1818: Rayleigh Scattering

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Rayleigh Scattering
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.
Title text: 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.


This comic suggests it is better to explain things in an easy-to-understand and intuitive manner, even if such explanations may not capture all of the scientific detail involved. 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 (as described explicitly in the article on Ignotum per ignotius).

The principle is demonstrated by the explanation on why the sky is blue. The commonly given explanation for this is, as the comic title says, Rayleigh scattering. However, in order to understand how Rayleigh scattering works to produce a blue color, one must go into 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" — that is, air molecules reflect blue light, in the same way blue paint reflects blue light — also, adequately explains the phenomenon and is much more understandable to less physically inclined listeners. When Jill asks Blondie (possibly Miss Lenhart) why the sky is blue, Megan walks in and starts to explain in a very scientific way involving quantum mechanics. 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, Jill asks another common question - how do planes fly? Megan starts again to give the traditional answer (airflow causing 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.

When Jill reacts like she believes Blondie's last comment about the planes, she could almost have been called April Fool. Although this comic was released one day too early for that, this was also the only year between April 1st of 2011 and April 1st of 2018 where no such comic was released. See more about this in the trivia section below.

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 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.

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 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 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").


[Jill asks Blondie a question which she answers while lifting her arm towards Jill.]
Jill: Why is the sky blue?
Blondie: Because air is blue.
[Megan walks in from behind Jill.]
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 Jill than Blondie who has thrown her arms out. Jill 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 Jill 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?
Jill: How do planes stay up?
[Blondie holds a finger up in front of Jill while Megan now is the one to throw out her arms.]
Megan: Well, the airflow–
Blondie: Tiny birds in the wings. Thousands. Flapping hard.
Jill: WOW!
Megan: NO!


No April Fools' Day comic in 2017[edit]

  • This comic was released the day before April Fools' day, but even though it could be said that Blondie makes up tales about physics that could cause her to call Jill an April Fool for believing her, this is definitely not one of Randall's April fools' comics.
    • The reason for this trivia is that this year, 2017, was the first since 2011 that Randall did not release a comic on April 1st, and neither this Friday comic from the end of March, or the first comic in April, 1819: Sweet 16 from Monday April 3rd, could be said to have any relation to such a joke.
    • As Randall took up the trend again in 2018 with 1975: Right Click, released on Sunday April 1st 2018, it showed that it was not because he just stopped with the April Fools' Day jokes.

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I keep trying to correct the misspelled joung Girl to Young Girl but it keeps reverting. I corrected the two non-capitalized sentences and they stay put. Does "joung" have a meaning i don't understand? ExternalMonolog (talk) 14:55, 31 March 2017 (UTC)

No, "joung" is only there, because of my limited english skills 06:59, 4 April 2017 (UTC)

There might be conflicting edits, that happens a lot with new comicsDontknow (talk) 15:34, 31 March 2017 (UTC)

Question - while I understand the intent of the comic is that overly complicated explanations can be confusing, isn't the title-text analogy incorrect? Doesn't chlorophyll scatter green light and absorbs other colors, whereas with the sky, it's really just different levels of scattering and very little absorbing (hence why a clear sky at dusk can appear red, the sky wasn't absorbing red light, it was just scattering it differently than blue light). Isn't that fundamentally different from the way most other common objects get their perceived color? (ps - I'm not a scientist, just curious, appreciate any feedback)

Sry, no answer to your question, but a second chlorophyll-related one: I doubt that chlorophyll "reflects" green light, "scattering" should be correct! Any other opinions???? milebrega, 14:38, 1 April 2017 (UTC)

"Why are leaves green?" "Well, the leaf absorbs most of the colors, but not the green light, which it scatters instead." "Why is my shirt black?" "Well the cloth absorbs most of the colors, but just scatters the black light... wait..." Andyd273 (talk) 15:46, 31 March 2017 (UTC)

I like to think this is Miss Lenhart, continuing her science teaching in the same vein as in 'Venus'. There's no proof in the comic, but it fits nicely. Potentially something to add as a possibility in the explanation? 16:38, 31 March 2017 (UTC)

