Talk:1145: Sky Color

2012-12-10T10:00:59Z

KeithyIrwin: Added comment.

Of course with '''''vertical''''' mirror vertical axis is selected: perceived switching of left and right (really close with far to mirror surface). When '''standing on''' '''''horizontal''''' mirror we will perceive switching bottom from top. --[[User:JakubNarebski|JakubNarebski]] ([[User talk:JakubNarebski|talk]]) 09:09, 10 December 2012 (UTC)

You're certainly correct, but I think that the original question is not really asking about text (or other things) which are perpendicular to the mirror, but rather text which is parallel to it (and thus the close vs. far doesn't come into it). For example, when reading signs in your rear view mirror or holding a book in front of your chest while looking in a mirror. I've added a little bit to the explanation to attempt to help clarify what's happening in that situation. I'm not sure if it really helps or not. [[User:KeithyIrwin|KeithyIrwin]] ([[User talk:KeithyIrwin|talk]]) 10:00, 10 December 2012 (UTC)

1145: Sky Color

2012-12-10T09:56:04Z

KeithyIrwin: /* Explanation */ Added a longer bit for the mirror explanation. Hopefully, it's all clear.

{{comic
| number = 1145
| date = December 10, 2012
| title = Sky Color
| image = sky color.png
| titletext = Feynman recounted another good one upperclassmen would use on freshmen physics students: 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?
}}

==Explanation==
{{W|Rayleigh scattering}} is the phenomenon that explains the color of the sky, where light of shorter wavelengths gets scattered by the air by the inverse of the fourth power of the wavelength as given in the comic. In the {{w|visibile spectrum}}, blue light has a wavelength of 450–495 nm while violet has a shorter wavelength of 380–450 nm. Violet light does indeed get scattered more than blue light, however the lower portion of the spectrum for sunlight consists of blue light and eyes are much more sensitive to blue light than violet light. This leaves the impression of a blue sky.

The title text refers to a {{w|mirror image}}. A mirror image is a virtual image produced by the reflection of light on a mirror. It does not have an inherent direction of inversion, in terms of horizontal vs. vertical (perhaps one could say it is inverted in depth). The "horizontal" aspect that we perceive is due to the fact that we normally swivel horizontally (about a vertical axis) to look behind us. The image of the world behind us has left and right switched, but not top and bottom. If we somehow normally swiveled vertically about a horizontal axis to look behind, we'd be accustomed to seeing up and down switched, but not left and right, in the view behind. We would then categorize the image in the mirror as inverted vertically, and perhaps wonder why it's not inverted horizontally.

To help understand why this effect happens, imagine that you are holding a sign which says "MIX" and facing a mirror. Initially, you face the sign towards you. The M is on the left and the X on the right. Now, you turn the sign around so that the sign faces the mirror. Now, even without paying any attention to the mirror, simply because you have turned it around, now the M is on the right and the X is on the left and if you could see through the back of the sign, it would say "XIM" from your perspective. When you look at it in the mirror, you are now able to see that orientation and it appears to read "XIM". If instead of turning the sign around horizontally to look at it in the mirror, you flipped it vertically and looked at it in the mirror, it would appear to say "WIX" in the mirror. Thus the mirror is only revealing how the text is oriented relative to your eyes.

{{W|Richard Feynman}} was a famous American theoretical physicist.

==Transcript==
:[Girl and her mother, Megan. Megan is at a desk and facing the girl.]
:Girl: Mommy, why is the sky blue?
:Megan: Rayleigh scattering! Short wavelengths get scattered ''way'' more (proportional to 1/''λ''<sup>4</sup>). Blue light dominates because it's so short.
:Girl: Oh.
:Girl: So why ''isn't'' the sky violet?
:Megan: Well, because, uh... ...hmm.

:Caption: My hobby: Teaching tricky questions to the children of my scientist friends.

{{comic discussion}}
[[Category:Comics featuring Megan]]
[[Category:Physics]]
[[Category:My Hobby]]

explain xkcd - User contributions [en]

Talk:1145: Sky Color

1145: Sky Color