1145: "Sky Color"

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obfpen
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Re: 1145: "Sky Color"

Postby obfpen » Mon Dec 10, 2012 11:02 am UTC

FourTael wrote:You know what's most interesting to me? Just how many different answers that question has gotten.

Now, we could use this to disprove science because clearly a random group of people (not entirely random, but enough for my point to stand) don't have 100% accuracy at answering questions, there is no answer and science must just be a bunch of people just coming up with random hypotheses that they make no effort to prove.

I came to read the forum to see if anyone else was as surprised as I was to see a character who knew enough to explain scattering as a function of the wavelength then fail to treat it as a continuous variable with the resultant spectrum typically perceived as blue, instead appearing to think in terms of physically discrete colours. So the breadth of answers intrigued me too. And I learnt something about the sun along the way.

In defence of my fellow theorisers, though, at least we mostly tended to point out that we weren't sure, or that our suggestion only could be the answer, or just part of the answer. And knowing to say "I don't know", rather than "well, um, because good people deserve blue skies", or whatever, makes a big difference.

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Re: 1145: "Sky Color"

Postby sameaspassword » Mon Dec 10, 2012 11:03 am UTC

Actually, it is quite sad to see that so far apparantly hardly anyone has even understood the mirror reversal problem. It's quite obvious that a mirror placed at [imath]y=0[/imath] performs the operation [imath]y ↦ -y[/imath]. This is equivalent to a 180 degree rotation about the [imath]z[/imath] axis (up and down), except for the fact that such a rotation also changes [imath]x[/imath] to [imath]-x[/imath] (left and right). So, by mirroring the [imath]x[/imath] axis, we obtain the correct results and therefore all text in the mirror appears switched left and right. Problem solved?

Not quite. The mirror only breaks the symmetry along the [imath]y[/imath] axis, so left/right and up/down should still be symmetrical. And indeed, we can perform the mirroring also by rotating around the [imath]x[/imath] axis, which will result in top and bottom being switched. Therefore, the mirror actually flips top/down the same way it flips left/right because the results are completely identical!

Now, what's the resolution? The best explanation that I've come across is that we sort of expect things to be more or less to be symmetric w.r.t. to left and right, and asymmetric w.r.t. to top and bottom (gravity is a natural explanation for the expected symmetry breaking along the [imath]z[/imath] axis). Therefore, it is mentally easier for us to think in terms of the first transformation with the [imath]x[/imath] axis being reversed, as it involves only familiar looking objects. But I'm open to better explanations.

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Re: 1145: "Sky Color"

Postby Dave » Mon Dec 10, 2012 11:35 am UTC

Lawton wrote:Also, the mirror question can be explained in simple terms. When you rotate a book to the mirror, you are the one who keeps up as up and down as down. If you flipped the book upside down then up and down would be flipped instead of left and right.


I'm not sure rotating the book is relevant, is it? The question in the alt text is why does a mirror invert only left and right, but not up and down. And the answer is that it does not - things on your left are still on your left in the reflection. It inverts on the Z axis (perpendicular to the surface of a mirror).

If you flip a book upside down, then show it to a mirror, it is still the same way up in the reflection as it is in reality - upside down.

edit - ok I think I get what you're saying - rotating the book puts the left hand side of it on your right hand side, which is then still on your right hand side in the reflection and so the text appears backwards. For me, "the reflection is inverted on the Z axis" to be a much simpler explanation!

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Re: 1145: "Sky Color"

Postby jonadab » Mon Dec 10, 2012 12:15 pm UTC

The sky is blue because of a WWII-era Pepsi advertising campaign. See here:
http://groups.google.com/group/rec.arts ... e3c13d6221

Alternately, the sky on the side away from the sun may actually be somewhat more violet than blue (depending on time of day and weather and how much light has been absorbed rather than merely scattered, which can get very complicated indeed), but in general it appears blue because it also has more green in it than yellow or red. You see, the human eye doesn't have separate receptors for every color of the spectrum. Most of us have only three types of receptors (a few people have four). The main difference in how we perceive violet light versus blue light is that while they both trigger the short-wavelength receptors to more or less the same extent, the violet light doesn't trigger the medium-wavelength receptors at all, and the blue light does, just a little. So blue looks just about the same as mostly violet with a little green. (This is an oversimplification. Actually, the amount of yellow and red light present is also partially relevant, at least potentially, because green light, while it triggers the medium-wavelength receptors most, also triggers the long-wavelength receptors just a little.)

