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anorlunda submitted a new PF Insights post
Rainbows are not Vampires
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Rainbows are not Vampires
Continue reading the Original PF Insights Post.
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OmCheeto said:Being that the rainbow in the "What caused the Quadruple Rainbow?" thread looks suspiciously like your "mystery" rainbow, I'm going to guess that yours was a "reflected" rainbow.
I think you missed the point, as astronomer Berman said, "Rainbows are not 3D objects." I too struggled with what that means.spareine said:Why denying virtual objects?
I am sure that, if rainbows didn't look so gorgeous, they wouldn't have got into folk lore and they wouldn't have that extra magical quality that seems to make people treat them differently from other optical phenomena.spareine said:Maybe it is useful to add that your statement that two persons cannot see the same rainbow is a denial of the concept of virtual objects.
I don't wee why it has to be a laser beam. The same thing would apply to any object that happens to be obscured from one viewer and visible to the other. One observer is aware of the object and the other is not.anorlunda said:The point may be clearer if we refer back to the article's analogy with a man pointing a perfectly collimated laser at my eye.
Because light reflected from an ordinary object goes in many directions. It is (at least partially) omnidirectional light.sophiecentaur said:I don't wee why it has to be a laser beam.
I can hold my hand in front of me so that I can see an object with one eye but not the other. Is there any (relevant) difference. Colimation is a way over the top requirement for this explanation. Speaking as one who did all the basic Physics learning in the absence of handy laser pointers, I often find that people reach for a virtual laser to prove points when simple shadows can do just as well. What did the 19th century opticians do when they wanted to explain things?anorlunda said:Because light reflected from an ordinary object goes in many directions. It is (at least partially) omnidirectional light.
Collimated light is unidirectional. A collimated beam can be aimed at your left eye and 0% of its light reaches your right eye.
So a man holding a paper with a red dot printed on it, is very different from a man holding a red laser pointer.
A big difference. Zero photons from the laser reach the other eye, with our without your hand.sophiecentaur said:I can hold my hand in front of me so that I can see an object with one eye but not the other. Is there any (relevant) difference
But in the case of the rainbow you are making a virtual image of the sun at infinity. Even if the different rays from this virtual image passes through different rain drops, it does not make them part of a different virtual image. When you construct virtual images of objects using lenses, the rays take different paths as well, passing through different parts of the lens. The important thing is that a virtual image is constructed. It is this virtual image which is reflected. Fine, the rays did not pass through the same rain drops, but the virtual image is not where the raindrops are.anorlunda said:A big difference. Zero photons from the laser reach the other eye, with our without your hand.
You could likely get similar effects with the virtual images created by lenses by placing a mirror beyond the lens, clearly the virtual image is not going to be reflected.anorlunda said:Now suppose that the rain just began so that the high drop is there but there is no low drop
anorlunda said:I think you missed the point, as astronomer Berman said, "Rainbows are not 3D objects." I too struggled with what that means.
The imaging forming structure in a rainbow is different from what happens in a lens - it's more like a multiplicity of lenses, with each lens contributing within a narrow angle. It's a bit like what happens with a lenticular screen or a fresnel lens. I think it's a bit pointless to try to make the rainbow fit in with the more straightforward images that we see. Of course the image is not 'real' because the light behaves as if it comes from way behind the image forming structure. There is no parallax against distant objects so it can be classed as infinitely far away.Orodruin said:When you construct virtual images of objects using lenses, the rays take different paths as well, passing through different parts of the lens.
I don't think they do, exactly. The distances are so large that it would be difficult to spot but when you move to the left, the bow moves to the left, with you. So it would be moving across the sky relative to the distant stars. The centre of the bow is in line with the Sun and the back of your head. But a rainbow at night? Weird idea! Perhaps it's an experiment you could do with the Moon - if you could arrange the rain to come at the right time of the day and month. But you would need to travel quite a distance sideways to see the effect against the moonscape as a background. (many km to observe a recognisable movement of a fuzzy thing like a rainbow.spareine said:Everybody sees the rainbow at the same location in the celestial sky.
sophiecentaur said:I don't think they do, exactly.
You are saying that the angle subtended by the Earth from the Sun is negligible. Yes, that sounds reasonable. But how does that relate to the fact that a rainbow moves against the Earth as you move? Ah - it moves by the distance you move, which means the apparent change in angle against the celestial sphere is zero.spareine said:It is pure geometry. The Sun's celestial location is the same for everybody, at a given time. Hence the celestial locations of the antisolar point and the 42° circle around it are the same for everybody.
Well, this is not strictly true. The difference is an angle of the distance between the people (measured orthogonal to the solar direction) divided by 1 AU. Miniscule and not noticable by eye, but it is there.spareine said:It is pure geometry. The Sun's celestial location is the same for everybody, at a given time. Hence the celestial locations of the antisolar point and the 42° circle around it are the same for everybody.
This doesn't make sense to me because the image is in front of me whilst the Sun is behind me. Can you explain it please?Orodruin said:Technically, the virtual image of the sun which is the rainbow is not located at infinity, but in the same plane as the sun
No, the virtual image is behind you, but the light is coming from in front of you. There is nothing strange here, it happens with lenses too.sophiecentaur said:This doesn't make sense to me because the image is in front of me whilst the Sun is behind me. Can you explain it please?
When I was learning about mirrors, lenses and images in School, we used to use long optical pins on a board and established the position of an image using parallax. What you are suggesting is that a similar method would place the image behind me? That is hard to square with my (beware!) intuition because I couldn't actually do this with pins on one side and an image on the other. I guess your point is that the Sums place the image behind me. Of course, the rainbow is so diffuse that one can't actually 'focus' on it for an extra clue about its distance.Orodruin said:No, the virtual image is behind you, but the light is coming from in front of you. There is nothing strange here, it happens with lenses too.
