# Why is the sky blue? **URL:** \ **Category:** Cecil's Columns/Staff Reports\ **Created:** [September 5, 2013, 10:16pm UTC](https://boards.straightdope.com/t/why-is-the-sky-blue/668120 "2013-09-05T22:16:43Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![jmrcm](https://avatars.discourse-cdn.com/v4/letter/j/7c8e57/32.png) [@jmrcm](https://boards.straightdope.com/u/jmrcm)\ **Post date:** [September 6, 2013, 6:39pm UTC](https://boards.straightdope.com/t/why-is-the-sky-blue/668120/21 "2013-09-06T18:39:25Z")
> [@wevets](#): > > Hi **jmrcm** , I think you misunderstand the culture here. This board has a large number of people working in or interested in the sciences who read or even publish papers as part of their daily grind.\* A cite does not have to be a peer-reviewed paper, but if you’re unwilling to provide some sort of reference to back up your claims, this board might not be a great fit for you. It’s not a unwillingness to provide a cite. The principles I’ve discussed are considered to be bedrock principles of modern physics. I’ve already mentioned Tyndall, Rayleigh and Compton. And Einstein and Planck. I don’t need to reference the respective papers. It’s textbook physics. (university texts - not children’s text). It’s not obscure physics. And I’m not making claims for the water buffalo population of Lake Victoria, without reference to publish research. Dr. Strangelove, accused me of being at odds with every other source out there. In away, it’s like talking about Australia, and someone saying I’m the only one who believes it exists, and it isn’t on any maps anywhere. I’m not even going to cite a textbook - the Wiki entries for Compton Scattering, Rayleigh Scattering etc are as good as any text book. Some Wiki answers of course are bad. > [@](#): > > - No, I don’t mean publishing daily 😉 I mean publication is an ordinary part of their work. I’m not publishing a radical groundbreaking paper. And I don’t want to make any contentious statements. I have had a few bad experiences. But I will say something. The blue sky business, fully and properly understanding it, is a key to understanding a lot of physics. If you have any of the joins wrong, it can also mean you misunderstand a lot of physics. I’ve known physics post-grads who’ve had some of the joins wrong. But it’s wonderfully weird. It has plenty of the weird classical/quantum thing going on. ---
**Author:** ![RitterSport](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rittersport/32/6326_2.png) [@RitterSport](https://boards.straightdope.com/u/RitterSport)\ **Post date:** [September 6, 2013, 6:58pm UTC](https://boards.straightdope.com/t/why-is-the-sky-blue/668120/22 "2013-09-06T18:58:06Z")
> [@jmrcm](#): > > It’s not a unwillingness to provide a cite. The principles I’ve discussed are considered to be bedrock principles of modern physics. I’ve already mentioned Tyndall, Rayleigh and Compton. And Einstein and Planck. I don’t need to reference the respective papers. It’s textbook physics. (university texts - not children’s text). It’s not obscure physics. And I’m not making claims for the water buffalo population of Lake Victoria, without reference to publish research. Just because you’re referencing “bedrock principles of modern physics” doesn’t mean they are the right ones. “You know, I’m 40 today, but I don’t feel a day over 35.” “Well, you’ve spent a fair amount of time on the 30th floor of a skyscraper, and one of the implications of relativity (a bedrock principle of modern physics) is that objects further from the center of mass of a large body will experience time dilation.” ---
**Author:** ![Irishman](https://avatars.discourse-cdn.com/v4/letter/i/b487fb/32.png) [@Irishman](https://boards.straightdope.com/u/Irishman)\ **Post date:** [September 6, 2013, 7:01pm UTC](https://boards.straightdope.com/t/why-is-the-sky-blue/668120/23 "2013-09-06T19:01:42Z")
> [@jmrcm](#): > > It’s not scattered in all directions. If you’re orbiting earth - and you can check this out by looking at the international space station webcam the blue sky we’re used to seeing is completely invisible - form outer space looking in, there is no blue sky. Actually, if you look at the horizon you can see that the atmosphere does look blue. However, the bright surface of the Earth overwhelms the slight blue glow of the light scattering, so the blueness of the scattered light is not really detectable. > [@](#): > > No, the blue is not shifted with more intensity than the other wavelengths. That disagrees