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| date      = December 10, 2012
 
| date      = December 10, 2012
 
| title    = Sky Color
 
| title    = Sky Color
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| image    = sky_color.png
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| 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?
 
| 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==
 
==Explanation==
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The point of this comic is that often, curious children ask their parents simple questions about understanding how the world works. Often, although the question is simple, the answer is not. "Why is the sky blue?" is a common example, since most parents are not familiar with {{w|Rayleigh scattering}}, and thus are unable to answer the question.
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The point of this comic is that often, curious children ask their parents simple questions about understanding how the world works. [[Randall]]'s hobby is to make those questions infinitely harder to stump the parents and make them uncomfortable.
  
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[[Randall]]'s hobby is to make those questions even harder, so that the parents who ''are'' familiar with the subject (scientists, for example) will be stumped.
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Another point of this comic is that we often think that we understand a scientific phenomenon (e.g. why is the sky blue?); however, a certain simple question (e.g. why isn't the sky violet?) can often uncover large gaps in our actual understanding.
  
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Another point of this comic is that we often think that we understand a scientific phenomenon (e.g., "Why is the sky blue?"). However, a certain simple question (e.g., "Why isn't the sky violet?") can often uncover large gaps in our actual understanding.
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{{W|Rayleigh scattering}} is the phenomenon that explains the color of the sky, where light of every wavelength gets scattered in the air by the inverse quartic (fourth power) of its wavelength as given in the comic. In the {{w|visible 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. A good explanation, including why blue and not violet, can be found in [http://math.ucr.edu/home/baez/physics/General/BlueSky/blue_sky.html Usenet Physics FAQ :: Why is the sky blue?] (but note that human color perception [http://blog.asmartbear.com/color-wheels.html is more complicated] than described there).
  
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{{W|Rayleigh scattering}} is the phenomenon that explains the color of the sky, where light of every wavelength gets scattered in the air by the inverse quartic (fourth power) of its wavelength as given in the comic. In the {{w|visible 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. Furthermore, the sunlight contains more blue than violet to begin with as a result of the surface temperature of the sun. This leaves the impression of a blue sky. A good explanation, including why blue and not violet, can be found in [http://math.ucr.edu/home/baez/physics/General/BlueSky/blue_sky.html Usenet Physics FAQ :: Why is the sky blue?], but note that human color perception [http://blog.asmartbear.com/color-wheels.html is more complicated] than described there.  xkcd later tackles the same question in [[1818: Rayleigh Scattering]].
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The title text refers to a {{w|mirror image}}, and is discussed by Richard Feynman in a famous BBC documentary [http://www.youtube.com/watch?v=6tuxLY94LXw], as one of the problems which he used have fun with first years.  
  
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The title text refers to a {{w|mirror image}} and is discussed by the famous American theoretical physicist {{w|Richard Feynman}} in a famous BBC documentary [http://www.youtube.com/watch?v=6tuxLY94LXw], as one of the problems which he used to have fun with first years (British English for first year student or fresher).
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A mirror image is a virtual image produced by the reflection of light on a mirror. In the mirror image, only front and back are switched around - like a printing press or a rubber stamp. Left and right are still left and right in an absolute reference frame - wave your left arm in front of the mirror and the "mirror person" also waves the arm on the left side. It is only when using "personal" reference frame - tied to the individual - that we can say that the "mirror person" is moving their right arm.  
  
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A mirror image is a virtual image produced by the reflection of light on a mirror. It's common to think of images in mirrors as being reversed left-to-right, as any text held in front of us will appear flipped. This is actually an issue of perception. In a plane mirror, images are reflected directly: the left side of your body will be reflected in the left side of the mirror, and vice-versa. The source of confusion is that people tend to think of a mirror image the way we would think of a person facing us. When another person faces us, they turn around the vertical axis, placing their right hand on our left side, so seeing our left hand on our left side in a reflection ''seems'' like an inversion, even though it's a direct representation. By the same token, in order to hold text up to a mirror, we generally flip it around the vertical axis, so that the start of the text is on right, and the end on the left (in English, at least). When the mirror reflects this, we see the text as backward, but the mirror hasn't reversed it, we reverse it when we turn it toward a mirror.
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The start of the word would still be closest to the door, the end of the word closest to the window, as in the real room, only the person in the mirror is facing the other way. The apparent inversion comes from the fact that the mind projects itself onto the person in the mirror, and the writing on his paper will be illegible (from "his" right to left instead of from "his" left to right)
  
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In other words, the vertical axis is only "special" because we're used to objects turning around it, so we come to expect that reversal, instead of a reflection.  
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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. (Or, to put it more succinctly: mirrors ''don't'' reverse left to right, ''turning around'' does.)
  
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You can induce a mirror to reverse left and right only --- by standing next to it instead of in front of it, facing along the plane of the mirror itself. If you lift your right arm, you can clearly see your image's left arm raising, without having to adjust for frame of reference. Similarly, you can induce a mirror to reverse top and bottom only by holding it flat above your head or laying it flat on the ground and standing on it (or perhaps standing under a suitably equipped bedroom ceiling). See [https://youtu.be/1t4dOPxKgrY this] video for a demonstration.
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{{w|Richard Feynman}} was a famous American theoretical physicist.
  
 
==Transcript==
 
==Transcript==
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:[Jill and her mother, Megan, but with her hair up. Megan is at a desk and facing the girl.]
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:[Girl and her mother. The mother is at a desk and facing the girl.]
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:Jill: Mommy, why is the sky blue?
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:Girl: Mommy, why is the sky blue?
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:Megan: Rayleigh scattering! Short wavelengths get scattered ''way'' more (proportional to 1/''<span title="lambda">&#955;</span>''<sup>4</sup>). Blue light dominates because it's so short.
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:Mother: Rayleigh scattering! Short wavelengths get scattered ''way'' more (proportional to 1/''<span title="lambda">&#955;</span>''<sup>4</sup>). Blue light dominates because it's so short.
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:Jill: Oh.
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:Girl: Oh.
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:Jill: So why ''isn't'' the sky violet?
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:Girl: So why ''isn't'' the sky violet?
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:Megan: Well, because, uh... ...hmm.
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:Mother: Well, because, uh... ...hmm.
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:[Caption Below the panel:]
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:My hobby: Teaching tricky questions to the children of my scientist friends.
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:Caption: My hobby: Teaching tricky questions to the children of my scientist friends.
  
 
{{comic discussion}}
 
{{comic discussion}}
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[[Category:Comics featuring Megan]]
 
[[Category:Comics featuring Megan]]
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[[Category:Comics featuring Jill]]
 
 
[[Category:Physics]]
 
[[Category:Physics]]
 
[[Category:My Hobby]]
 
[[Category:My Hobby]]

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