For the same reason, the sun (high in the sky on a clear day) looks yellow rather than red or orange because the medium-wavelength receptors are triggered somewhat, not just the long-wavelength ones. There's a lot of red light coming from the sun, but it would look quite a bit more red to the human eye if there were no green and not much yellow. This happens sometimes when there's heavy cloud cover -- most of the yellow light gets scattered away and you see a red disc. Make the cloud cover even thicker, and the red starts to get scattered as well, resulting in a red smear. This is of course most likely to happen when the angle is shallow, so that there's more total atmosphere to pass through -- hence sunrise or sunset.

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Re: 1145: "Sky Color"

Postby peewee_RotA » Mon Dec 10, 2012 12:22 pm UTC

While looking for an amusing anecdote that I heard in one of Richard Feynman's lectures I accidentally stumbled upon a video of the mirror question ... in question:
http://www.youtube.com/watch?v=msN87y-iEx0


So the actual anecdote is interesting and I can't do it justice but I'll try. Long story short if anti-matter is mirrored "left-to-right" and you are visited by a space alien from another galaxy and this alien somehow looks exactly like you and acts exactly like you and reaches out to greet you with his left hand, don't shake it. He's made of anti-matter.
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jonadab
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Re: 1145: "Sky Color"

Postby jonadab » Mon Dec 10, 2012 12:46 pm UTC

(From the Feynman video): What keeps the train on the track?

Well, duh. Engineers. No, I don't mean railroad drivers, I mean railroad _designers_. They specifically engineered both the train and the track in such a way that the train would stay on the track.

Specific details vary depending on exactly what kind of train and track you're talking about. Gravity is one of the more common mechanisms, but there are rail systems that don't rely on gravity and instead utilize some other device or method to keep the train on the track.

As for the mirror, Feynman's explanation is correct in general, but there's one thing it doesn't (specifically) explain about the way the question was stated here. Namely, when you turn the paper around to face the mirror, you flipped it horizontally, rather than vertically. You do this because we're accustomed (for physiological reasons) to implicitly assuming that vertical orientation is significantly more important to preserve than horizontal orientation.

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Re: 1145: "Sky Color"

Postby Duban » Mon Dec 10, 2012 12:49 pm UTC

I'll try to explain this in a more relatable way. Ok, place a mirror behind you and make sure you're looking in the same direction as the mirror "with the mirrior on your back". We can agree that the right side of the mirror shows the right side of your back and the left side of the mirror shows the left side of your back. Now, turn around. Touch the mirror, your right hand will touch the image of your right hand and your left hand will touch the image of your left hand. There is no reversal of the mirror's image, left is left right is right. You'll notice that the mirror hasn't changed any special properties when you turn around, but the letters seem inverted. The items on your left side are still on the left side of the mirror, and the designs on the right are still on the right.

Which way did you turn around, left to right or head over feet upside down? Left to right i bet. Do the same thing with a fiiend and touch your friends hands with yours. Your right hand will touch his left and your left hand will touch his right. You'll notice that a mirror doesn't flip any image. When you turn around to look at an image/word YOU are the one who gets inverted, left to right, by turning around. When you turn around to look at words you are seeing the words themselves inverted when you don't use a mirror. When you look at something in a mirror the image isn't inverted, like you would normally see them, and thus those words appear backwards.

Make sense?
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Re: 1145: "Sky Color"

Postby nick012000 » Mon Dec 10, 2012 12:59 pm UTC

Dave wrote:
Lawton wrote:Also, the mirror question can be explained in simple terms. When you rotate a book to the mirror, you are the one who keeps up as up and down as down. If you flipped the book upside down then up and down would be flipped instead of left and right.


I'm not sure rotating the book is relevant, is it? The question in the alt text is why does a mirror invert only left and right, but not up and down. And the answer is that it does not - things on your left are still on your left in the reflection. It inverts on the Z axis (perpendicular to the surface of a mirror).

