Insights Rainbows are not Vampires - Comments

[spareine]

And may be this helps: each raindrop of the rain curtain is represented by a single pinhole in the screen.

 

[sophiecentaur]

just as you cannot see an object behind a wall if there is no window in the object's direction.

A nearer parallel could be the limited field of view through a small mirror.

each raindrop of the rain curtain is represented by a single pinhole in the screen.

Yes. That, again, is fairly obvious at this stage in the discussion. In fact I really don't see where the last few posts have been going. A straightforward rainbow was explained to me when I was 15 years old or less and the whole class got the whole thing except the position of the image (not discussed ). Non-standard rainbows have been dealt with.
There are many images that work similarly to how a rainbow works. The result of smearing a lens or a window is due to an array of separate elements. A starburst lens filter is another example. In some cases, there is a real image and in some cases it will be virtual.

 

[Orodruin]

A nearer parallel could be the limited field of view through a small mirror.

Only if you put a lot of other mirrors arranged with their surfaces parallel to the first small mirror such that you never really lose sight of the object.

Edit: I just realized I have the mathematics skills to actually compute the angle of the rainbow caustic ... I wonder why I never thought about that in the 17 years since I learned calculus ... Anyway, it is a neat little exercise in geometry and calculus I can recommend to anyone who has not tried it yet. It also shows why there is no rainbow from the light exiting the raindrops without an internal reflection in between.

 

[Buckleymanor]

I will have to think about this some more.
Perhaps someone should start a survey: Where is the image of the rainbow?
To make it simpler, perhaps we should reduce it to one raindrop, as natural rainbows are a mess of parallax, angles, a 3D refraction screen, and a non-point source of light.
I was thinking about someone's comment; "No two people see the same rainbow". This got me thinking, and I came up with the question; "At what distance, do each of our eyes not see different rainbows"? Perhaps we should shut one eye when talking about rainbows. That would cut the confusion in half, at least for me.

From anorlunda's image, It is my vote, that the "rainbow image" should be located at the surface of the south pole of the raindrop, where the light finally exits.

I'm fairly certain that the water hose "water shape & flow selection device" was set on "mist", so the globs of water would have been nearly spherical.
Likewise with my "Hippies & Rainbows" image. Everyone was standing under a "mister", as it was somewhat hot that day.

The picture of my 90° intersecting rainbows was caused by a reflecting source.

The second bow being created by the sun reflecting off of the convex back window on one of my cars.

So according to anorlunda; "I think you mean "reflection" rainbow, not "reflected""
It is an example of a natural, and a reflection rainbow, captured in a single image.

Sorry about the late reply computer down.
The comment about no two people see the same rainbow is interesting in as much as that no one person sees the same rainbow or is it that we all see the same rainbow effect but from a different perspective.
I don't know if you have ever walked along the see shore with the sun shining low in the sky.You get a bright band of sunshine reflecting off the see from the horizon up towards your feet, as you walk along the bright band follows.So like the rainbow you can deduce that the bright band actually covers the whole of the see depending on which position the individual observer or observers are placed.Some one further up the beach would see a bright band just like you do but if you looked back towards him you would not see his band just the same as he would not see yours.

 

[sophiecentaur]

The comment about no two people see the same rainbow is interesting

Yes, it is interesting; more so because the form of the rainbow is exactly the same for everyone. This contrasts with the different view that two people have of most other things they may both be observing. We say that we all see the same Moon but the views from different parts of the Earth is measurably different. This conversation says more about People than about the Physics of the situation.

 

[Orodruin]

Yes, it is interesting; more so because the form of the rainbow is exactly the same for everyone.

But this is precisely what you expect for an object at infinity! So why have you (seemingly) been arguing that the rainbow (as a virtual object) is not at infinity?

 

[sophiecentaur]

I can put the image of a microscope sample at infinity. Can anyone else see that? What has the image distance got to do with it?
I really don't know where you are going with this.
We can all see the same image on TV and the image is a few metres away.?

 

[Orodruin]

I can put the image of a microscope sample at infinity. Can anyone else see that?

This has to do with the aperture, not the location of the virtual object. Just like seeing the rainbow is dependent on the aperture in the form of the raindrops. The rainbow itself as a virtual object is not at the raindrops, but at infinity. There is a family of rays which are parallel, corresponding to an image at infinity, whether you see it or not depends on the aperture.

I really don't know where you are going with this.

I am only trying to understand how you can have the view that the rainbow as a virtual image is at the raindrops at the same time as you consider it to not change depending on the observer - a typical property of an image at infinity.

