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It said to use red colur objects as a sign of danger because it is scattered at least and hence can be seen from far?

But if use suppose blue colour bulb in a open area (where no objects to absorb light) , I agree that its rays will not reach us when far because all colour is already scattered and hence we cannt see it.

But why we cannt see even that scattered blue light from far(very) , although we can see scattered light from sky that is much farther than blue bulb?

It is said that during time of sunset or sunrise, appears red as all blue colour is scattered very before, My doubt is why cannt we see that blue colour scattered light? Although it is said our eye can watch at infinity.

Just because scattered light is far, we cannt see it, Can't scattered light reach us from that far?

  • You see the blue light from the sky exactly because it is scattered. It's sunlight, scattered in all directions, including yours. That said, could you provide some references to using red danger signals rather than blue? My naive guess would be that on such short distances that danger signals are used on, scattering can be neglected. – pela Oct 06 '21 at 14:08
  • No i am asking, although when light (any short wavelength one) is scattered far away ok take an ex. Of sunset, sunrays have to travel a long distance so, almost all blue light is scattered at starting of atmosphere and finally in the more inner atmosphere now remaining red light is can scatter. (I thibk u know all this) so come to doubt, why i cannt see that blue scattered light far in atmosphere? Just because it is far? Why cannt that blue scattered light reach my eyes? Although eye can recieve light from infinity! –  Oct 06 '21 at 14:19
  • I am just making a guess! Is it because that blue scattered (already) rays at starting, keep on scattering while coming to inner atmosphere and finally it is that much scattered when it reach our eyes, whose intensity is too low to be recognized???? Is it like that or something else? –  Oct 06 '21 at 14:21
  • This may help. Is there more to the the conventional explanation of 'Why is the sky blue?'? – mmesser314 Oct 06 '21 at 14:24
  • @mmesser314 thanks a lot, but i know all this, i think u don't got my doubt? My doubt is not only based on sunlight and all, that was just an example i gave to help u guys understand more. I m just asking light which is scattered at far place why cannt we see it from a large distance although light rays can travel to a long distance,? Read my comment to pela –  Oct 06 '21 at 14:29

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Suppose you placed a bulb very far away(say infinity). Now if the bulb is turned on, you can't see the light at infinity, because of the intensity, as you can see in this image, The farther the light travels the more area it has to cover, now a bulb shining at infinity have to cover an infinite area where you are standing(at infinity from bulb).
Thus the density of rays decreases very much, and become invisible.
Note- You can see a bulb shining when you are close to is because it's intensity and light ray density is high, but as you move away both decreases and at a point you can't see the bulb anymore.