with everything else on the topic. For instance, on the [Tyndall Effect](http://en.wikipedia.org/wiki/Tyndall_effect): > [@](#): > > The Tyndall effect, also known as Tyndall scattering, is light scattering by particles in a colloid or particles in a fine suspension. It is named after the 19th century physicist John Tyndall. It is similar to Rayleigh scattering, in that the intensity of the scattered light depends on the fourth power of the frequency, so blue light is scattered much more strongly than red light. If you don’t like Wikipedia, here is a [Georgia State University](http://hyperphysics.phy-astr.gsu.edu/HBASE/atmos/blusky.html)website describing light scattering: > [@](#): > > The blue color of the sky is caused by the scattering of sunlight off the molecules of the atmosphere. This scattering, called Rayleigh scattering, is more effective at short wavelengths (the blue end of the visible spectrum). Therefore the light scattered down to the earth at a large angle with respect to the direction of the sun’s light is predominantly in the blue end of the spectrum. Also: > [@](#): > > Rayleigh scattering can be considered to be elastic scattering since the photon energies of the scattered photons is not changed. Scattering in which the scattered photons have either a higher or lower photon energy is called Raman scattering. [QUOTE=jmrcm] There is no lunar sky - something that is transparent by the grace of not being there is not black. [/QUOTE] “Sky” does not equal atmosphere. The “sky” is the expanse overhead. It includes any atmosphere, but also the “heavens” beyond, i.e. space and any astronomical bodies. The lunar surface does have a sky - it is black, not blue, because the lunar sky does not include an atmosphere, but there is still an expanse of space above the lunar surface that is visible. [QUOTE=jmrcm] The Tyndall effect (the blueness of milk and glass), Rayleigh and Compton scattering, they are all in principle the same thing. [/QUOTE] Arguable depending on how loosely you are using the words “in principle”. They are different enough that they have different terms named after different discoverers. They behave with different characteristics and the effects are calculated with different formulas. [QUOTE=jmrcm] If someone asks you why is the sky blue, if you respond that the light is scattered in all directions - and then you’re asked the question why is the sky white(gray) on a cloudy day - answering the light is scattered in all directions, is in fact more correct for a gray sky than a blue sky. [/QUOTE] From that GSU website above: > [@](#): > > The almost white scattering near the sun can be attributed to Mie scattering, which is not very wavelength dependent. > > Clouds in contrast to the blue sky appear white to achromatic gray. > > The water droplets that make up the cloud are much larger than the molecules of the air and the scattering from them is almost independent of wavelength in the visible range. The key factor is that particle size in relation to the wavelength of light is crucial in determining whether the scattering will be directional or evenly dispersed. Rayleigh scattering is wavelength dependent (shorter wavelengths scatter more) but nondirectional (scatters evenly). Mie scattering is not wavelength dependent, but scattering is more directional. Tyndall scattering occurs from particles roughly the same size as the wavelength of the light, and is wavelength dependent. Again from wikipedia: > [@](#): > > Rayleigh scattering is defined by a mathematical formula that requires the light-scattering particles to be far smaller than the wavelength of the light. For a dispersion of particles to qualify for the Rayleigh formula, the particle sizes need to be below roughly 40 nanometres; and the particles may be individual molecules. Colloidal particles are bigger, and are in the rough vicinity of the size of a wavelength of light. Tyndall scattering, i.e. colloidal particle scattering, is much more intense than Rayleigh scattering due to the bigger particle sizes involved. The importance of the particle size factor for intensity can be seen in the large exponent it has in the mathematical statement of the intensity of Rayleigh scattering. There is no equivalent mathematical statement for Tyndall scattering. But, if the colloid particles are spheroid, Tyndall scattering is mathematically analysable in terms of Mie theory, which admits particle sizes in the rough vicinity of the wavelength of light. [QUOTE=jmrcm] I’m