https://www.explainxkcd.com/wiki/api.php?action=feedcontributions&feedformat=atom&user=84.37.93.254explain xkcd - User contributions [en]2026-07-06T15:03:57ZUser contributionsMediaWiki 1.30.0https://www.explainxkcd.com/wiki/index.php?title=1145:_Sky_Color&diff=223841145: Sky Color2012-12-10T09:55:44Z<p>84.37.93.254: /* Transcript */</p>
<hr />
<div>{{comic<br />
| number = 1145<br />
| date = December 10, 2012<br />
| title = Sky Color<br />
| image = sky color.png<br />
| 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?<br />
}}<br />
<br />
==Explanation==<br />
{{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.<br />
<br />
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.<br />
<br />
{{W|Richard Feynman}} was a famous American theoretical physicist.<br />
<br />
==Transcript==<br />
:[Girl and her mother, Megan. Megan is at a desk and facing the girl.]<br />
:Girl: Mommy, why is the sky blue?<br />
:Megan: Rayleigh scattering! Short wavelengths get scattered ''way'' more (proportional to 1/''λ''<sup>4</sup>). Blue light dominates because it's so short.<br />
:Girl: Oh.<br />
:Girl: So why ''isn't'' the sky violet?<br />
:Megan: Well, because, uh... ...hmm.<br />
<br />
:Caption: My hobby: Teaching tricky questions to the children of my scientist friends.<br />
<br />
{{comic discussion}}<br />
[[Category:Comics featuring Megan]]<br />
[[Category:Physics]]<br />
[[Category:My Hobby]]</div>84.37.93.254https://www.explainxkcd.com/wiki/index.php?title=1145:_Sky_Color&diff=223831145: Sky Color2012-12-10T09:50:55Z<p>84.37.93.254: /* Explanation */</p>
<hr />
<div>{{comic<br />
| number = 1145<br />
| date = December 10, 2012<br />
| title = Sky Color<br />
| image = sky color.png<br />
| 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?<br />
}}<br />
<br />
==Explanation==<br />
{{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.<br />
<br />
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.<br />
<br />
{{W|Richard Feynman}} was a famous American theoretical physicist.<br />
<br />
==Transcript==<br />
:[Girl and her mother, Megan. Megan is at a desk and facing the girl.]<br />
:Girl: Mommy, why is the sky blue?<br />
:Megan: Rayleigh scattering! Short wavelengths get scattered ''way'' more (proportional to 1/''λ''<sup>4</sup>). Blue light dominates because it's so short.<br />
:Girl: Oh.<br />
:Girl: So why ''isn't'' the sky violet?<br />
:Megan: Well, because, uh... ...hmm.<br />
<br />
{{comic discussion}}<br />
[[Category:Comics featuring Megan]]<br />
[[Category:Physics]]<br />
[[Category:My Hobby]]</div>84.37.93.254