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Why is the sky blue? (Cojones)

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11385.  Mon Nov 29, 2004 2:46 pm Reply with quote

The usual explanation is the so-called Rayleigh scattering.

John William Strutt, Lord Rayleigh, discovered that the shorter the wavelength of light, the more of it was scattered about by the oxygen and nitrogen molecules in the sky.

Red light has a longer wavelength (630-760 nanometres) than blue light (450-490 nanometres).

So blue light is scattered 4.92 times as much as red light, so that much more blue light reaches our eyes, and so the sky appears blue.

You can find something along very similar lines in numerous places on the net though usually minus the specific nanometre lengths, which I found here:

Last edited by JumpingJack on Mon Nov 29, 2004 3:22 pm; edited 1 time in total

11386.  Mon Nov 29, 2004 2:47 pm Reply with quote

The problem with this is, that blue light doesn't have the shortest wavelength of the visible spectrum, violet light (380-450 nanometres) does.

So if Rayleigh is correct, the sky either is, or should be violet.

11388.  Mon Nov 29, 2004 2:55 pm Reply with quote

Although the Rayleigh scattering is invariably mentioned on pretty well all the many, many net sites purporting to explain why the sky is blue, a much smaller proportion of such sites mentions the anomaly above.

Of the ones that I have found that do raise the question "Why isn't the sky violet?" I have seen only two reasons given.

These, roughly paraphrased, are:

1. The sky is actually violet but 'our eyes aren't as good at detecting violet light as they are at detecting blue light'.

2. The sky isn't violet (or at least not very violet) because although it's true that violet light scatters better than blue light, there is less violet than blue light in white light (ie light as a whole).

11389.  Mon Nov 29, 2004 3:00 pm Reply with quote

These strike me as remarkably feeble excuses.

They are generally mumbled off in a cursory fashion right at the end of an otherwise enormously technical explanation as to the reason why the sky is blue.

Does anyone know the correct answer?

11490.  Mon Nov 29, 2004 5:59 pm Reply with quote

I've always been bothered by just that question, and I don't know any more than the Rayleigh scattering argument. Smoke is blue for the same reason, but why it (and the sky) aren't violet is somewhat of a mystery to me.

Perhaps anything with a shorter wavelength than blue light is actually absorbed by the atmosphere, or the water vapour in it...

11495.  Mon Nov 29, 2004 6:04 pm Reply with quote

Thank God, Gray.

I was really hoping you didn't know the answer to that one.

There must be a cracker of an Alan Davies trap in there somewhere if we can only formulate it...


11496.  Mon Nov 29, 2004 6:06 pm Reply with quote

Meanwhile, see if you can make head or tail of this chap...

11499.  Mon Nov 29, 2004 6:08 pm Reply with quote

The trick questions only work if the panel know the right wrong answer.

11500.  Mon Nov 29, 2004 6:10 pm Reply with quote

Okay, I've come across several explanations, all of which seem reasonable. The sky is a bit violet, but it does not appear violet to us because:

1. Our eyes aren't very receptive to violet light; they are far more responsive to the ROYGB section of the spectrum. This is demonstrably true - try 'seeing' the violet end of a rainbow. Colour is very important to primates, simply to recognise ripe leaves, fruit, or bright scarlet bottoms - whatever takes your fancy.

2. The sunlight we receive on earth is actually rather yellowish, and so lacking somewhat in the violet part of the spectrum.

3. The upper atmosphere absorbs some of the violet light.

So, to birds and bees the sky is almost certainly violet. Maybe that's why they're used to describe the pinkly passionate moments...

11503.  Mon Nov 29, 2004 6:26 pm Reply with quote


So what you're saying is that the sky IS violet but it doesn't look that way to people. That'll do me, squire.

Another question, though. I thought sunlight was white. Now you're telling us it's yellowish?

11504.  Mon Nov 29, 2004 6:28 pm Reply with quote


We'll see about how the trick questions work, or not. It's our show remember we can rewrite the rules, especially if it improves things.

The question for example:

What colour is the sky?

....might produce something, might it not?

In any case not everything has to be formualted as General Ignorance, or, as you know, it gets repetitive and/or people don't make an effort...

11505.  Mon Nov 29, 2004 6:52 pm Reply with quote

What colour is the sky? has the merit of resonating back to the current series, the way the multiple moons did.

As to the colour of sunlight, it looks pretty yellow to me. But our eyes do bugger us about in the matter of colour. If you use an objective judge like a photographic film, it reads tungsten as orange and strip lighting as green, when they both look more or less white to us.

11512.  Tue Nov 30, 2004 5:39 am Reply with quote

Well, quite. You get those problems all the time on film sets.

Still, if sunlight ISN'T white, then what light IS white?

As to:

What colour is the sky?

I'm fumbling towards a formulation, not giving the definitive question.

At any rate, the fact that all the theory points to the fact that the sky OUGHT to be violet is about as QI as it gets.

And personally I don't find the "are they/aren't they" moons a big problem.

(Though I've started to worry I may have lost that defensive email I sent to that chap about 'satellites'. You didn't keep it on file did you?)

11517.  Tue Nov 30, 2004 6:22 am Reply with quote

Stars come in all sorts of colours - large, young ones are blue, middle-aged, medium-sized ones (like ours) are yellow, and old ones are red. These characteristics were drawn up and presented on the Herzsprung-Russell diagram.

So yes, the light from our sun - which is intimately related to the star's surface temperature - is yellow. Our sun's surface is about 5700 degrees Centigrade, which when linked to the element hydrogen (which is what's at the sun's surface) corresponds to a yellowish colour. So the light from the sun is not quite 'white', which would mean an equal proportion of all frequencies of visible light, but has a bit of a peak in the yellow part.

Photographic film is specifically made to react to different temperature/frequency light so that you can reproduce 'actual' colours on a transparency or print. Most film is set to 'outside' temperature sensitivity, so if you shoot pictures inside under an incandescent lamp (a lighbulb), they'll turn out yellow, simply because incandescent lights are low-temperature, and don't give out much blue or green.

Similarly, shooting under flourescent lights, which are quite high-temperature, will give you greeny-blue pictures. Halogen lights (whose gasses most resemble those responsible for the colour of outside light) give the best colour reproduction on film when used inside. Which is why they're nice and expensive!

What's annoying is that your brain automatically compensates for every lighting condition in which you find yourself, whereas film does not (can not). When you look into a shadow outside, you think everything's the same colour, but in fact (as a photo of things in shadow reveals), they are actually blue because they're lacking the yellow light that comes directly from the sun, and are lit only by the blue/cyan from the sky.

This is a pretty good article on it. Of course, if you have a digital camera/scanner and Photoshop, you can correct colour-casts afterwards if you've forgotten to set your camera's white-balance appropriately.

11521.  Tue Nov 30, 2004 7:04 am Reply with quote

incandescent lights are low-temperature

but, perversely, are described as "warm" toned, as opposed to the higher-temperature blue casts which are called "cool".

Jack, I'm sending you that e-mail about the moons.


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