 

[sophiecentaur]

how you can have the view that the rainbow as a virtual image is at the raindrops

I don't have that view. Where did I say that? Was it because of my comments about seeing a rainbow 'in front of' a field of grass? I would argue that I do and I assume most people do, also (due to the stories of pots of gold etc.) Where we place an image (mentally) need have nothing to do with the optics of the situation because our brains don't just use optics when we build up a scene in our minds. We 'see' a rainbow in a field because out brain tells us that we could not actually be seeing 'through' solid Earth and grass. We perceive the rainbow moving across the field as we walk. Interestingly, it is quite the opposite when we see an image in a mirror because we are quite prepared to believe there's a whole roomfull of stuff in that solid brick wall.
I think we would not be having any of this discussion is we could be totally objective about what enters out eyes. If you want to discus just the Physics of the Rainbow then that's ok but the thread would need to have been terminated way back.

 

[Orodruin]

I don't have that view. Where did I say that? Was it because of my comments about seeing a rainbow 'in front of' a field of grass?

Perhaps, I just got that impression over the course of the thread.

I think we would not be having any of this discussion is we could be totally objective about what enters out eyes. If you want to discus just the Physics of the Rainbow then that's ok but the thread would need to have been terminated way back.

Of course we cannot be objective, our brain plays tricks on us all the time, which is why I have always focused on the physics aspect. If you have been talking about the perception all the time this would explain why we talk around each other.

 

[sophiecentaur]

If you have been talking about the perception all the time this would explain why we talk around each other.

Yes. You are right. The reason for my treating that rainbow as 'something different' is that it's treated by many (most) people as such. It's in the same neck of the woods as Colour. So many people treat that as a matter of Physics when it's psychology and perception.
Do you think we could draw a line under this one? I have found that airing the topic has been very useful. Placing the virtual image was particularly good.

 

[spareine]

There is an objective physics aspect that somehow got lost in this thread. If the Sun was replaced by a point-like light source with a line spectrum the rainbow would become a focusable object. Photos of that rainbow would be blurred unless the camera was focused at infinity.

 

[Janus]

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.

That's how I make them.

I am not sure if they are not 3D objects.

Ha ha! The semantics of science. The two "nots" cancel and your phrase becomes; "I am sure if they are 3D objects".
Which, if you throw out the "if", looks suspiciously like "I am sure they are 3D objects".
I suspect they are 3D objects.
Where the hell is @Janus when you need a ray-trace guru??!?

If you stood in the same position and took a series of photos of the rainbow would the rainbow appear as a sphere when the photos were combined together.Provided there was enough time to do it.

Now that sounds like a grand experiment!

ps. Rainbows are complicated. I like that about them.

Running a true rainbow effect in POV-ray would be quite a challenge (POV-ray has an rainbow object, but is just a simulation). It has its limitations. While it can deal with refraction and reflection, in order to get effects like light bouncing off a reflective object illuminating another surface or for light passing through a refractive material to produce accurate caustics, you need to use a feature called "photons", which adds a great deal to the processing time. As a result, I had to limit myself to working with single objects, and even then I had to keep the dispersion samples (which determines how many colors the prismatic effect produces) down to a fairly low number which means the rainbow effects aren't all that smooth.

Anyway, this is what I was able to come up with. the first is a cylinder seem end on, with the light coming from the right. The second is a "raindrop" or sphere. I don't know how illuminating they are towards the discussion, but they interesting to look at.

One thing I should mention is that these are not perfectly accurate physical representations. One limitation is that once the white light is broken up into its spectra, the refractive index for each color does not vary. In reality, each color would refract slightly differently. For instance, in the POV-ray model after passing light through a prism, passing it through a second prism will not reassemble the spectrum back into white light.

 

[A.T.]

If the Sun was replaced by a point-like light source with a line spectrum the rainbow would become a focusable object. Photos of that rainbow would be blurred unless the camera was focused at infinity.

Does the focus of the camera affect how sharp natural rainbows appear in the picture?

 

[OmCheeto]

...I don't know how illuminating they are towards the discussion, but they interesting to look at.
...

As long as they are inspirational, they are illuminating.

I just watched Walter Lewin's "Rainbows and Blue Skies" for the third freakin' time, and his "The Hidden Beauty of Rainbows" for at least the second, because of your images.
I also reread the insight article. It was better the 4th time around.
I will have to read the comments again, after my nap, as I still don't know where the rainbow is.

One thing I should mention is that these are not perfectly accurate physical representations. One limitation is that once the white light is broken up into its spectra, the refractive index for each color does not vary. In reality, each color would refract slightly differently. For instance, in the POV-ray model after passing light through a prism, passing it through a second prism will not reassemble the spectrum back into white light.

Mere technical difficulties.
Have you ever started, or been involved in a ray-tracing thread?
I worked out, mentally, how to optimize them, decades ago.
Unfortunately, life got in the way, and disrupted implementation.
Kind of like, naps...

 

[sophiecentaur]

Does the focus of the camera affect how sharp natural rainbows appear in the picture?