I just realized that we are wasting effort debating semantics instead of science. Think of one person in New York viewing a rainbow, and another person in Chicago viewing a rainbow. Spareine would argue that both are looking at a virtual image of the sun. But no sane person would argue that these are the same rainbows.spareine said:Why denying virtual objects?
anorlunda said:But no sane person would argue that these are the same rainbows.
spareine said:Instead of calling someone insane, we could verbalize a distinction to avoid confusion: for some people rainbow means the 'rainbow cone' within the shower, for others it means the 'rainbow circle' in the celestial sky. Then:
- two persons see the same rainbow circle, but different rainbow cones.
- Through the water surface a reflected-rainbow circle can be seen, it is another circle in the celestial sky.
- the reflected-rainbow cone and the rainbow cone are not the same cones.
Calm down, dear boy. No one really believes that you're mad (no worse than the rest of us, at least). It really is just a way of looking at this, whether you say we all have our own rainbow or we 'share one'. Anorlunda's point is totally valid because you have to ask yourself which group of raindrops are you using. Is it the same as my group of raindrops? If we both happen to be looking at a few 'common' raindrops then you are seeing a different colour from the colour I am seeing. Either we all have our own rainbow or we alll share the same one. (Does anybody care?)spareine said:Instead of calling someone insane, we could verbalize a distinction to avoid confusion: for some people rainbow means the 'rainbow cone' within the shower, for others it means the 'rainbow circle' in the celestial sky. Then:
- two persons see the same rainbow circle, but different rainbow cones.
- Through the water surface a reflected-rainbow circle can be seen, it is another circle in the celestial sky.
- the reflected-rainbow cone and the rainbow cone are not the same cones.
You will have to give a reference or derivation of that, I'm afraid. Whilst we can see part of the rainbow proper 'in the surface' of the lake, the drops producing that are between the surface of the water and your eye (just the same as when we see rainbows against the grass in a field. I tried to decide how the reflection rainbow (not 'reflected rainbow') would be formed and the 3D geometry needed to describe what happens in a spherical droplet, off axis, is hard. But I can't see how any of this can explain the horizontal spectrum that's shown in the 'boat picture' without a formal ray tracing calculation. My diagram in post 26 shows how the bow must appear below and closer to the horizon - but can it really be that near to the horizon?spareine said:- Through the water surface a reflected-rainbow circle can be seen, it is another circle in the celestial sky.
The point that there are different raindrops causing the rainbow for different observers is a point well taken. But I do not think this is very different from two people looking at the same object through different mirrors. I would still call it the same object and if one of the mirrors disappear, one person will not see the object.sophiecentaur said:Anorlunda's point is totally valid because you have to ask yourself which group of raindrops are you using.
If you are talking about where the drops are which make the reflection, this is not true. It depends on how far away they are. The rainbow on the celestial sky as mentioned by spareine is a more useful concept and for that it the the same (virtual) image at infinity which is relevant (which is then also reflected in the lake). As to where the raindrops are which make you see this image, they are all in a cone and (for the reflected image) in the reflection of that cone in the water surface.sophiecentaur said:My diagram in post 26 shows how the bow must appear below and closer to the horizon - but can it really be that near to the horizon?
sophiecentaur said:You will have to give a reference or derivation of that, I'm afraid. Whilst we can see part of the rainbow proper 'in the surface' of the lake, the drops producing that are between the surface of the water and your eye (just the same as when we see rainbows against the grass in a field. I tried to decide how the reflection rainbow (not 'reflected rainbow') would be formed and the 3D geometry needed to describe what happens in a spherical droplet, off axis, is hard. But I can't see how any of this can explain the horizontal spectrum that's shown in the 'boat picture' without a formal ray tracing calculation. My diagram in post 26 shows how the bow must appear below and closer to the horizon - but can it really be that near to the horizon?
Why do you have the sun coming off-axis in your diagram, I wonder?spareine said:No need to be afraid, a flat mirror is basic geometrical optics. Rays from the raindrops contributing to the reflected rainbow could be extrapolated to A'. And there is photo evidence like this.
View attachment 94219
(About reflected/reflection rainbow: your diagram seems to be about the reflected rainbow. The picture of the boat shows a reflected rainbow, as anorlunda said. A reflection rainbow is a rare phenomenon seen in the sky.)
You don't have to arrange the rain to come at the right time of the day and month you could make a spray with some with water and a hose.sophiecentaur said:The imaging forming structure in a rainbow is different from what happens in a lens - it's more like a multiplicity of lenses, with each lens contributing within a narrow angle. It's a bit like what happens with a lenticular screen or a fresnel lens. I think it's a bit pointless to try to make the rainbow fit in with the more straightforward images that we see. Of course the image is not 'real' because the light behaves as if it comes from way behind the image forming structure. There is no parallax against distant objects so it can be classed as infinitely far away.
I don't think they do, exactly. The distances are so large that it would be difficult to spot but when you move to the left, the bow moves to the left, with you. So it would be moving across the sky relative to the distant stars. The centre of the bow is in line with the Sun and the back of your head. But a rainbow at night? Weird idea! Perhaps it's an experiment you could do with the Moon - if you could arrange the rain to come at the right time of the day and month. But you would need to travel quite a distance sideways to see the effect against the moonscape as a background. (many km to observe a recognisable movement of a fuzzy thing like a rainbow.