If you flip a book upside down, then show it to a mirror, it is still the same way up in the reflection as it is in reality - upside down.

edit - ok I think I get what you're saying - rotating the book puts the left hand side of it on your right hand side, which is then still on your right hand side in the reflection and so the text appears backwards. For me, "the reflection is inverted on the Z axis" to be a much simpler explanation!

Except that that's not really it at all. When you raise your left hand at a mirror, your reflection is raising its right hand, which is on your left side. As for "why is the left-right reflection special", it's because there's a 180-degree rotation about the z-axis (where x is left-right, y is forward/backward, and z is up/down), and the inversion on the x/z plane is needed to maintain make your reflection match up to the real object; that is, when you touch the mirror, the inversion is necessary to make your reflection touch the same spot on the mirror you are.

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Re: 1145: "Sky Color"

Postby nick012000 » Mon Dec 10, 2012 1:02 pm UTC

Duban wrote:I'll try to explain this in a more relatable way. Ok, place a mirror behind you and make sure you're looking in the same direction as the mirror "with the mirrior on your back". We can agree that the right side of the mirror shows the right side of your back and the left side of the mirror shows the left side of your back. Now, turn around. Touch the mirror, your right hand will touch the image of your right hand and your left hand will touch the image of your left hand.

Yes, but the image of your right hand is actually the reflection's left hand, and vice versa.

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Re: 1145: "Sky Color"

Postby Barstro » Mon Dec 10, 2012 1:19 pm UTC

[quote="Drowsy Turtle"]We don't see much indigo in the sky because there isn't much indigo in sunlight (plus as others have described, indigo is readily absorbed in the atmosphere).

Image

I'm a non-science guy who believes he's heard things before;

I heard that night vision goggles and similar things use green because we are genetically coded to perceive differences in greens better than other colors (you know; since we were tree-swinging monkeys before creative design kicked in :wink: ). Maybe that's also because there is more green in our visible light. So, maybe, with a sky made up of shorter wavelengths, the purple gets washed out by the colors we are more inclined to see.

I'd also read that it is our atmosphere that makes the sky blue, and planets with different atmospheres could have different colors (but I have a feeling that was read in a sci-fi book, instead of a science article).

Back to the mirror;
It reverses in and out. If we are looking at ourselves in the mirror and move our right hand, the hand on the right side of the mirror moves as well. We are just more used to looking at another person, as opposed to a mirror. If we face another person and raise our right hand, and that person raises the hand that for us is on the right-hand side, we know that the person is raising the left hand.

This reminds me of the time I was with my parents at a pizza parlor that had mirrors all along the back wall. My mother couldn't understand why she was able to read the store's sign through the mirror, since mirrors always reverse lettering. Then she finally realized that the window sign was 1) meant to be read from the outside, 2) reversed to anyone on the inside looking through the window, and 3) reversed back by the mirror.

Nobody ever asks why windows change lettering from left and right but not up and down.

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Re: 1145: "Sky Color"

Postby jonadab » Mon Dec 10, 2012 1:24 pm UTC

A related one, it's clear the sun's output peaks in the green, so why isn't the Sun green?


Green has a medium wavelength, between yellow and blue, so it gets scattered more than the yellow but less than the blue. If the light that's more scattered is dominant (e.g., when looking at the sky away from the sun), we see the short end of the spectrum more than the middle. If the less-scattered light dominates (e.g., looking at the sun), we see the long end of the spectrum. (Why this often appears yellow rather than red, I explained already upthread.) If they're balanced, however, we would see... white, of course, because if more-scattered and less-scattered light are balanced, coming from a black body, it's because not enough scattering is happening to change what color we're seeing. That's just the physics.

In terms of perception, it is highly relevant to note that green light is dead-center on our medium-wavelength receptors, and that's why it's special for the human eye. Blue light is also in the middle, between violet and green, but we don't have any cones that detect violet better than blue, so it's possible for violet to perceptually fall off the end of the spectrum. Yellow light is in the middle between green and orange, but we don't have any receptors that detect orange (or red) very much better than they detect yellow, so it's possible for the sun to look "yellow" even if we're actually getting more orange light than yellow light, as I explained a few moments ago upthread.

Blue is the color of oxygen.


Even if that were true (and I've never seen it claimed anywhere else), it's irrelevant here. We can rephrase the question as "Why is the sun yellow?" and the correct answer still works.