I second the take that this is Miss Lenhart; I guess she's not in a classroom setting but she's been in similar situations. Someone should mention that the girl's second question is the same from 803: Airfoil (also with Miss Lenhart). Articles have mentioned sort of "series" of themes before; that article, 1145: Sky Color, and this have an ongoing theme of "how to explain science to kids". 04:04, 1 April 2017 (UTC)

Yesssss <3 I had the exact same thought the first time Rayleigh scattering was explained to me: "isn't that just a specific mechanism of air being blue?" For some reason such explanations majorly tend to insist that the air is not in fact blue, and it has always bothered me. 16:41, 31 March 2017 (UTC)

Maybe the explanation should point out that the real reason the planes "stay up" is that the tiny birds are on the underside of the wings.-- 17:20, 31 March 2017 (UTC)

If air is blue how come a sunset, with LOTS of air, is red? I know the answer but it is the obvious next question with this explanation. 17:22, 31 March 2017 (UTC)

During the day the Sun heats the air. At sunset you see the result of this heating, the air glows red-hot or orange-hot and starts to quickly cool down. You can't see it glowing during the day because of the very bright Sun.-- 17:40, 31 March 2017 (UTC)
Oh ok. Than why is the sun-rise also red? ;-). --DaB. (talk) 23:40, 31 March 2017 (UTC)
https://www.reddit.com/r/ExplainLikeImCalvin/ 10:31, 1 April 2017 (UTC)

Can someone add this to the comics featuring those respective characters? Dontknow (talk) 17:41, 31 March 2017 (UTC)

New here, probably not following proper form in this commend, but, if I may ask, is that thing about mountains appearing blue actually true? (Unsigned)

If I do remember it correctly from my theoretical part of thesis about telepresence (so not exact domain, but I used good sources I think) the point behind why we see distant objects as bluish is: we see them colorless - maybe grayish - because the more the object is distant from us, the less reflected photons bounce into our eye. That means less energy of the radiation. As you may know, we have two sets of photoreceptor cells in the eye retina: one type for brightness perception (bacilli) and the other for multiple colors detection (coni). The later is active only in sufficient light conditions (photopic vision) - not in the dark. So that's why you don't see much colors in the night (scotopic vision) - not enough photonic energy gets into your eyes so only brightness is perceived. The same applies for the less energy from distant object photons - only brightness detected. So why it is it seems blue and not gray only? That is because bacilli cells are most sensitive to the blue color (500nm) contrariwise to the cone cells color sensitivity maximizes at green color (550nm). So the answer is bacilli cells are most sensitive to blue color, therefore the bacilli stimulus interpreted by our brain as blueish color, and only bacilli cells are active for low energy visual input - distant objects or in dark. Sources: sorry only in Czech language pdf-section 3.1.1: https://goo.gl/hOnWoi Gr4viton (talk) 06:59, 19 April 2017 (UTC)

Also to the question in hover-comment "Why does chlorophyll scatter green light?". Only lately (if I do remember correctly), in the year 2016 "scientist" found out why. From all of the electromagnetic spectra of the sun radiation, the most energy efficient frequencies are the non-greenish for the chemical reaction of photosynthesis. So the answer is: photosynthesis is more effective without green color, plants need photosynthesis to survive, Darwin principle of the strongest survives exists, plants evolved to diminish green color intake on leaves. Why there are plants with not only green colored leaves? Other reasons may drive the evolution - animals eating plants, environment temperature and other conditions may lower the importance of photosynthesis effectivity. Gr4viton (talk) 07:12, 19 April 2017 (UTC)

I received the 'rayleigh scattering' explanation myself, and it served me well. Even without knowing anything about quantum mechanics or how the human eye works, knowing that there's an optic principle at work other than simple pigmentation explains why the light is golden early and late in the day, and why dust or smoke can have such diverse effects on the colour of the light beaming down, especially at dusk and dawn.