The Space Guy
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  • So suppose i place a blue bulb, so doesn't this could be reason that light is scattered and kerp on scattering as it move farther and farther, and one point reach when the angle of scattering is much more because of continuous scattering, that most of blu light cannt travel into 90° with bulb but most of light goes either in bulb direction or some random direction, nit direct to our eyes!!??? –  Oct 07 '21 at 05:12
  • Or can u explain tge blue rays that are scattered at top starting atmosphere during sunset or sunrise cannt reach us, we say it is scattered 'away' so what does it mean, is that mean what i write in @mmesser314's comment, look! Is that why blue light scattered at starting cannt reach us?? Or other reason. BTW nice answer –  Oct 07 '21 at 05:24
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    During sunset or sunrise light has to travel more through the atmosphere to reach us, and as blue light is scattered most, blue light experience multiple scattering as a result it is highly deviated from it's path and fails to reach us, where as red or orange lights experiencing least deviation reaches us. – The Space Guy Oct 07 '21 at 06:10
  • So if we look like this than if we light a bulb(white light) and look from very far, it should appear red yellowish and air around it should glow in blue colour(same like sky), but this doesnt happen in real world, when we see a white bulb (ON) we can either see it dim from far but not a bluish glow in air!??? Nor bulb appear yellowish like sun during noon? Why? Bulb also has white light, air can scattered its blue light!! Than why we dont see such things happening –  Oct 07 '21 at 06:44
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    You are comparing sun with a bulb...Bulb doesn't have even any fraction of sun's intensity, for light to scatter that much(what happens with sun's ray) it should be able travel through 1000's of km through atmosphere, where as for a light emmited by bulb can't travel that much, thus no scattering....[by can't travel I mean intensity minimizes to 0] – The Space Guy Oct 07 '21 at 07:13
  • O... But i watch a vedio in that,tank is filled with water and some crystals added of some solution (i dont remember name, u can search it on khan academy scattering exp. On u tube), so after that tge sol. Is ready for scattering, now they keep rectangular tub and look from its length part, solution look blue and torch glown on opp. Length look yellow, now they saw through breadth part of tank while keeping torch to another breadth part, now solution and torch appear red, here light traveks only a very very small distance still scattered!!!! –  Oct 07 '21 at 07:23
  • Does amt. Of scattering depend on medium? –  Oct 07 '21 at 07:35
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    Yes...scattering depends on medium – The Space Guy Oct 07 '21 at 07:57
  • Hey just one more, last one, in ur previous comment 'you are comparing sun and bulb' this one, so you said 'no scattering happen ' i think u mean ' very less scattering hapoen to BE NOTICED' because scattering is MUST! It should happen!! As air allow scattering , More or less that depends on circumstances. So see what i understand.... "Sun's rays intensity is far... More than a small bulb! So when we take suppose a unit area in that if intensity is high more no. Pf scattering took place so.. Sun's intensity is too.... High and hence so much scattering happen that sky appear blue.. –  Oct 07 '21 at 08:41
  • Now in light bulb intensity is very low as compared to sun. So scattering happen in unit area is very low to be noticed as blue! So when we look at bulb it appear just same and no glow of blue like in sky. When we look from far also it appear blue because from what max. distance bulb is visible i am sure upto that time all blue light must not have finished like in case of sunset and sunrise because that distance is tooooooo long. So still max. Distance we can see bulb we will ee white light only and scattering is too low to be noticed much (except a little blue glow very small) –  Oct 07 '21 at 08:54
  • And finally at what point blue light from spectrum of bulb is scattered away and red left like sunrise, than at that distance the rays intensity is extremely low.... Nearly zero. So that even red light left in spectrum is not visible. Hence bulb is not visible. Am i correct in understanding?? –  Oct 07 '21 at 08:56
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    Bulb light also scatter as sun's ray but it's intensity decreases!! – The Space Guy Oct 07 '21 at 15:07
  • really thanks a lot, because of you i m able to know many things and also that i have to know many things really great answer sir Can you tell in which class I will learn these all in depth? –  Oct 08 '21 at 03:49
  • Are you in 10th? – The Space Guy Oct 08 '21 at 03:51
  • Yes. How u know? –  Oct 08 '21 at 05:24
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    Guessed it by the type of questions you were asking!!...By the way I am in 12th – The Space Guy Oct 08 '21 at 06:47
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Red things are used as stop signs because of our vision, not because red light travels farther. Red things appear bright to us because of the way our brain works. Not all that many things are red, so a red object is not likely to blend in to the background.

In clear air, scattering works over longer distances. At noon, light travels straight down through the atmosphere. The atmosphere gets thinner and fades away with altitude, but about half of it is below 3 miles high. At sunset, light travel a longer path through the atmosphere.

Blue light scatters more often and at a larger angle than red light.

At sunset, almost all the blue light from the sun has been scatter away from its original path. So when you look directly at the sun, red predominates.

Likewise, when you look near the sun, you are looking at an angle almost directly at the sun. A lot of what you see is red light that was almost aimed at you, and has been deflected a little. You will get some blue, but a lot of the blue was deflected at a sharper angle.

When you look at $90^o$ from the sun, you will see the sky is still blue, particularly if you look up. Light that was headed somewhat in your direction, but higher gets overhead before it is deep in the atmosphere. There is it scattered. Blue light is likely to be scattered $90^o$. Red light is not. So the sky appears blue.

mmesser314
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  • But sir why we sannt see that blue light scattered at far atmosphere? What do u mean by blue has be deflected from its original path, does it mean before blue light could reach us, it has been so much scatrered that its actual path is changed means most of light is turn towards space and some in 180° overall it is not straightly entering to eyes except only a little bit??? So intensity does got low!? Just because it has faced many scattering one after the other due to long path its got scattered is high angles (most not coming straightly to eyes) so do you mean more and more scattering.. –  Oct 06 '21 at 14:54
  • Cause... Blue light to scatter at high angles like going east, west, back in soace direction and directly nit coming to earth direction(straight to eye) while less scattering like in afternoon , due to short patg, it is not scattered by much particles hence most of it enter directly into eye instead of going random direactions? –  Oct 06 '21 at 14:56
  • Is my understanding right? Sorry i cannt say it in technical language, i am just a small kid –  Oct 06 '21 at 14:57