not really sure what a Texan can or cannot understand, but if you can’t explain it to a Texan - or an example a great scientist who can’t remember the name of; an Oklahoman corn farmer, then you probably don’t understand it yourself. [/QUOTE] That comment was what is referred to as “a joke”. [QUOTE=jmrcm] Saying the blue portion of sunlight is scattered with greater intensity than the other wavelengths, is, completely and utterly wrong. [/QUOTE] And yet that is what the literature says. Again from wikipedia on Tyndall effect: > [@](#): > > Under the Tyndall effect, the longer-wavelength light is more transmitted while the shorter-wavelength light is more reflected via scattering. An analogy to this wavelength dependency is that longwave electromagnetic waves such as radio waves are able to pass through the walls of buildings, while shortwave electromagnetic waves such as light waves are stopped and reflected by the walls. The Tyndall effect is seen when light-scattering particulate-matter is dispersed in an otherwise light-transmitting medium, when the cross-section of an individual particulate is the range of roughly between 40 and 900 nanometers, i.e., somewhat below or near the wavelength of visible light (400–750 nanometers). [QUOTE=jmrcm] The blue portion of light is scattered with less intensity than other wavelengths. This is the fundamental principle of the Tyndall effect - why glass appears bluish. [/QUOTE] That is entirely backwards. The glass appears blue because the blue light is being scattered and is therefore eminating from the sides of the glass to travel to your eyes, whereas the main flow of light is emerging out the other side as determined by refraction. [QUOTE=jmrcm] How about, Arthur Compton. He won the 1927 Nobel Prize for Physics, for the Compton effect (it’s named after him). His paper was peer-reviewed, and his peer’s even nominated him for a Nobel, which he won. [/QUOTE] The Compton Effect does relate to X-ray scattering and shifting wavelengths, but that does not explain any connection to how the atmosphere scatters light. Compton scattering is described as “inelastic scattering”, versus Rayleigh and Tyndall scattering that are termed “elastic scattering”. [QUOTE=jmrcm] Your bluff has been called. There isn’t a single reputable modern physics book that will contradict me. [/QUOTE] So you dispute the Georgia State University website I cited? [QUOTE=jmrcm] So, under your principle (whatever you want to call it - did anyone ever win a Nobel for it?), blue light scatters more because it has a shorter wavelength then red, but X-rays scatter less because they have a shorter wavelength than blue light. [/QUOTE] You are misreading. He did not say that wavelength alone was the determining factor, rather the wavelength in relation to the size of the scattering particles. Blue light scatters more because it is about the same size, red light scatters less because it is longer wavelength, X-rays scatter less because they are much smaller. ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [September 6, 2013, 8:23pm UTC](https://boards.straightdope.com/t/why-is-the-sky-blue/668120/24 "2013-09-06T20:23:05Z")
> [@jmrcm](#): > > It’s not a unwillingness to provide a cite. The principles I’ve discussed are considered to be bedrock principles of modern physics. This is just by way of advice, not a specific instruction. As you have already found, this approach is not going to work here. Unless you can cite something more specific than the “bedrock principles of modern physics,” you’re not going to make much headway on this site. This is a tough crowd - bald assertions aren’t going to convince them. > [@](#): > > I have had a few bad experiences. Based on what I’ve seen so far, this statement does not surprise me in the least.😉 ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [September 6, 2013, 8:24pm UTC](https://boards.straightdope.com/t/why-is-the-sky-blue/668120/25 "2013-09-06T20:24:26Z")
