1145: Sky Color - Revision history

81.179.199.253: /* Explanation */ Minor changes.

2026-02-08T00:38:32Z

‎Explanation: Minor changes.

81.179.199.253: /* Explanation */ It needed a better "why does it reverse left and right?" than was stated. Deleted the "invoking the up and down mirroring by standing on it" paragraph, that's just the actual "in and out of the mirror's surface" as now stated.

2026-02-08T00:35:53Z

‎Explanation: It needed a better "why does it reverse left and right?" than was stated. Deleted the "invoking the up and down mirroring by standing on it" paragraph, that's just the actual "in and out of the mirror's surface" as now stated.

81.179.199.253: /* Transcript */

2026-02-07T23:48:50Z

‎Transcript

74.111.172.164: /* Transcript */

2026-02-07T23:06:15Z

‎Transcript

Kynde: This is not Megan and no reason to assume Jills mother would be Megan! Megan has loose hair. Ponytail has hair in a ponytail. And Blondie is not Ponytail so a black hari3ed woman with hair in a ponytail is not Megan!

2025-03-31T18:21:51Z

This is not Megan and no reason to assume Jills mother would be Megan! Megan has loose hair. Ponytail has hair in a ponytail. And Blondie is not Ponytail so a black hari3ed woman with hair in a ponytail is not Megan!

172.69.194.205: /* Explanation / I believe the last correction missed the intent of the aside, which is that a US "freshman" might be an early secondary-school age in the UK school system. But (SFAIK) only* "Fresher" is used, and only for 1st-year-undergrads, so...?

2025-01-29T14:24:01Z

‎Explanation: I believe the last correction missed the intent of the aside, which is that a US "freshman" might be an early secondary-school age in the UK school system. But (SFAIK) *only* "Fresher" is used, and *only* for 1st-year-undergrads, so...?

162.158.74.115: /* Explanation */ Changed the explanation claiming that Feynman's prank was aimed at year 7s (12 year olds) because that's frankly ridiculous. British english (currently) uses freshman exclusively for first year university students

2025-01-29T11:45:10Z

‎Explanation: Changed the explanation claiming that Feynman's prank was aimed at year 7s (12 year olds) because that's frankly ridiculous. British english (currently) uses freshman exclusively for first year university students

StapleFreeBatteries at 08:08, 13 November 2024

2024-11-13T08:08:35Z

172.69.194.187: /* Explanation */ Really not sure why the comments about the British system, but trying to make it actually make sense. (Brit here: Years 1-5 in Primary; Years 1-3 (lower school) and 4-5 (upper school) in Secondary; two "6th Form" years in college, etc.)

2024-09-11T22:27:10Z

‎Explanation: Really not sure why the comments about the British system, but trying to make it actually make sense. (Brit here: Years 1-5 in Primary; Years 1-3 (lower school) and 4-5 (upper school) in Secondary; two "6th Form" years in college, etc.)

Jacky720: It's saying that "first year" is British English for what Americans call "freshman". I'm not sure who calls them "freshers" but as Randall (and whoever originally wrote "freshman") is American, I think that's the intent of the sentence.

2024-09-11T18:46:23Z

It's saying that "first year" is British English for what Americans call "freshman". I'm not sure who calls them "freshers" but as Randall (and whoever originally wrote "freshman") is American, I think that's the intent of the sentence.

@@ Line 18: / Line 18: @@
 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''.
+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 it is ''yourself'' having flipped the text left-to-right (as could be seen, without a mirror, if the ink has permeated the paper or else if you had written upon a an actually 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).

@@ Line 18: / Line 18: @@
 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. 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.
+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''.
-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.
+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).
-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.
+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==

@@ Line 25: / Line 25: @@
 ==Transcript==
-:[Jill addresses her mother with drawn 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 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.

@@ Line 25: / Line 25: @@
 ==Transcript==
-:[Jill adresses her mother drawn 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 addresses her mother with drawn 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.

@@ Line 8: / Line 8: @@
 ==Explanation==
-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.
+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.
@@ Line 25: / Line 25: @@
 ==Transcript==
-:[Jill and her mother, Megan, but with her hair up. Megan is at a desk and facing the girl.]
+:[Jill adresses her mother drawn 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?
-: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.
+: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?
-:Megan: Well, because, uh... ...hmm.
+:Mother: Well, because, uh... ...hmm.
 :[Caption Below the panel:]
 :My hobby: Teaching tricky questions to the children of my scientist friends.
@@ Line 36: / Line 36: @@
 {{comic discussion}}
 [[Category:Comics featuring Jill]]
 [[Category:Physics]]
 [[Category:My Hobby]]

@@ Line 16: / Line 16: @@
 {{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 (relating to a British English context, probably a student in their initial year of university, more commonly referred to as a "fresher"). 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.
+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. 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.

@@ Line 12: / Line 12: @@
 [[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 question (e.g., "Why isn't the sky violet?") can often uncover large gaps in our actual understanding.
+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]].