinedPlane wrote: post #8

Mirror's don't flip left and right. We implicitly compare a mirror image to the original by rotating the object, and naturally we maintain top/bottom orientation, this gives the appearance of "flipping" the object left to right. However, if you use vertical rotation to super-impose the object on its mirror image you'll see that it seems to have been flipped top-to-bottom instead.

It would also be easier to see with an object that had near-perfect vertical bilateral symmetry but no horizontal bilateral symmetry; the opposite of a human in a normal posture.

Re: A mirror flips left and right, but not up and down

#22

Post by doyoulikeworms »

Assuming a mirror's face is its front:

Photons bounce off the right side of you, strike the left side of the mirror, and then reach your eyes. Similarly for the left side.

Photons bounce off the top side of you, strike the top side of the mirror, and then reach your eyes. Similarly for the bottom side.

Re: A mirror flips left and right, but not up and down

#25

Post by Knacker_Hughes »

InclinedPlane wrote: post #8

Mirror's don't flip left and right. We implicitly compare a mirror image to the original by rotating the object, and naturally we maintain top/bottom orientation, this gives the appearance of "flipping" the object left to right. However, if you use vertical rotation to super-impose the object on its mirror image you'll see that it seems to have been flipped top-to-bottom instead.

The way I see it, the mirror does flip up and down, at least by the measure of the same experiment used to prove that it flips left and right.

To prove that it flips left and right, imagine a vertical pole between you and the mirror. The pole has a card attached to it with the front facing the mirror. The two dots, red and blue are painted on the card and you can see their reflection in the mirror.

In the reflection, the blue dot is on the right and the red dot is on the left.

If you rotate the pole so that is facing you and you can now see the front of the card you see that the blue dot is really on the left and the red dot on the right.

This proves that the mirror flips left and right.

To prove that it flips up and down, imagine the same pole in front of you with the card facing the mirror, but this time the pole is horizontal.

in the reflection you see that the blue dot is at the top and the red dot is at the bottom.

If you rotate the horizontal pole so that the card is now facing you, you see that the blue dot is really at the bottom and the red dot at the top.

Re: A mirror flips left and right, but not up and down

#28

Post by hammock »

JonnieCache wrote: post #6

The various feynman "fun to imagine" videos, one of which is linked by the answerer, are amazing. I advise you to watch all of them as soon as you can. They're all up on youtube as linked on stackexchange, or you can see them on the BBC's site here: http://www.bbc.co.uk/archive/feynman/ Also, feynman's explanation is much more intuitive and more easily understood than the one given on stackexchange. As you might imag…

Feynman says some interesting stuff but none of these explanations have yet satisfied me. Sure, I get it that a mirror technically does not rotate the image, it does the near/far thing instead, flips the nose, flips the north/south, however you want to put it.

But, this the fact remains: I hold an object in front of me (there is no mirror anywhere). IF I want to see/simulate what the object will look like in a mirror, I rotate it LEFT TO RIGHT (on the vertical plane, around the line that is parallel to my body no matter whether I am standing or laying down). I cannot rotate it any other way and get the same effect. WHY is that?

EDIT: Whoops, you guys are all right, I'm an idiot. Here I am sitting at my desk with quite literally nothing but bisymmetric objects around me- for a second there I went a little crazy. Thanks for setting me straight

Re: A mirror flips left and right, but not up and down

#29

Post by kenjackson »

JonnieCache wrote: post #6

The various feynman "fun to imagine" videos, one of which is linked by the answerer, are amazing. I advise you to watch all of them as soon as you can. They're all up on youtube as linked on stackexchange, or you can see them on the BBC's site here: http://www.bbc.co.uk/archive/feynman/ Also, feynman's explanation is much more intuitive and more easily understood than the one given on stackexchange. As you might imag…

The video says, "Not available in my area"! I don't think they realize I'm in the USA. The U-S-A! Everything is available to me. How could anyone let this happen?!

Re: A mirror flips left and right, but not up and down

#30

Post by numlocked »

JonnieCache wrote: post #6

The various feynman "fun to imagine" videos, one of which is linked by the answerer, are amazing. I advise you to watch all of them as soon as you can. They're all up on youtube as linked on stackexchange, or you can see them on the BBC's site here: http://www.bbc.co.uk/archive/feynman/ Also, feynman's explanation is much more intuitive and more easily understood than the one given on stackexchange. As you might imag…

It's funny - that series of videos is exactly the sort of video that would make someone a YouTube star today.

In the same way a teenage girl gains a following after making videos of how to apply makeup, I could easily see Feynman becoming a similar YouTube phenomenon had he lived now and released videos like this on a regular basis.

Always fun to imagine people from another era who you know would have been MASSIVE on social media (another one: Robert Townsend -> Twitter).

Post reply on HN