> [@jmrcm](#): > > I’m not even going to cite a textbook - the Wiki entries for Compton Scattering, Rayleigh Scattering etc are as good as any text book. Some Wiki answers of course are bad. Let’s see: [Rayleigh scattering](http://en.wikipedia.org/wiki/Rayleigh_scattering) _The strong wavelength dependence of the scattering (~λ^−4) means that shorter (blue) wavelengths are scattered more strongly than longer (red) wavelengths. This results in the indirect blue light coming from all regions of the sky. Rayleigh scattering is a good approximation of the manner in which light scattering occurs within various media for which scattering particles have a small size parameter._ To a first approximation, the Compton effect doesn’t apply to visible light. See an explanation [here](http://www.trentu.ca/academic/physics/jbeda/PHYS202H/PHYS202H-2004-Fall/HW-03-202H-solutions.pdf) (problem 1). The Compton effect is much stronger with X-rays and gamma rays. > [@jmrcm](#): > > But I will say something. The blue sky business, fully and properly understanding it, is a key to understanding a lot of physics. If you have any of the joins wrong, it can also mean you misunderstand a lot of physics. I’ve known physics post-grads who’ve had some of the joins wrong. But it’s wonderfully weird. It has plenty of the weird classical/quantum thing going on. I don’t suppose you’re an advocate for “tired light” and a steady state universe instead of the Big Bang? ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [September 6, 2013, 10:23pm UTC](https://boards.straightdope.com/t/why-is-the-sky-blue/668120/26 "2013-09-06T22:23:18Z")
Out of curiosity, **jmrcm** , how do you explain a picture like [this](http://upload.wikimedia.org/wikipedia/commons/0/0b/Why_is_the_sky_blue.jpg)? As you can see, the transmitted light is orange, while the scattered light is blue. This is perfectly consistent with the idea that blue light is scattered more: the excess blue light that is scattered to the side is removed from the portion passing straight through, leaving only orange. The picture is not consistent with your explanation, to the degree that you have even described it clearly. The scattered light is the same color of blue all the way to the left part of the stone, and yet the unscattered portion that hits the ground is a consistent orange. If instead the frequency was shifting as it scattered, we would see a color gradient across the image instead of a distinct split. Actually, that makes me realize there is another fundamental problem with your explanation. If light in a sunset was reddish because it underwent a greater frequency shift due to passing through more atmosphere, then it must be the case that there was a larger number of atomic interactions. With each interaction, the path of the photon must change somewhat. Because of this, objects beyond the atmosphere should appear blurry. However, they don’t: they are perfectly clear, aside from the unrelated atmospheric refraction. The standard scattering explanation has no problem with this, because most photons don’t interact with the atmosphere at all, and the ones that are transmitted pass through completely unperturbed. The scattered photons are completely randomized, but this adds only to an ambient glow and not a blurriness to the image. ---
**Author:** ![jmrcm](https://avatars.discourse-cdn.com/v4/letter/j/7c8e57/32.png) [@jmrcm](https://boards.straightdope.com/u/jmrcm)\ **Post date:** [September 7, 2013, 3:29pm UTC](https://boards.straightdope.com/t/why-is-the-sky-blue/668120/27 "2013-09-07T15:29:35Z")
> [@Colibri](#): > > And now Marshall Dillon walks into the bar to see what all the fuss is about…😉 Howdy, Marshal…We seen the same movies…I had a feeling you might show (which is why I left your line blank for you to write your own entrance) I will abide by the rules. ---
**Author:** ![jmrcm](https://avatars.discourse-cdn.com/v4/letter/j/7c8e57/32.png) [@jmrcm](https://boards.straightdope.com/u/jmrcm)\ **Post date:** [September 7, 2013, 3:52pm UTC](https://boards.straightdope.com/t/why-is-the-sky-blue/668120/28 "2013-09-07T15:52:23Z")
> [@RitterSport](#): > > **jmrcm** , the only one in this thread who seems rude to me is you. > > Anyway, NASA says you’re wrong: [http://spaceplace.nasa.gov/blue-sky/](http://spaceplace.nasa.gov/blue-sky/) You’re waving NASA children’s education web page at me. It’s not a good explanation, and there’s plenty of misconception in there. > [@](#): > > There’s my cite. Where’s your cite? I don’t think cite would fly with many scientific journals should you chose to submit a paper. > [@](#): > > Also, if it appears blue because the light is losing momentum due to all the collisions with the air, why don’t stars appear blue? > > Why doesn’t the sun itself appear blue? Ah haw…You thought you got me…But the sun appears yellow to us. Though from outer space the sun appears white. I can’t embed and image here. But here’s a link to the spectrum [http://www.redorbit.com/media/uploads/2004/10/6\_c9b95f985acbe749f13bf6301a3180a85.jpg](http://www.redorbit.com/media/uploads/2004/10/6_c9b95f985acbe749f13bf6301a3180a85.jpg) There is no such thing as white light. What white light is, is the eye and the brain mixing colors in a certain way to give us the impression of whiteness. If you look at the diagram I have posted it has the visible spectrum with lowest energy(which is longest wavelength) on the left and blue to the right. If you saw all the visible spectrum at once, it would appear white. But if the entire spectrum lost some energy (through collisions, whatever) the spectrum would be moved left (more red) …That is why the sun appears yellow. If you have a light dimmer switch, you can observe that at low energy the tungsten filament is red, slowly turn the power up and it becomes whiter, if you can go higher, which most lights won’t let you, the tungsten filament will glow blue (you shouldn’t even look at this - you’ll see it with arc welding) ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [September 7, 2013, 4:47pm UTC](https://boards.straightdope.com/t/why-is-the-sky-blue/668120/29 "2013-09-07T16:47:10Z")
> [@](#): > > Ah haw…You thought you got me…But the sun appears yellow to us. It does? News to me. I just glanced out the window, and yup, it’s white, just like it normally is. As for sending you to the NASA children’s page, well, you’ve got to start somewhere, and you already rejected a quote from Feynman. ---
**Author:** ![jmrcm](https://avatars.discourse-cdn.com/v4/letter/j/7c8e57/32.png) [@jmrcm](https://boards.straightdope.com/u/jmrcm)\ **Post date:** [September 7, 2013, 5:00pm UTC](https://boards.straightdope.com/t/why-is-the-sky-blue/668120/30 "2013-09-07T17:00:00Z")
> [@Dr.Strangelove](#): > > Let’s see: > [Rayleigh scattering](http://en.wikipedia.org/wiki/Rayleigh_scattering) > _The strong wavelength dependence of the scattering (~λ^−4) means that shorter (blue) wavelengths are scattered more strongly than longer (red) wavelengths. This results in the indirect blue light coming from all regions of the sky. Rayleigh scattering is a good approximation of the manner in which light scattering occurs within various media for which scattering particles have a small size parameter._ > > To a first approximation, the Compton effect doesn’t apply to visible light. See an explanation [here](http://www.trentu.ca/academic/physics/jbeda/PHYS202H/PHYS202H-2004-Fall/HW-03-202H-solutions.pdf) (problem 1). The Compton effect is much stronger with X-rays and gamma rays. Okay, now I’m sorry for referring to Wikipedia. Every Photon in the universe is alike. Their only variation is in their energy, frequency, and wavelength - a photons energy is a function of its wavelength or frequency. The only difference between a photon from the visible spectrum, and one that’s in the x-ray range, is its’ energy. > [@](#): > > I don’t suppose you’re an advocate for “tired light” and a steady state universe instead of the Big Bang? You’re being rude. ---
**Author:** ![ZenBeam](https://avatars.discourse-cdn.com/v4/letter/z/3ab097/32.png) [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)\ **Post date:** [September 7, 2013, 5:37pm UTC](https://boards.straightdope.com/t/why-is-the-sky-blue/668120/31 "2013-09-07T17:37:48Z")
Still no cites from you that actually support what you’re claiming, **jmrcm**. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [September 7, 2013, 5:52pm UTC](https://boards.straightdope.com/t/why-is-the-sky-blue/668120/32 "2013-09-07T17:52:44Z")
> [@](#): > > Every Photon in the universe is alike. Their only variation is in their energy, frequency, and wavelength - a photons energy is a function of its wavelength or frequency. > > The only difference between a photon from the visible spectrum, and one that’s in the x-ray range, is its’ energy. Yes, we know that. What does that have to do with your argument? ---
**Author:** ![jmrcm](https://avatars.discourse-cdn.com/v4/letter/j/7c8e57/32.png) [@jmrcm](https://boards.straightdope.com/u/jmrcm)\ **Post date:** [September 7, 2013, 7:44pm UTC](https://boards.straightdope.com/t/why-is-the-sky-blue/668120/33 "2013-09-07T19:44:40Z")
> [@Chronos](#): > > Yes, we know that. What does that have to do with your argument? It means, you cannot have low scattering at a low frequency, and then high scattering at a higher frequency, but magically back to low scattering at an even higher frequency. ---