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Reecer6
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Re: 1145: "Sky Color"

Postby Reecer6 » Mon Dec 10, 2012 1:26 pm UTC

Screw these two questions that have been answered thoroughly in this thread, what I really want to know is this. Why did the kid emphasize "isn't" instead of "violet?"

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Re: 1145: "Sky Color"

Postby phlip » Mon Dec 10, 2012 1:31 pm UTC

nick012000 wrote:Except that that's not really it at all. When you raise your left hand at a mirror, your reflection is raising its right hand, which is on your left side. As for "why is the left-right reflection special", it's because there's a 180-degree rotation about the z-axis (where x is left-right, y is forward/backward, and z is up/down), and the inversion on the x/z plane is needed to maintain make your reflection match up to the real object; that is, when you touch the mirror, the inversion is necessary to make your reflection touch the same spot on the mirror you are.

But the reason you refer the reflection of your left hand as "its right hand" is that you're imagining your reflection as another person facing you... and how does a person who's standing like you are become a person who's facing you? By rotating around the vertical axis. And so the rotation comes back into it.

In reality the important time to consider is whenever you're facing another person (without a mirror) and your right is on their left, but your up is still their up... this is the time that the horizontal is reversed and the vertical left unchanged. The mirror only appears to reverse the left and right because you're expecting them to be reversed and they aren't, thus making them reversed from what you're expecting, if you get my drift.

If you were used to people turning around to face you by rotating around a horizontal axis, then their left would still be your left and their right your right... but their up would be your down and vice versa. And then if you looked at your reflection in a mirror then you'd think it was reversed vertically than what you were used to. This, of course, doesn't happen, because "up" and "down" are very important directions in an environment with constant gravity, and so most things we do will keep "up" and "down" the same, and just rotate around a vertical axis.

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Re: 1145: "Sky Color"

Postby cellocgw » Mon Dec 10, 2012 1:48 pm UTC

SEE wrote:The trick on the mirror one is that there is no such reversal.

(Doubt me? Write something on clear plastic, then hold it up to a mirror.)

(Alternatively, hold a piece of paper horizontally, top of the text closest to a vertical mirror. Hey, now the letters are all upside-down!)


The real answer is: mirrors reverse front -to-back. Think about it :P
For even more fun, take a look at the reflection from a French Horn bell (or similar curve). It does, in fact, reverse left-to-right, but NOT top-to-bottom, so you see yourself upright and when you raise your right hand, so does the reflection.
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Re: 1145: "Sky Color"

Postby Harry Voyager » Mon Dec 10, 2012 1:48 pm UTC

mrb4 wrote:Yet another unnecessarily complex and long answer to "why is the sky blue", *sigh*... The answer is simply:

Blue is the color of oxygen. (Yes, gaseous oxygen is very, very slightly blue, to a point it is sometimes incorrectly described as colorless.)

Similarly, when a kid asks "why are plants green", you reply "green is the color of chlorophyl, a pigment in plants", you don't talk about wavelengths, photons, absorption, scattering, etc.


A fun demonstration:
Warning: Addictive: Oxygen

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Re: 1145: "Sky Color"

Postby conorjh » Mon Dec 10, 2012 1:54 pm UTC

A fun variant: using the The On-Line Encyclopedia of Integer Sequences to help you children with their "2,4,8,16" what is the next entry in this sequence?" style maths homework, eg 30

http://oeis.org/A027423

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Re: 1145: "Sky Color"

Postby keithl » Mon Dec 10, 2012 2:02 pm UTC

We have a counter-to-ceiling 90 degree corner mirror in our bathroom, which DOES swap left and right. Except for a thin black line down the middle, we can see what others see looking at us. Faces are slightly asymmetrical, so this is subtly spooky for those accustomed to looking in a flat mirror.

Because of the double reflection, the corner mirror is somewhat dimmer.

Our bathroom is also a bit red (not blue). I rigged up a red LED strip over the toilet, helps with night-time aim. Red light at night doesn't wake you up as much as blue light does.

We don't have kids, but it is fun to imagine the strange questions about mirrors and light they would ask teachers and scientists.

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Re: 1145: "Sky Color"

Postby orthogon » Mon Dec 10, 2012 2:27 pm UTC

WIMP wrote:... It's not actually possible to reproduce what the mirror's doing, because it's pulling your front through your back, not doing any rotations that you can mimic in reality.