If I'd been told air was blue, I'd have a lot more questions, and I'd still think that gemstones like alexandrite that look different colours in different light were somehow magical, instead of just having unique physical properties. So, I'd like to assume Randall's just making a outlandish joke, not really trying to say that it's wrong to give children the phrase 'rayleigh scattering' and explain what the consequences of it are, even without describing the mechanisms behind it--something that still goes way over my head.Namaphry (talk) 04:59, 1 April 2017 (UTC)

Air is not blue - certainly not in the same way that leaves are green or blue-dyed liquids are blue. This is evident when observing the Moon - a large chunk of rock much farther away than any mountains. Does all the air we're looking through at it make it appear blue? Of course not. If anything, the Moon can appear orange near the horizon. (But clearly, the reason is not that "air is orange".)

""Air is not blue - certainly not in the same way that leaves are green or blue-dyed liquids are blue. ""

Have You ever seen liquid air or oxygen? 06:29, 2 April 2017 (UTC)

Actually no. And a google image search for Liquid Oxygen doesn't give an image I'm totally convinced is actually of Oxygen (Best match seems to be a flask of Ozone, although it is most certainly a very deep blue).
Air in the Earth's atmosphere is not liquid, nor does it contain liquid oxygen. While liquid oxygen is pale blue, gaseous oxygen is colorless. Feel free to check the Wikipedia page on oxygen for a quick reference. 08:21, 3 April 2017 (UTC)

What is happening with the sky is that air can appear to glow with various colors when illuminated with a strong directional source of light, such as the sun. The color depends on multiple factors, including the angle of illumination and observation. The glow can be commonly seen being blue, white, yellow, or red - with blue hues generally observable on clear days, and reddish hues at sunrise or sunset. This only works with a directional source of light; when the source of light is diffuse, such as under a large cloud cover, the sky doesn't appear blue, and neither do distant mountains, readily disproving the notion that air itself would be blue.

There. That didn't involve any quantum mechanics. 07:07, 1 April 2017 (UTC)

Is there no xkcd April Fool's this year? ~Luc [talk] 22:27, 1 April 2017 (UTC)

Current explanation all backwards?

I seriously doubt the point of the comic is to tell children wrong answers just because they might not understand the real answer. That is just horrible. "The child's reaction in this comic, "Wow!", suggests that not only she understood, but is also excited about learning, which could be more important to her development than hearing the "correct" answer up front." That doesn't make any sense. I'm sure he doesn't want to say, tell children planes fly because of birds in their wings. There are easy and simple ways to give the correct answer, that children will understand, and it's definitely not better for their development and interest to tell them absolut BS. It's the other way around, he makes fun of this answer, and so wants to make the point to NOT tell children things like that just because you're too lazy to explain, or because you think they might not understand. The point is not: Give an easy and possibly wrong answer to children. The point is rather: While for blue sky the easier (and TRUE) answer might be enough for kids, for other things that's not the best solution, and definitely don't oversimplify so much that your answer actually is wrong. 08:53, 1 April 2017 (UTC)

I completely agree. I find the current explanation to be 100% the opposite of what Randall has been trying to say in earlier comics. The joke is those that think it is better to tell false stories rather than try to tell the truth. --Kynde (talk) 18:28, 1 April 2017 (UTC)

Look, there's really no question about whats going on in this comic, and people need to stop overcomplicating and overanalyzing things. Here's how it breaks down: Science Girl asks a science question. Blondie gives a simplified but still correct answer, while Megan gives the traditional but more complicated answer. Blondie then makes the point that's also the comic's intended moral - that we can view things at different levels of detail, and that just because an explanation is less detailed than you're use to doesn't mean it's actually wrong.

Blondie is doing the child a disservice. It's debatable if "Because air is blue" is accurate as others gave the examples of with sunrise and sunset. Also dubious is her statement, "Blue light bounces off it and hits our eyes". This makes it sound like simple reflection which is a very distinct mechanism from scattering. Things that reflect light vs. things that allow some light to pass through them (glass, air, water, irises) get wavelengths to our eyes in different ways. If you give the completely useless "because it makes it so blue light gets to our eyes" you'd be correct, but saying that the light reflects rather than is scattered is false. 15:25, 3 April 2017 (UTC)

Then in the last panel, Blondie takes her valid point to its illogical conclusion by giving an explanation that isn't merely simplified, but straight up wrong, which frustrates Megan. This panel has no moral and is not trying to make a point. It's just there to be the punchline. Anyone reading any kind of moral into it should probably remember why they're called "comics" in the first place. 02:04, 2 April 2017 (UTC)

Hommage to Calvin and Hobbes?