**Author:** ![jmrcm](https://avatars.discourse-cdn.com/v4/letter/j/7c8e57/32.png) [@jmrcm](https://boards.straightdope.com/u/jmrcm)\ **Post date:** [September 7, 2013, 8:11pm UTC](https://boards.straightdope.com/t/why-is-the-sky-blue/668120/34 "2013-09-07T20:11:12Z")
> [@RitterSport](#): > > Wait a minute – if the losing momentum story was correct, wouldn’t we see a red sky? Red light is lower energy than blue light, right? That’s kind of interesting. Let’s look at the sky of Mars [![](http://d1jqu7g1y74ds1.cloudfront.net/wp-content/uploads/2008/05/mars_panorama.jpg) ](http://d1jqu7g1y74ds1.cloudfront.net/wp-content/uploads/2008/05/mars_panorama.jpg) [![](http://t1.gstatic.com/images?q=tbn:ANd9GcRZSvmBBR-xk179w4XSRj5TBvvvbF-5oal8IL9iyRuz5v0oBUNqKg) ](http://t1.gstatic.com/images?q=tbn:ANd9GcRZSvmBBR-xk179w4XSRj5TBvvvbF-5oal8IL9iyRuz5v0oBUNqKg) Not that I have sat down and rigorously worked it out. But, atmospheric density, distance from the sun, should effect the color of the sky. But this is a Straight Dope comment thread, not the Annals of Physic. I have a pet theory, that regardless of the planet you are on, the sky is mostly blue. (A pet theory, that would require a lot of pen and paper work, some tricky computer modeling, then spectrographic results from Mars. In other words, lots of work ) Color is just a phenomena that exists for humans. A single illumination, of a single wavelength of red, blue, green - like from a laser, we’ll see as a single color. Whites are more complicated - we need to see a specific distribution of wavelengths to see white. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [September 7, 2013, 8:11pm UTC](https://boards.straightdope.com/t/why-is-the-sky-blue/668120/35 "2013-09-07T20:11:46Z")
> [@jmrcm](#): > > It means, you cannot have low scattering at a low frequency, and then high scattering at a higher frequency, but magically back to low scattering at an even higher frequency. That’s an absolutely false statement, aside from the fact that there’s nothing magical about it. Scattering is a very complex phenomenon with multiple sources, and these sources can most certainly combine to form a curve with a peak–or even many peaks. Even within the realm of Mie scattering alone, [there are many local maxima](http://en.wikipedia.org/wiki/File:Radar_cross_section_of_metal_sphere_from_Mie_theory.svg). ---
**Author:** ![C\_K\_Dexter\_Haven](https://avatars.discourse-cdn.com/v4/letter/c/b2d939/32.png) [@C\_K\_Dexter\_Haven](https://boards.straightdope.com/u/C_K_Dexter_Haven)\ **Post date:** [September 8, 2013, 1:20pm UTC](https://boards.straightdope.com/t/why-is-the-sky-blue/668120/36 "2013-09-08T13:20:30Z")
> [@jmrcm](#): > > You’re being rude. MODERATOR COMMENT: **jmrcm** , that’s a comment that only a Moderator can make. If you think someone is being rude or insulting, the proper response is to report the post: click on the little red triangle with the !, in the upper right corner. Then let the Moderator handle it. As it happens, I think that **Dr Strangelove** ’s comment is slightly snarky, but not actually a direct insult. We want to have polite discussion here, without personal insults, but we don’t want to squash a sense of humor or a slight bit of “edge” (commonly called “snark”.) It’s often a fine line. ---
**Author:** ![Fear\_Itself](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/fear_itself/32/19637_2.png) [@Fear\_Itself](https://boards.straightdope.com/u/Fear_Itself)\ **Post date:** [September 8, 2013, 1:49pm UTC](https://boards.straightdope.com/t/why-is-the-sky-blue/668120/37 "2013-09-08T13:49:08Z")
> [@jmrcm](#): > > That’s kind of interesting. > > Let’s look at the sky of Mars To what do you attribute the color of the sky on Mars? ---
**Author:** ![Irishman](https://avatars.discourse-cdn.com/v4/letter/i/b487fb/32.png) [@Irishman](https://boards.straightdope.com/u/Irishman)\ **Post date:** [September 9, 2013, 4:26pm UTC](https://boards.straightdope.com/t/why-is-the-sky-blue/668120/38 "2013-09-09T16:26:23Z")
Testing…1…2…3. Is this thing on? > [@jmrcm](#): > > If you have a light dimmer switch, you can observe that at low energy the tungsten filament is red, slowly turn the power up and it becomes whiter, if you can go higher, which most lights won’t let you, the tungsten filament will glow blue (you shouldn’t even look at this - you’ll see it with arc welding) Yes, bodies emit radiation at different wavelengths based upon their temperature. I’m not sure how that is supposed to relate to the scattering of light that causes the sky to look blue. > [@jmrcm](#): > > Every Photon in the universe is