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Drove through a rainbow

  1. Nov 16, 2005 #1

    Mk

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    Is it physically possible to drive through a rainbow? Why or why not?
     
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  3. Nov 16, 2005 #2

    Mk

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    From the Wiki:
     
  4. Sep 24, 2011 #3

    A.T.

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    When the rain is uniform, the rainbow that you see is a cone with your eye in the apex. So you will never have the impression of approaching the rainbow, because it always extends from your eye to very far away.

    But if there is only a stripe of rainy area, the rainbow is just a slice of a cone: an actual arc. When you approach the stripe of rain, you will see the near boundary of the arc on the ground coming closer (because it actually is). So even though the arc is not a fixed object, you can get closer to it, and drive trough its base.

    When you get into light rain that sprays your windows with tiny water drops, with the sun low, you will see a lot of nice color effects. (However, this can be annoying when you are trying to land a glider, which don't have wipers.)
     
    Last edited: Sep 24, 2011
  5. Sep 24, 2011 #4
    It was a bit more distinct than this to my memory, however I do not discount the possibility of wishful thinking playing a role in what I saw. I'm not willing to discard what I vividly saw, which was the colorization of the interior. There are actually a lot of videos surfacing on youtube of people getting close to the foot of rainbows, so it seems to have happened enough times to accrue a number of eyewitness accounts. A lot of other people are saying they witnessed a change in colors as well. However, from the videos, it does appear that it is less manifestation than I thought, so I am inclined to believe that I am wrong. However, being able to "pass" and "approach" a rainbow, to the degree that one is capable of is quite different than one would expect from messing with standard rainbows made in a mist.
     
  6. Sep 25, 2011 #5

    A.T.

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    Link some that are similar to what you saw.
    That is no problem as I decribed above. In fact, once you drive into the rain, the foot of the rainbow (intersection of the cone and the ground) starts right outside of your car.
     
  7. Sep 25, 2011 #6

    sophiecentaur

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    The above statements about the Physics of how a rainbow forms are correct, of course. A rainbow is a virtual image that is not, actually, anywhere . Mostly (when in the sky) it will appear to be at infinity. But it can often appear in front of a distant hill (your brain tells you that it can't be behind the hill!) or even in the grass of a nearby field.

    If you spray a garden hose near you and the Sun is strong, you can produce a 'rainbow' which will show up vividly against a nearby dark surface. It is even possible to place your hand (which is not where you eye is) so that it appears to be 'in' the rainbow. To get this effect you really need things to be just right. You can also get rainbow-like effects from smears on a windscreen, too, with the colours appearing to be quite close.

    However 'clearly' one remembers a really stunning rainbow, I think that the memory of your exact visual experience will very easily become distorted. A particularly vivid rainbow is a pretty overwhelming experience and I wouldn't necessarily trust even myself to make a highly accurate witness statement. Without a photograph of one of those experiences, I think you can't be absolutely sure of what you saw.
     
  8. Sep 26, 2011 #7

    A.T.

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    The water drops that send light of a certain color to your eye have a well defined position. They form a cone surface with your eye in the apex, and an axis patallel to the sun rays.

    Not really. It will appear where the water drops are. It can extend from your position to a few kilometers away.

    It doesn't just appear to be in front of a distant hill. It is in front of it, if the rainy area is closer than the hill.

    Whether it extends beyond the hill depends on whether it also rains behind the hill. So the brain might sometimes be right here.
     
  9. Sep 26, 2011 #8

    sophiecentaur

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    The mirror that you gaze at yourself in also has a defined position, on the wall. That doesn't mean that your (virtual) image appears in that position. Is your image in a mirror 'in front' of the wall that the mirror is attached to? The mirror is not a projector screen and neither is a water droplet.

    The only absolute position that you could justifiably assign to a rainbow would be the position of the Sun, from which the light originates. You said, yourself, that the 'rays come , via the water drops, into your eye; they start diverging when they leave the Sun and they keep diverging, putting their apparent source a long way away. The rays are never 'focused' onto a point, which they would have to be, for a real image to be produced at a certain point.