Eurgh. I feel unusual.
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Re: 1145: "Sky Color"

Postby benefluence » Mon Dec 10, 2012 2:44 pm UTC

The way I've always though about the mirror one is that what the mirror actually does (a reflection across the plane of the mirror) is equivalent to a 180 degree rotation about the vertical axis of the mirror followed by a left-right reflection. We perform the rotation subconsciously because we see the reflection as a person standing in front of us, and that's how we would get behind the mirror if such a thing were possible. Up and down ARE flipped, if you think about 'getting behind the mirror' as rotating around the horizontal axis parallel to the mirror, as such a rotation followed by an up down reflection is also equivalent to the mirror's reflection. we just don't think of it that way.

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Re: 1145: "Sky Color"

Postby dp2 » Mon Dec 10, 2012 2:56 pm UTC

Dave wrote:
Lawton wrote:Also, the mirror question can be explained in simple terms. When you rotate a book to the mirror, you are the one who keeps up as up and down as down. If you flipped the book upside down then up and down would be flipped instead of left and right.


I'm not sure rotating the book is relevant, is it? The question in the alt text is why does a mirror invert only left and right, but not up and down. And the answer is that it does not - things on your left are still on your left in the reflection. It inverts on the Z axis (perpendicular to the surface of a mirror).

If you flip a book upside down, then show it to a mirror, it is still the same way up in the reflection as it is in reality - upside down.

edit - ok I think I get what you're saying - rotating the book puts the left hand side of it on your right hand side, which is then still on your right hand side in the reflection and so the text appears backwards. For me, "the reflection is inverted on the Z axis" to be a much simpler explanation!

I agree -- yours is the best answer, and one of the few that seems to get the question.

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Re: 1145: "Sky Color"

Postby dotancohen » Mon Dec 10, 2012 3:10 pm UTC

Dryhad wrote:Clearly the sky is blue because it's moving towards us. Of course, it reverses direction every night, as can be briefly seen at sunset or dawn.


This is the best comic that we've seen in a long time, and here is the best comment that I've seen in ages. I will certainly use this line in conversation. Thanks!

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Re: 1145: "Sky Color"

Postby neremanth » Mon Dec 10, 2012 3:11 pm UTC

My take on the mirror question (nothing that others haven't already said in this thread, but just put how it makes most sense for me):

A mirror simply reproduces what is in front of it, except that the direction which is away from the mirror in the real world appears as into the mirror.

Try this experiment: write something on a piece of glass or clear plastic. For example,
normal.png
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.

Now stand in front of a mirror, holding this up in front of you, with the right side towards you. In the mirror you should see
normal.png
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. So it looks exactly the same as what you see. (Actually, it won't be exactly the same: you will see the pen marks on top of the plastic while the mirror shows the plastic on top of the pen marks - but at any rate, the mirror isn't rotating).

Ok, but we don't usually compare what the mirror shows with the reverse view of an object (probably because most objects aren't see through). What we really want to know is how the mirror view compares to the view of the side it's looking at.

How do we look at that side? We need to come round the front of the piece of plastic.

First, keeping the plastic where it is, and holding it in just one arm, step out sideways from behind it so you are standing next to it, your arm extended to your side. Now, still keeping your arm in exactly the same position, bring your body round so that the plastic is again in front of you, but you're looking at it from the other side - your back to the mirror. You should see
horizontalrotate.png
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. In other words, what you would see if you wrote the word on a piece of paper and held it right side to the mirror.

Now return to your original position facing the mirror and holding up the piece of plastic with the right side towards you. This time, holding the plastic still crane your head over the top to see the other side. You should see
verticalrotate.png
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. In other words, a "mirror image" where the words have been flipped from top to bottom instead of right to left.

We normally regard
horizontalrotate.png
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as the mirror image of
normal.png
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because when we want to compare the mirror's view of the writing to what we would see if we were looking at it from that angle, we either come round to the front of it ourself (preserving the vertical direction and rotating in a horizontal plane), or we rotate the writing round towards us (again, preserving its vertical orientation and rotating horizontally). If we instead preserved the left-right axis and rotated the writing (or ourselves) in a vertical plane, then we would regard
horizontalrotate.png
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as the mirror image of
verticalrotate2.png
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, and note that the left-right direction was preserved but the vertical had been flipped.