alike. Their only variation is in their energy, frequency, and wavelength - a photons energy is a function of its wavelength or frequency. > > The only difference between a photon from the visible spectrum, and one that’s in the x-ray range, is its’ energy. Did you even read the second link **Dr. Strangelove** provided? It’s a .pdf, so I’ll copy the text for you. > [@](#): > > (From Eisberg & Resnick, Q 1-12, pg 51) Do you observe a Compton effect with visible light? Why or why not? Limit your discussion to about 50 words or so. Hint: this is related to Eisberg & Resnick, Q 1-10, pg 51 in that the reason for the independence of Compton scattering on the composition of the scatterer is related to the frequency of the inciden photons. [10] > > Solution: There are a number of reasons why the Compton effect is not observed with visible light. They all boil down to the fact that Δλ for visible light is much much much smaller than the wavelength of the light itself - basically the wavelength shift is undetectable. The Compton effect is independant of the wavelength of the incoming photon, and is of order equal to the Compton wavelength λ[sub]C[/sub] = 0.243 Å for scattering off of a free electron. Since visible light has wavelengths in the range of 400 nm to 700 nm (which is 4000 Å to 7000 Å) this shift is hard to detect, being a change of only a couple parts in ten thousand. For cases where the electron is bound to an atom and the incident photon does not have enough energy to free it, the Compton shift is even smaller since the effective mass of the system is not the entire atomic mass. Thus for visible light, Rayleigh scattering does an excellent job of describing the behaviour of the scattered photon. > [@jmrcm](#): > > It means, you cannot have low scattering at a low frequency, and then high scattering at a higher frequency, but magically back to low scattering at an even higher frequency. You are still neglecting that the cause of the scattering is interaction with the air molecules, and thus the relationship of the wavelength to the size of the molecules affects the scattering process. ---
**Author:** ![Irishman](https://avatars.discourse-cdn.com/v4/letter/i/b487fb/32.png) [@Irishman](https://boards.straightdope.com/u/Irishman)\ **Post date:** [September 9, 2013, 4:30pm UTC](https://boards.straightdope.com/t/why-is-the-sky-blue/668120/39 "2013-09-09T16:30:47Z")
Ignore the stupid-ass Q in the title block. It was an accident. I tried to edit my post, but the buffer didn’t want to hold the post text. I didn’t feel like copy/paste and then recoding all the quote boxes and such. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [September 13, 2013, 9:19pm UTC](https://boards.straightdope.com/t/why-is-the-sky-blue/668120/40 "2013-09-13T21:19:48Z")
_The Feynman Lectures on Physics, Volume I_, have now been put on the web (legally). Chapter 32-5, which I cited earlier, can be found [here](http://www.feynmanlectures.info/docroot/I_32.html#Ch32-S5). Here is a slightly longer quote (but I still recommend reading the whole thing): > [@](#): > > Let us look at some examples. First, if we go to a very low natural frequency ω[sub]0[/sub], or to completely unbound electrons, for which ω[sub]0[/sub]=0, then the frequency ω cancels out and the cross section is a constant. This low-frequency limit, or the free electron cross section, is known as the Thomson scattering cross section. It is an area whose dimensions are approximately 10[sup]−15[/sup] meter, more or less, on a side, i.e., 10[sup]−30[/sup] square meter, which is rather small! > > On the other hand, if we take the case of light in the air, we remember that for air the natural frequencies of the oscillators are higher than the frequency of the light that we use. This means that, to a first approximation, we can disregard ω[sub]2[/sub] in the denominator, and we find that the scattering is proportional to the fourth power of the frequency. That is to say, light which is of higher frequency by, say, a factor of two, is sixteen times more intensely scattered, which is a quite sizable difference. This means that blue light, which has about twice the frequency of the reddish end of the spectrum, is scattered to a far greater extent than red light. Thus when we look at the sky it looks that glorious blue that we see all the time! [Previous page](https://boards.straightdope.com/t/why-is-the-sky-blue/668120.md?page=1) [Next page](https://boards.straightdope.com/t/why-is-the-sky-blue/668120.md?page=3)