    Your brain tries to make what it can out of the light arriving at the eye. Optical illusions occur all over the place. The point of the scientific approach is to 'see' past these illusions and to try to explain them. To be successful, you need a certain amount of rigour. Note (Google) the difference between real and virtual images.

    I wonder whether you have, in your mind some idea that the water droplets behave as if they had some sort of pigment in them. That would account for your idea that the rainbow is actually 'located' somewhere. But remember, as you move about, a droplet that was sending you red light could be a droplet that is now sending you yellow light.
     
  10. Sep 26, 2011 #9

    A.T.

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    The term "virtual image" makes no sense in the case of a rainbow, because there is no focused virtual image of the sun, like in the case of a simple mirror.

    And neither is a water droplet a simple mirror. And a single water droplet doesn't produce a rainbow, the collective of many water droplets does. And within that collective there is a subset for each each color which forms a cone. It's that cone determines where you see the rainbow, not some "virtual image" of the sun.
     
  11. Sep 26, 2011 #10

    sophiecentaur

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    As you seem so sure of this, perhaps you would draw a simple ray diagram to show how this image of a rainbow forms on each water drop.
    There is no focussing involved in order to see a virtual image.
    Come on, this is all simple schoolboy stuff.
     
  12. Sep 26, 2011 #11

    A.T.

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    There is focussing involved in seeing anything. That is why your eyes have lenses.

    - In the case of a single flat mirror the lenses are focused on the virtual image.

    - In the case of a rainbow there is no distinct virtual image. Each water droplet produces a real image of the sun (for each color), within the droplet itself. But that image is quite irrelevant seeing the rainbow.
     
    Last edited: Sep 26, 2011
  13. Sep 26, 2011 #12

    sophiecentaur

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    Of course your eyes focus becaus they form a real image of anything you see.
    My issue is that you are claiming that there is an image 'in' the raindrops and that there is focussing involved in the formation of a virtual image there. This is nonsense which you will discover (using schoolboy ray tracing) when you try to draw a diagram of what goes on. When you have failed, take a look on Wiki or any of the other explanations you can find.
     
  14. Sep 26, 2011 #13

    xts

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    You may make an easy experiment (hurry! Autumn is coming!): on a sunny day make a shower from garden sprinkler. You see mini-rainbow in it (it seems to be within a shower of droplets). Then move your head - you'll see that the rainbow moves accordingly - now your brain (trained to interprete perspective) changes the interpretation of the picture: you now see the rainbow very far away, and the shower is just a window, allowing you to watch it. Your brain may be only a bit confused by the shrubs in the background, as the perspective is such that rainbow is in bigger distance than shrubs, but shrubs do not block the view. That is something confusing to your brain, which is used to transparent windows and transparent showers, but not to transparent shrubs.

    EDIT>
    Ouch :blushing: - apologies for skipping over some previous posts... Mea culpa!
     
    Last edited: Sep 26, 2011
  15. Sep 26, 2011 #14

    sophiecentaur

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    Been there. Did it. See my earlier post. The illusion of the actual 'position' of that image can soon be resolved. The parallax as you move your head from side to side will show you that the image is, in fact, at the Sun' s distance. You are always at the centre of that cone. If the image really was where you say it is, the cone would no longer have its axis through your eye.
    As I said earlier, draw the diagram, rigorously, and don't rely on what you think you see as 'evidence'.
     
  16. Sep 26, 2011 #15

    A.T.

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    Yes, assuming a spherical raindrop the real image for the reflection on the inner surface is in the raindrop. (I had a typo in my previous post, calling it "virtual"). But the key point I was making is, that there is no distinct virtual image here.

    What is nonsense, is your claim that a virtual image is somehow produced here. A concave mirror cannot produce a virtual image of an object which is placed outside of the mirrors focal length.
     
