Editing 1818: Rayleigh Scattering

{{comic
| number    = 1818
| date      = March 31, 2017
| title     = Rayleigh Scattering
| image     = rayleigh_scattering.png
| titletext = If you ask "why are leaves green?" the usual answer is "because they're full of chlorophyll, and chlorophyll is green," even though "why does chlorophyll scatter green light?" is a great question too.
}}

==Explanation==
This comic suggests it is better to explain things in an easy-to-understand and intuitive manner, even if such explanations may not be completely accurate.  This is especially the case for children whose ability to grasp abstract physics has not yet fully developed.  Giving the most complete and physically accurate explanation would make the concepts much more elaborate than necessary, and would cause major confusion in inexperienced listeners.

The principle is demonstrated by the explanation on {{w|Diffuse sky radiation|why the sky is blue}}.  The commonly given explanation for this is, as the comic title says, {{w|Rayleigh scattering}}.  However, in order to understand how Rayleigh scattering works to produce a blue color, one must go into {{w|quantum mechanics}} and deal with properties of molecules in air and their effects on different wavelengths of light.  Even then, one will also need to know about the inner workings of human visual perception to realize why the color we perceive isn't the wavelength that's being most strongly scattered (see [[1145: Sky Color]]).  The child is not likely to understand this kind of explanation.

On the other hand, a much simpler explanation, such as "because air is blue", also adequately explains the phenomenon, and is much more understandable to less physically inclined listeners. When [[Science Girl]] asks [[Blondie]] (possibly [[Miss Lenhart]]) why the sky is blue, [[Megan]] walks in and starts to explain in a very scientific way. This is criticized by Blondie, who then convinces her that the simpler explanation is sufficient, as there is a quantum mechanical explanation for every color, there is no need to elaborate on the sky's color any more than any other object's color.

Megan implicitly accepts this, but then in the final panel, Science Girl asks another common question - how do planes fly? Megan starts again to give the traditional answer (airflow causing {{w|Lift_(force)|lift}}) but is interrupted by Blondie saying that it's because the wings of an airplane are full of small birds.  While this might not be as ridiculous as it first seems (the child might later learn that the "tiny birds" are actually air molecules, and "flapping wings" are actually pressure differentials), it is certainly over-simplified to a staggering extent.  Thus Megan and Blondie illustrate the two extremes of education philosophy: where one chooses to teach the complete truth with no regard for whether it's understandable, the other chooses to make up understandable explanations with no regard for whether it's true.  Arguably, neither approach is in the student's best interest and a balance needs to be achieved.

The title text refers to another common question as for why leaves are green. This is commonly explained by the fact that they are filled with {{w|chlorophyll}}, a chemical used by plants for photosynthesis. Randall points out that it would be an equally valid question to ask why chlorophyll is green. This poses an interesting contrast to the answer to the question about the color of the sky, since even physicists are usually satisfied with the general explanation for leaves and don't feel the need to jump into describing quantum phenomena that cause chlorophyll to reflect green light.  Also, "Why does chlorophyll scatter green light" may be a great question because chlorophyll reflects, not scatters, light and this challenges Megan-types to coherently explain the difference before they go challenging little children with pedantry. Or because green light is less efficient during photosynthesis, and explaining that is similar to explaining Rayleigh Scattering.

[https://what-if.xkcd.com/141/ What-if 141] also mentions the simpler explanation to the original question: Sunbeam has this relevant text: "Normal light interacts with the atmosphere through Rayleigh scattering. You may have heard of Rayleigh scattering as the answer to 'why is the sky blue.' This is sort of true, but honestly, a better answer to this question might be 'because air is blue.' Sure, it appears blue for a bunch of physics reasons, but everything appears the color it is for a bunch of physics reasons." There is also a footnote in that comment with an additional example: "When you ask, 'Why is the {{w|Statue of Liberty|statue of liberty}} green?' the answer is something like, 'The outside of the statue is copper, so it used to be copper-colored. Over time, a layer of copper carbonate formed (through oxidation), and copper carbonate is green.' You don't say 'The statue is green because of frequency-specific absorption and scattering by surface molecules.' "

Randall himself has published [https://xkcd.com/thing-explainer/ Thing Explainer] which gives simplified descriptions of complex scientific and technological objects.  Even in his book, some of the more advanced details have been simplified to a toy model (such as calling liquid oxygen "cold wet air" and a nuclear reactor "box of burning metal").

==Transcript==
:[Science Girl asks Blondie a question which she answers while lifting her arm towards Science Girl.]
:Science Girl: Why is the sky blue?
:Blondie: Because air is blue.

:[Megan walks in from behind Science Girl.]
:Megan: No, the sky is blue because of Rayleigh scattering–
:Blondie: Nah, it's because air is blue. Blue light bounces off it and hits our eyes. Same as why anything is any color.

:[Zoom in on Blondie's face.]
:Blondie: It's why far-off mountains look blue – because of all the blue air in the way.

:[Zoom out to Megan standing longer from Science Girl than Blondie who has thrown her arms out. Science Girl is facing directly out towards the reader.]
:Megan: There's a specific quantum mechanism by which–
:Blondie: Yeah but there's a physics mechanism for every color. You don't have to get all quantum right away.

:[Frameless panel with Science Girl looking up at Blondie who stands holding her hands on her sides. Megan speaks from off.panel.]
:Megan (off-panel): ...OK, I guess.
:Blondie: Any other questions?
:Science Girl: How do planes stay up?

:[Blondie holds a finger up in front of Science Girl while Megan now is the one to throw out her arms.]
:Megan: Well, the airflow–
:Blondie: Tiny birds in the wings. Thousands. Flapping Hard.
:Science Girl: WOW!
:Megan: ''NO!''

{{comic discussion}}

[[Category:Comics featuring Science Girl]]
[[Category:Comics featuring Blondie]]
[[Category:Comics featuring Megan]]