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Re: 1145: "Sky Color"

Postby Aelfyre » Mon Dec 10, 2012 3:15 pm UTC

phlip wrote:There's also the fact that oxygen is (faintly) blue, which has a nonzero effect on the whole thing. But IIRC this isn't the major component to what we see.


I always thought Oxygen was faintly green? From all those deep space nebula photos where they enhance it so it is visible to the naked eye? And also the Aurora Borealis.. I have been told the green light comes from excited oxygen.
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Re: 1145: "Sky Color"

Postby Chezzik » Mon Dec 10, 2012 3:38 pm UTC

Mirror reversal question is easy.

Grab a piece of paper, and write "T" near the top edge, "B" near the bottom, and "L" and "R" near the right and left sides.

Now walk to a mirror while holding it, noticing how easy it is to keep it oriented correctly.

Now, you are at the mirror. Turn the paper around so that it faces the mirror, keeping the letters in the right orientation. You quickly realize that is impossible. You can flip it over the left edge, but then L and R are backwards. Or, you can flip it over the top edge, but then the T and B are reversed.

Mirror symmetry is different than rotational. The mirror symmetry always has an axis of symmetry. You choose where the axis lies when you flip the paper around.

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Re: 1145: "Sky Color"

Postby Twitchkidd » Mon Dec 10, 2012 3:41 pm UTC

On the subject of mirrors, mirror image script with the left hand (for rightys) is a great way to increase your ambidexterity :D
On the subject of the sky, it's not blue if you live in L.A. End pollution, save the whales.

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Re: 1145: "Sky Color"

Postby cyberneticentomologist » Mon Dec 10, 2012 3:42 pm UTC

Many years ago, my mother called me up to say hello while I was in the shower, so my roommate answered the phone. In order to kill time, he asked my mom "Why is the sky blue", completely not expecting the half-hour science lecture that followed. So that's been kind of an inside joke for us ever since. Naturally, this comic was immediately forwarded to my mother, who sent the following reply, because my mom is awesome like that.

"the answer is that there is a sudden drop in actual scattering (reading from long to short wavelengths) at about 450 nm, a slight rise at about 410 nm, then a very steep dropoff to almost nothing at 400 nm – down to less than the scattering in the yellow and red wavelengths. Interestingly, the magnitude of scattering depends on the angle between the sun and the look direction, and there is much more violet at 90 degrees, which is why I suppose you get a more violet tinge to the sky at the zenith near sunrise and sunset, or near the horizon at noon. The human eye’s blue cones are maximally sensitive at 445 nm (corresponding to max blue scatter), still substantial at 400, but virtually 0 at 350. So, the sharp dropoff I will take as a function of atmospheric composition and the specific molecular sizes present, and that’s why the sky isn’t violet – not that we couldn’t see it if it were!"

And there you have it. That's why the sky isn't purple.

Why does my mother know this off the top of her head? She's a professor of graduate-level remote sensing, and gets to play with lasers and satellites because she has an even nerdier job than my boring IT existence. Nerd runs deep in my genes.

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Re: 1145: "Sky Color"

Postby JoeZ » Mon Dec 10, 2012 3:44 pm UTC

When I saw this comic at 3AM last night, my sleep-deprived brain was completely unable to comprehend the mirror question. One measly hour of sleep later and I got it immediately. Clearly I should have gotten in a little more shut-eye before finals, but hey, what can I do?

cantab314 wrote:Yeah, my immediate answer to "Why isn't the sky violet?" was that the Sun doesn't emit as much violet. But the response of human vision I'm sure is also important.

A related one, it's clear the sun's output peaks in the green, so why isn't the Sun green? It turns out that a black body can never be green, whatever its temperature, so why not? My assumption is that there's enough red and blue as well so it looks white, but I have no fuller answer.


The reason for that is because a blackbody continues emitting red well beyond the point that it starts emitting green. Because of the way our retinal nerves and visual cortex truncate the data coming out of our eyes, we see a combination of red light and green light as yellow. When the blackbody starts emitting significant amounts of violet, all three of our cone cell types are stimulated, and we see white. It's not until you get an object that's very, very, hot that the blue end of the spectrum starts to dominate the red end.