    Last edited: Sep 26, 2011
  17. Sep 26, 2011 #16

    sophiecentaur

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    I can only refer to this and hundreds more sources.
    http://rebeccapaton.net/rainbows/tir.htm

    This is how it is generally understood to work. Can you point our where, in the raindrop diagram, there is any focussing of a real image that you could possibly see? Rays of different wavelengths may cross over in the drop but your eye only intercepts a ray of one wavelength (say, red) from each drop. The guy next to you may see another ray (say yellow) from the same drop but that's not the yellow part of the image that you see.
    As I keep saying, we all learned this in School and it's still right. (Most diagrams ignore the apparent focusing inside the drop because it is not relevant to what you experience.)
    The problem with your interpretation of the effect is that it implies you can sit 'in' the rainbow. Why do you think the daft story about a pot of gold came from, if not because you can't actually get there?

    Have you an answer for my 'parallax' objection to your idea about where the rainbow actually is situated? If you move your head from side to side and an image always moves as if it's a long way away then that image is a long way away. Wherever the drops happen to be, this is the effect that you see. I claim that, without any other visual clues, you could have no opinion whether the rainbow was formed by a nearby cloud or by one that is miles away.

    btw, I don't claim that a virtual image is formed in the drop. Where did you deduce that from?
     
  18. Sep 26, 2011 #17

    A.T.

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    I made it pretty clear that the real image inside the raindrop is not relevant for seeing the rainbow. The only reason I even mentioned that image is because you claimed that we see a virtual image.

    There is no virtual image at all here. Concave mirrors don't create virtual images of objects that are outside of their focal length:

    http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/mirray.html#c3
    http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/mirray.html#c4

    The sun is hardly inside the focal length of a raindrop.
     
  19. Sep 26, 2011 #18

    sophiecentaur

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    Where is there a "concave mirror" that is, in any way, relevant to this? If you say the focussing of some rays inside the drop is not relevant then why should you be considering the formation of an image of the Sun 'inside the drop"? I certainly am not.

    The easiest way to look at this is that one ray, of just one wavelength, per drop (sloppy terminology if you like but it can suffice) comes to your eye. This arrives because of total internal reflection at the back surface of the drop. The curvature of this surface is quite irrelevant because we are only considering a tiny arc of the circle; we can treat it as a plane mirror, in the same way that one can construct the usual image formed by an ordinary curved mirror - by breaking it down into a polyhedron. Only one face of this polyhedron is relevant in each drop and for each wavelength.

    My problem with reading your posts is that you bring up ideas and then say they are not relevant. It's very hard to follow because of that. What I would like is for you to point out just how the image you 'see' of the rainbow appears to be at or in the drops. Can you really believe that my parallax (or even the binocular effect) can place the image anywhere other than at a great distance? And are you really dismissing the effect of other visual clues in the way the image 'position' is determined by the brain? (Do you understand what I am saying about Parallax?)
     
  20. Sep 26, 2011 #19

    A.T.

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    You brought up the concept of "virtual images" which is not relevant here. I just pointed out that the only image here is inside the drop and I said that it is not relevant right in the same post.
     
  21. Sep 26, 2011 #20

    sophiecentaur

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    I'm afraid the whole point about a rainbow is that it is virtual. The light does not come from a point that is in line with your vision. It comes from the Sun, which is behind you. It is precisely the same as an image in a mirror. You have not proved it's not.

    The light of different wavelengths just comes from different directions - not different points. How is that anything but a virtual image? It is you who first introduced the idea of focussing and image formation. What exactly is your modified stance on this matter? You seem to keep shifting.
    And where is your rebuttal for my Parallax point? It all hangs on that. You have not yet said how you could actually tell where this cloud is positioned, without other clues. In fact you really haven't answered any of my points, I think. So, how about it? How can you construct an "image" inside the drop (image of what??), bearing in mind that the rainbow covers several degrees of your vision and a drop covers a fraction of a second of arc?
    I think you are having a problem with the common definitions that we use in optics.
     
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