I believe this comic could be an hommage to Calvin and Hobbes comics, where Calvin asks his dad stuff about nature, and his dad replies with completely nonsensical explanations. 21:20, 1 April 2017 (UTC)

Calvin and Hobbes did not invent children asking science questions or parents who can't answer them. 02:04, 2 April 2017 (UTC)
Lot's of homages exist to things that didn't invent other things. 15:27, 3 April 2017 (UTC)
Sure, but if it were an homage to Calvin and Hobbes, it would include something that's recognizably Calvin and Hobbes, not just a child asking science questions or a parent who can't answer them. References and homages are never this subtle. 16:07, 9 April 2017 (UTC)

The other side of the argument[edit]

The other side of the argument is basically that by giving an overly simple and inaccurate explanation, one can be very misleading. There is another take on the following sentence:

The child's reaction in this comic, "Wow!", suggests that not only she understood, but is also excited about learning, which could be more important to her development than hearing the "correct" answer up front.

That is that the child may be impressed by a mere superficiality. Did she really understand, or was the explanation in some way impressive? (Impressiveness does not imply correctness.)

A person might say, "Wow!" to some Hollywood special effects. Does that really mean that the person is interested in how to create such effects? Does the girl in the strip really care to learn? If she does, starting off by telling her something wrong seems a rather odd way to go about it.

When one starts in a field of study, one often needs to have simple explanations. Otherwise, the sheer mass of detail can be overwhelming. It is more useful to give an explanation that is more or less correct and to mention that there are special cases. An example is Einsteinian physics which obsoleted Newtonian physics, but the latter is still close enough to be useful in everyday situations.

Additionally, if one gets in the habit of simplifying everything without regard to correctness, where does it end? The final frame gives an example of this. 00:26, 2 April 2017 (UTC) (Gene Wirchenko <[email protected]>

+1 14:45, 6 April 2017 (UTC)

April fools day comic absense[edit]

Where's the 2017 April fools day comic?? 11:50, 2 April 2017 (UTC)

I noticed that too... maybe its hiding. 15:33, 2 April 2017 (UTC)

This is extremely reminiscent of Terry Pratchett's concept of "lies to children".[edit]

Defined as, "a statement which is false, but which nevertheless leads the child’s mind towards a more accurate explanation, one that the child will only be able to appreciate if it has been primed with the lie."

Basically, these are simplified stories we tell – and not just to children – when we want to begin to explain something, but feel that our audience doesn’t have the background information necessary to understand the “full” story. These can be anything from “the stork brought you” to “Columbus wanted to prove that the Earth was round” to “atoms look like little solar systems” to “evolution is the survival of the fittest.”

The full answer[edit]

The fundamental issue that this comic vaguely touches on, but doesn't really address, is that any given object actually has three different colours and people often overlook this.

  • Emission colour - This is the colour that the object appears without external illumination.
  • Transmission colour - This is the colour that the object imparts to a (white) light source when illuminated from behind.
  • Reflection colour - This is the colour that the object imparts to a (white) light source when illuminated from in front.

Obviously, an object that doesn't emit light has an emission colour of "black". An object that is opaque has a transmission colour of "black". Because many objects have these properties, and most of the non-opaque objects have similar transmission and reflection colours, it's easy for people to overlook the three different colours of objects and say for instance "plants are green" when technically they mean "plants are black/green/green" or "the sun is white" when technically they mean "the sun is white/black/black".

The thing that makes air stick out is that it has a reflection colour of pale blue, but a transmission colour of orange/red. The reason why is that its colour derives from Rayleigh scattering rather than absorption. (Air's emission colour is black, unless strongly heated.) Magic9mushroom (talk) 13:16, 8 December 2019 (UTC)