This has been worrying me. The raindrops can be, indivudually, very small (<1mm) and diffraction could be relevant but I can't be sure of the appropriate calculation, bearing in mind the enormous range of distances involved - between a couple of metres and a km. The diffraction limit for a 1mm aperture at 600nm is about 6 e-6 (in radians). How this could relate to the effect on focussing the line (arc), I can't be sure. Also, as Walther Lewis says in his lecture 'Rainbows and Blue Skies' in the above post that there is a 'narrow peak' in the range of angles which are wavelength selective. He makes the point that the region where the bow occurs is the only place where the wavelengths are individually selected. The sharpness of an arc will depend on how narrow this peak happens to be.
If anyone has access to a laser and a hose, they could do an experiment for us and tell us (or use a camera to show us) the sharpness of the arc and the effect of focussing at different distances.

 

[anorlunda]

This picture is reported to have gone viral today, from Glen Cove, NY. Most people would say that it is four rainbows, but since there is only one virtual image at infinity, others would argue that it is only one.

Edit: Also from today's news https://www.washingtonpost.com/news...ical-phenomena-captured-in-one-amazing-photo/

 

[sophiecentaur]

Most people would say that it is four rainbows, but since there is only one virtual image at infinity, others would argue that it is only one.

I refuse to be drawn in on this one! All those pictures are very pretty, though. I wish I could have been there.

 

[Orodruin]

Most people would say that it is four rainbows, but since there is only one virtual image at infinity, others would argue that it is only one.

I do not think anyone would argue that. The one image at infinity is predicated on there being one point source of light at infinity, which is not always the case. Optical effects may lead to several images (as in the later photos) being reflected by the raindrops.

On a side note, I saw sun dogs for the first time (at least as far as I have noticed) just about a year ago. I was amazed at how strong they can actually become.

 

[OmCheeto]

This picture is reported to have gone viral today, from Glen Cove, NY.
...

Does it mean I'm obsessed with rainbows, if I admit that I recognize this one?

What caused the Quadruple Rainbow? [PF]
21 April, 2015

I think it's true what Walter Lewin said in the last two minutes of his "The Hidden Beauty of Rainbows" lecture, regarding a new fascination, once you know how they work. (Or at least, think you know how they work.)

"This is like a disease
You can't resist it
And it's all my fault
I cannot cure you any more
It's too late for that
It is a disease for life"

--- Walter Lewin​

 

[Keith_McClary]

http://www.atoptics.co.uk/rainbows/seabow.htm

 

[anorlunda]

http://www.atoptics.co.uk/rainbows/seabow.htm

Wow! It gets more and more interesting. Bows in salt water. Bows in glass. It makes me wonder what other mists, what other solids, can make bows.Thanks for sharing.

 

[chemaster]

I wonder how a physicist can struggle so much with such an incredibly simple phenomenon as rainbows.

 

[sophiecentaur]

I wonder how a physicist can struggle so much with such an incredibly simple phenomenon as rainbows.

It's refreshing to read of someone who finds the topic so straightforward. I hope your confidence is well founded.

 

[spareine]

Wow! It gets more and more interesting. Bows in salt water. Bows in glass. It makes me wonder what other mists, what other solids, can make bows.

Rain made of diamond droplets could not make a primary rainbow. Only higher bows, because of the high index of refraction. [1]

 

[OmCheeto]

Has anyone else noticed that the peephole lens in their front door

makes rain-bowish spots, on the chimney of their wood stove?

 

[anorlunda]

Here's another something that vaguely resembles a rainbow. At least it is bow shaped with the roughly 42 degree bow size, but the explanation is very different.

Image Credit & Copyright: Dave Lane; Rollover Annotation: Judy Schmidt

http://apod.nasa.gov/apod/ap160127.html

Explanation: Why would the sky look like a giant fan? Airglow. The featured intermittent green glow appeared to rise from a lake through the arch of our Milky Way Galaxy, as captured last summer next to Bryce Canyon in Utah, USA. The unusual pattern was created by atmospheric gravity waves, ripples of alternating air pressure that can grow with height as the air thins, in this case about 90 kilometers up. Unlike auroras powered by collisions with energetic charged particles and seen at high latitudes, airglow is due to chemiluminescence, the production of light in a chemical reaction. More typically seen near the horizon, airglow keeps the night sky from ever being completely dark.

That's interesting, but on PF gravity waves usually mean something else. The explanation also does not explain why it is bow shaped.

 

[sophiecentaur]

but on PF gravity waves usually mean something else.

I was confused when I first read the term in a non-GR context. All it means is a surface wave on a fluid (often water) where the restoring force ( to make the water level) is gravity - just plain old waves. They can happen at the interface between any two fluids, aamof. I think it's what you can see when you look from a mountain top at a layer of cloud, below you (when there's no detectable wind) and the cloud layer seems to form slow motion waves which appear to 'crash' on the mountain slope. Good excuse to stop climbing and just enjoy it.

 

[Keith_McClary]

Rainbow with spokes:

Snapped from moving car in BC, Canada .
Explanation of a very similar image here .