Here's a link full of nice images to help you visualize it, in case my exhausted ramblings aren't doing it justice; http://chemistry.beloit.edu/BlueLight/pages/body.html

So it's basically a combination of the nature of blackbody radiation and the poor spectral resolution of the human visual system.
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Re: 1145: "Sky Color"

Postby FrostBlast » Mon Dec 10, 2012 3:52 pm UTC

A mirror doesn't reverse left and right; it reverses front and back. In other words, the reversal doesn't happen on the X (height) or Y (width) axis, but on the Z axis (depth)

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Re: 1145: "Sky Color"

Postby MadH » Mon Dec 10, 2012 3:59 pm UTC

The way I think of a mirror is this. (This is essentially a more complicated way to explain "reverses front and back" to a person who may not be able to interpret that)
Imagine you are standing in front of a piece of glass. Then, imagine your body flying forward and through the glass. As you pass through, the first molecule to hit the glass sticks and stays there, essentially creating a flat image of you as you pass through it.

If you are standing on top of a mirror, your feet would have passed through first, so they are the first molecules to hit the mirror. Your head would pass through last, hence why it is "behind" everything else when you look at the final picture on the glass.

That's exactly what you're seeing on the surface of the mirror. It doesn't flip anything in the vertical or horizontal axis, it just shows you as you would look like if you were literally passing straight through the mirror, except it has a surface that bounces the molecules back at you. The problem with the human mind interpreting that is we encounter regular people every day. We're used to that 180 degree rotation of our left is their right, and that muddles with our perception of what looks like another human standing across from us in a mirror. We expect everything to be flipped when really, nothing is flipped the way we expect it. And it never helped when as kids we were shown the "write in reverse then look in a mirror" trick, without any explanation as to how that worked.

I could have explained all that more "properly" with photons but I find "your first molecules stick to the mirror" is easier for a layman to understand than all the bouncing involved with photons.

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Re: 1145: "Sky Color"

Postby borkabrak » Mon Dec 10, 2012 4:01 pm UTC

AvatarIII wrote:
rhomboidal wrote:If the sky was violet, the poem would go "roses are red, violets are violet," which would be redundant instead of romantic, and eventually the entire species would go extinct because of its crappy love poetry.


Roses are Red
Violets are Violet
I am your shoelace
and you are my eyelet.

yeah not hugely romantic


I feel I would be remiss if I didn't point out the laudable attempt to fix the "violet/blue" issue attempted by the incredible Mr. Roger Miller:

Roses are red
Violets are purple
Sugar is sweet
And so's maple syrple.

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Re: 1145: "Sky Color"

Postby finity » Mon Dec 10, 2012 4:03 pm UTC

lemonickous wrote:Obviously the answer to the Feynman conundrum is gravity. It works downwards, so up and down is special. Whereas horizontal is an equipotential plane so anything can happen.


This is awesome. I want this question to come up in conversation sometime so I can give this answer. It's like blaming things on the Coriolis effect...

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Re: 1145: "Sky Color"

Postby Coolfusis » Mon Dec 10, 2012 4:09 pm UTC

My GED in physics wants me to say that the mirror just sends the light from its perspective right back, so it what appears in the mirror is the same as what you would see if you were looking through the back of the page. What is written on your right appears on your right, and vice versa. The mirror hasn't changed anything, you're just looking at the words from a different angle.

Print something on a clear sheet, then turn it around and look at it. It'll look "mirrored" because what is usually on your left is now on your right - you turned it around.

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Re: 1145: "Sky Color"

Postby Klear » Mon Dec 10, 2012 4:14 pm UTC

finity wrote:
lemonickous wrote:Obviously the answer to the Feynman conundrum is gravity. It works downwards, so up and down is special. Whereas horizontal is an equipotential plane so anything can happen.


This is awesome. I want this question to come up in conversation sometime so I can give this answer. It's like blaming things on the Coriolis effect...


I'd blame it on a weather balloon.

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Re: 1145: "Sky Color"

Postby Shadowman615 » Mon Dec 10, 2012 4:22 pm UTC

jonadab wrote:(From the Feynman video): What keeps the train on the track?

Well, duh. Engineers. No, I don't mean railroad drivers, I mean railroad _designers_. They specifically engineered both the train and the track in such a way that the train would stay on the track.

