Does a rainbow exist?
December 16, 2017 10:07 AM   Subscribe

You'll often see/hear someone saying "Did you see the rainbow?" Or "Oh yeah I saw that same rainbow". But did they?

The rainbow is simply the defracted light entering our eyes. It's appearance to you is based on your location in relation to the position of the sun and the water droplets in the air. So if I'm in one place and someone is a mile or two to my right or left...

Do we see the same rainbow? Does a rainbow really "exist"?
posted by humboldt32 to Science & Nature (14 answers total) 2 users marked this as a favorite
You could also ask, "Did you see the same parade?". If you think of a rainbow as an event rather than a thing, it makes a difference. Maybe?
posted by amtho at 10:10 AM on December 16, 2017 [9 favorites]

Do rivers exist? A river is just a flow of water molecules in a certain pattern. A rainbow is a flow of photons/light waves in a certain pattern.

In fact, if we're entertaining silly philosophical arguments, I could argue that rainbows are one of the few things you can actually see. When you look at, say, a toaster, what you are actually seeing is light bouncing off of the toaster. When you see a rainbow, you see the actual rainbow because it is made of light.
posted by Zalzidrax at 10:22 AM on December 16, 2017 [10 favorites]

Of course you don't see the whole of a rainbow at once, but you never see the whole of anything at once.
posted by Zalzidrax at 10:22 AM on December 16, 2017 [3 favorites]

If they live on your block and you discuss the rainbow of the last evening, bets are it's close enough for jazz. But really we don't even see the same universe, any of us, even identical or co-joined twins.
posted by Oyéah at 10:27 AM on December 16, 2017

Can two people at different locations see the same cloud? It's appearance to you is based on your location in relation to the position of the sun and the water droplets in the air.
posted by Homeboy Trouble at 11:08 AM on December 16, 2017

Rainbows are surprisingly complicated physical phenomenon. But, they extend a few degrees in viewing angle and often occur thousands of meters above the ground. So, anyone within a few km is probably seeing the same specific phenomenon. If you're hundreds of km away, it's probably a different rainbow. Between the two, it's hard to say without lots of detailed measurements and math.

(I know of one fantastic rainbow physics summary, but it assumes a lot of technical background in optics. I'm afraid I don't know of one that's more useful for people who haven't already spent years majoring in physics.)
posted by eotvos at 11:14 AM on December 16, 2017 [9 favorites]

I know it's breaking the guidelines to disagree with another commenter, but glancing over the link eotvos posted it looks super awesome and I'm going to read it in more detail when I'm not on my phone -- and I've only had the two semesters of physics that were a basic requirement at where I went to school. So don't be put off if you've never studied optics!

posted by yohko at 11:37 AM on December 16, 2017 [2 favorites]

I say you can definitely see the same rainbow, and it does exist as a perceived thing. A friend was married in Boston, and several of us traveled to it from the west in different cars. All of us went down Belmont Hill on Rte 2, which affords a panoramic view of the Boston skyline. We all arrived at the wedding and started talking about the awesome triple rainbow* we'd seen over the city. We were seeing it at slightly different times, from the same location.

* Three rainbows, one inside another. I have read of doubles, and seen some kind of tepid photos, but this sucker was stunning - very distinct in all three parts, with the area inside the inner arc very dark gray.
posted by Kirth Gerson at 11:47 AM on December 16, 2017 [1 favorite]

@yohko, indeed I wasn't put off at all. My dad was an optomechanical engineer for over 40 years, he was one of the original developers in the LASER field and Scientific Americans filled our house. I've been reading the article already. Thank you eotvos.
posted by humboldt32 at 12:01 PM on December 16, 2017 [3 favorites]

Strictly speaking, no, not really. The focus point of the rainbow we each see is unique (and changes when we move, as well as from person to person). What we think of as a rainbow is more like "the conditions that allow a rainbow to be seen", and without a lens of some sort, the rainbow as we think of rainbows does not exist.
posted by notquitemaryann at 12:04 PM on December 16, 2017 [6 favorites]

Warning: plate of beans

I think one could argue for the sameness of rainbows roughly based on the contiguity of locations (in space and time) where a rainbow can be seen.

Say you're looking at a rainbow and you step a foot to the left. It's still visible from there, and it was continuously visible throughout your movement. It seems natural to say that's the same rainbow. More precisely, you're still in the same rainbow-viewing region.

Let's say you move 10 miles to the left. After 1 mile you no longer see a rainbow. After 9 miles you see a rainbow again. It seems natural to say that's a different one.

This gets a little questionable though. Suppose you move only 10 feet to the left and the rainbow disappears and comes back along the way. I think most people would say that's the same rainbow. And possibly both points are indeed in the same rainbow-viewing region--although you took a path that temporarily exited the region, there exists some other path that you *could* have taken (e.g. 5 feet forward, 10 feet to the left, 5 feet back) where the rainbow would have been continuously visible.

But I guess you probably wouldn't have such a well-behaved region most of the time. If you drew a map of the region you wouldn't have a nice solid shape, you'd have a splatter of smaller shapes, disconnected from each other, but still close enough that most people would say the rainbows are the same. So connectedness can't be a requirement, but nonetheless you can informally say that this "cluster" of regions should all count as one big region and therefore the same rainbow.

If I understand right (not guaranteed :)) the regions for different people might have very slightly different borders based on the geometry of their eyes(?). But surely you can say whether their regions have enough overlap in time and space to be considered the same.

So although you'd have a hard time coming up with a rigorous mathematical definition, IMO "same rainbow" is a legit concept.
posted by equalpants at 1:56 PM on December 16, 2017 [3 favorites]

I would say it's the same as seeing a reflection of the sun in the panes of glass on a large office building. It "exists" in the sense that light is physically being cast onto the street below but the "image" has a position relative to your own eyes. Any nearby observers are seeing the same image but each in a different window.
posted by bonobothegreat at 10:28 PM on December 16, 2017 [2 favorites]

Hmm, of course I am at home and this not actually with my copy of Light and Color in the Outdoors, which is my favorite atmospheric phenomenon book. It is a little less technical than eotvos’ link (although still assumes significant physics background) and has some possibly relevant comments— especially for equalpants’ plate of beans above. In particular, it mentions that the generic distance of drops forming a rainbow is c. 1-1.5 km away. So, if anyone knows about how wide the sort of rainstorm is that also allows for a clear view of the sun, then we can figure out how far you can walk while still seeing “the same” rainbow. We might also care to know the rough angle at which a rainbow appears, because surely it’s still the same rainbow by equalpants’ definition even if you only see a part of it— so even if I go far enough that I can see only the left part of a rainbow, while you go so far you can only see the right, they’re still the same thing by that definition.
posted by nat at 1:13 AM on December 17, 2017 [1 favorite]

A rainbow exhists because you can make an image of it.

The light from the sun is reflecting and refracting at a very particular angle to your eyeballs, and you’re looking through an ever-changing mix of raindrop lenses as they fall to earth, but the physical phenomenon can be captured by a mechanical eye of the camera. It exhists.

But even if you are standing next to your friend, the angles of light distortion making the optical phenomenon are particular to each of you, so actually anytime we are looking at a rainbow, each person looking is seeing their own.

Sort of a beautiful metaphor for the human experience that way I have always thought.
posted by wowenthusiast at 7:59 AM on December 17, 2017

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