Editing 1145: Sky Color

{{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==
The point of this comic is that often, curious children, like [[Jill]], 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.

[[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.

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 follow-up question (e.g., "Why isn't the sky violet?") can often uncover large gaps in our actual understanding.

{{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&nbsp;nm while violet has a shorter wavelength of 380–450&nbsp;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]].

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 freshmen (in a British English context, that would be a student in their initial year of university, more commonly referred to as a "fresher", but the term may elsewhere be applied to those in the first year of other stages of education). If not for this direct quote, the apparent age of the potential student pictured, and the scientific detail of the question, ''could'' easily apply to lessons in the later years of primary education.

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. The actual direction being flipped is the forwards-backwards one (i.e. anything pointing towards the mirror will appear to be pointing back out of the mirror). In turning a piece of paper round to face the mirror, one normally keeps it orientated in the same vertical axis, so having ''yourself'' having flipped the text left-to-right (as could be seen if the ink has permeated the paper, or if you wrote upon a transparent surface), it is no surprise that the image in the mirror now returns your text in this apparent left-to-right-flipped state while now ''facing you''.

If instead you were to present the paper to the mirror by turning it over along a horizontal axis, you preserve the side of the paper that each end of the text appears (after reflection), but the text is inverted top-to-bottom. Also, flipping it around any ''diagonal'' angle maintains the original relationship between points along the direction of that chosen diagonal (flipping everything that has any relative positional component perpendicular to that axis), and still the ''only'' the thing that a mirror then does in addition is reverse the direction from facing away from you (into the mirror) to become facing towards you (back 'through' the mirror).

In other words, the vertical axis is only "special" because we're used to turning objects around it, so we choose to invoke that particular transformation for ourselves, and mistake that for what the mirror actually does.

==Transcript==
:[Jill addresses her mother, drawn with black hair in a ponytail. The mother was sitting at her computer at her desk, but has turned around on her office chair and is now facing the girl.]
:Jill: Mommy, why is the sky blue?
: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.
:Jill: Oh.
:Jill: So why ''isn't'' the sky violet?
:Mother: Well, because, uh... ...hmm.
:[Caption Below the panel:]
:My hobby: Teaching tricky questions to the children of my scientist friends.

{{comic discussion}}

[[Category:Comics featuring Jill]]
[[Category:Physics]]
[[Category:My Hobby]]