Specific details vary depending on exactly what kind of train and track you're talking about. Gravity is one of the more common mechanisms, but there are rail systems that don't rely on gravity and instead utilize some other device or method to keep the train on the track.

As for the mirror, Feynman's explanation is correct in general, but there's one thing it doesn't (specifically) explain about the way the question was stated here. Namely, when you turn the paper around to face the mirror, you flipped it horizontally, rather than vertically. You do this because we're accustomed (for physiological reasons) to implicitly assuming that vertical orientation is significantly more important to preserve than horizontal orientation.


This is the video about the train question...
http://www.youtube.com/watch?v=y7h4OtFDnYE

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Re: 1145: "Sky Color"

Postby rubseb » Mon Dec 10, 2012 4:22 pm UTC

The answer to the mirror conundrum, as I think Feynman mentions in the same anecdote (or at least in the version I recall from "Surely you're joking, Mr. Feynman"), is very simple: it's not left and right that are reversed, but front and back. If you stand in front of a mirror and raise your left hand, on what side of the mirror's midline do you see a hand being raised? On the left. So left is still left, and right is still right. It's just that with respect to the person you see in the mirror, it appears to be their right hand. And *that* is because that person is flipped back-to-front with respect to you, the original. (Not rotated, as people can be are in real life, but flipped, which they can't be, and which I guess is where some of the confusion comes from.)

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Re: 1145: "Sky Color"

Postby cellocgw » Mon Dec 10, 2012 4:26 pm UTC

dotancohen wrote:
Dryhad wrote:Clearly the sky is blue because it's moving towards us. Of course, it reverses direction every night, as can be briefly seen at sunset or dawn.


This is the best comic that we've seen in a long time, and here is the best comment that I've seen in ages. I will certainly use this line in conversation. Thanks!


Possible enhancement: The sky is being driven away from the sun by the solar wind, so when it's daytime the sky is driven towards us, and at night away from us.
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Re: 1145: "Sky Color"

Postby RRob » Mon Dec 10, 2012 4:35 pm UTC

"Why is the sky blue?"
What color should it be?
"Ummm..."
Invisible. Like it is at night.

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Re: 1145: "Sky Color"

Postby Kinami » Mon Dec 10, 2012 4:40 pm UTC

Reecer6 wrote:Screw these two questions that have been answered thoroughly in this thread, what I really want to know is this. Why did the kid emphasize "isn't" instead of "violet?"

Finally, a question for a linguist!
Short answer:
It's contrastive - "is" from the first question ("Why is the sky blue?") versus "isn't" in the second "Why isn't the sky violet?"

Longer answer:
In the first question, the most important word to convey is the color - why is the sky blue, not yellow or green or any other color - so "blue" would be the word with the most emphasis.
In the second question, the form of the question has changed, and it's important to pull the listener's attention to that difference - "why is...?" has become "why isn't...?". Therefore, "isn't" will receive the most emphasis. "Violet" is also strongly emphasized, to contrast it with "blue" from the first question. However, the most notable new information - the most drastic change in the sentence structure - is the change from "is" to "isn't", so that contrast is slightly stronger.

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Re: 1145: "Sky Color"

Postby Lawton » Mon Dec 10, 2012 4:41 pm UTC

honnza wrote:There is. The weak interaction violates the parity symmetry. See http://en.wikipedia.org/wiki/Parity_(physics)#Parity_violation

So,
up = negative direction of the gravity force
front = an arbitrary direction defined by the center of the visual field.
left = the cross product of both, such that the coordinate system is chosen such that two specific interactions violate two of (CP symmetry, P symmetry, C symmetry) in predetermined ways.
I don't quite buy into this, but I'll admit I don't have a solid argument against it.

I find it odd that in a thread about a comic about explaining science to children, everyone is objecting to my simplistic explanation because it doesn't involve the Z-axis.

Okay, here's an even shorter explanation: Left and right are relative to the observer, but up and down aren't.

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Re: 1145: "Sky Color"

Postby wagr » Mon Dec 10, 2012 4:58 pm UTC

Come up in these Colorado mountains on a dry day. The sky is violet. In Cleveland, where I used to live, the day-time sky is usually gray or white; it is rarely blue.


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