I can understand it if it was just a straight column; light travels in a straight line from the sun to us so that’s intuitive. I can even understand it if its only a slight curve parallel to the curvature of the earth if yada yada yada science.
But what I don’t understand is why it has the curve angle that it does. And do all rainbows have the same curvature?
Because you are only seeing the part that gets reflected to you. All the rest goes elsewhere.
Read this for a scientific explanation: Why are Rainbows Curved as Semicircles? | Physics Van | Illinois
Water droplets are spherical. I presume that the angle at which light can form a prismatic effect reflected against a spherical droplet is at a slight angle, and consequently forms a ring around the light source. That ring is then, presumably, cut off by the Earth, so you just see an arc.
The angle the rainbow makes, measured at the observers eye, and measured against the antisoloar point (the point on the dome of the sky that lies directly opposite the sun – it is below the horizon when the sun is above it) is about 42 degrees, the minimum deviation angle for l,ight passing through a spherical raindrop. as others have noted, that circle of light is cut off by the ground, but if you’re high enough in an airplane, or are looking downwards toward the antisolar point (as, for instance, whenm you’re standing at the top of Niagara falls looking down to the foot of the falls) you can see more of the circle, and potentially a full ciurcle.
The double rainbow makes an angle of about 53 degrees with the antisolar point.
There are other meteorological phenomena that have different radii, and may be centered on different points. The tertiary and quarternary rainbows, for instance, are both tightly centered on the sun, rather than the antisolar point, and are therefore virtually never seen in nature. There are various ice crystal haloes that are centered on the sun and the moon. The Glory is a rainbow-like phenomenon seen around one’s shadow on a cloud below (you can often seen them when you’re on an airplane). It makes an extremely small circle around the antisolar point.
There are some ice crystal arcs not centered on the sun or the antisolar point – the parhelic circle, the circumzenth arc, the Lowitz arcs. There have been rare and pathological reports of off-center rainbows of ucertain causation. There are rainbows created by the reflection of the sun from bodies of water (or, nowadays, mirror-sided buildings) that produce arcs centered about the antisolar point of the reflected sun. And there are elliptical bows and haloes due to pollen grains (which aren’t spherical) and therefore don’t have a single angle defining them.
Because, like me, it’s not straight. 
In a complex way, what CM is saying is that the rainbow is a reflection (and there are additional reflections or refraction effects). Basically, you stand with a light source behind you, and that light goes into the water droplets and is reflected back to you. Because the droplets are spherical, the light is reflected at multiple angles - due to the refraction effect of the spheres. If X is the angle between you and the water droplets with yellow reflected, then X+(a bit) is the angle for red, and X-(a bit) is the angle for blue.
So if you were floating high enough in the sky, you’d see a rainbow circle formed in front of you, centered on the point where the light source (sun) is directly behind you. Essentially whichever way you look, up, down, left, right, you se the same colour at the same angle between you and the light.
You only see an arc because on the ground, there’s no droplets between you but below the horizon to reflect the rainbow. If the water droplets in the distance are limited, you may only see on side of the rainbow arc. If the angle upward is too high, you may not be seeing the top of the rainbow because there are no droplets there.
Bingo, except that I wouldn’t call the rainbow a “reflection”, but purely for historical reasons – the Greek philosophers called it a Reflection, mainly because they saw that it always bore the same relationship to the sun and the observer. But calling it that got them into trouble when they tried to derive its cause and properties. It wasn’t until al-Farist and al-Shirazi and (independently) Theodoric of Freibourg did their own experiments with water-filled glass globes, and observed the sequence of refraction-reflection-refraction that the true mechanism was discovered. They even discovered how the secondary rainbow was produced by refraction-reflection-reflection-refraction. Theodoric drew up p[ictures showing how the different colors were produced. Al farisi and al Shirazi observed tertiary and quarternary rainbows in the laboratory.
Then everybody promptly forgot what they did, and a good explanation of the rainbow had to wait another couple of centuries to become widely known.
You can see the entire “ring” effect by putting a backyard hose on the mist setting and spraying between you and the sun. You’ll be able to find a piece of the arc in the mist and then trace it around in a complete circle.
Quite right, except that you don’t want the spray to be between you and the sun. You want yourself to be between the spray and the sun.
That may not sound right, but it is. Try it and see.
Doh, you’re right. Need to have the sun at your back.
The OP appears to believe that a rainbow should be straight, because light travels in straight lines. But no light is traveling along the arc of the rainbow. The light is traveling in a straight line from the Sun to each drop of water, and then in another straight line from the drop to your eye.
There is a phenomenon called the Rainbow Pillar, which appears to be a straight vertical section of a rainbow rising from the point where another rainbow strikes the ground. This picture shows one:
The “piullar” is actually a curved rainbow due to light from a reflection of the sun, as I mentioned in a post above. It doesn’t appear curved at first, because it’s in an unexpected rotation and location. But if you look at thsat image above closely, you’ll see that it does curve.
Here are pillars for both the primary and secondary rainbows:
In this image you can very clearly see that it’s curved:
https://images.search.yahoo.com/images/view;_ylt=AwrB8p.6nhBVGVwAigCJzbkF;_ylu=X3oDMTIyZWJsY2gwBHNlYwNzcgRzbGsDaW1nBG9pZAMyZGNmMjRmMmEwM2Y4YTFlMTFiNWFhMjUyYWJhMjlhZQRncG9zAzIEaXQDYmluZw--?.origin=&back=https%3A%2F%2Fimages.search.yahoo.com%2Fsearch%2Fimages%3Fp%3DReflected%2BRainbow%26fr%3Dyfp-t-901%26tab%3Dorganic%26ri%3D2&w=958&h=834&imgurl=th01.deviantart.net%2Ffs71%2FPRE%2Ff%2F2011%2F089%2F7%2F8%2Freflection_rainbow_by_darksilverflame-d3csq7f.jpg&rurl=http%3A%2F%2Fwww.rainbowgeeks.com%2F2012%2F08%2Freflection-rainbow.html&size=74.6KB&name=It%26%2339%3Bs+called+a+%26%2339%3B<b>reflection<%2Fb>+<b>rainbow<%2Fb>%26%2339%3B+and+it%26%2339%3Bs+produced+in+between+...&p=Reflected+Rainbow&oid=2dcf24f2a03f8a1e11b5aa252aba29ae&fr2=&fr=yfp-t-901&tt=It%26%2339%3Bs+called+a+%26%2339%3B<b>reflection<%2Fb>+<b>rainbow<%2Fb>%26%2339%3B+and+it%26%2339%3Bs+produced+in+between+...&b=0&ni=160&no=2&ts=&tab=organic&sigr=11rmqa6a9&sigb=12vn2a628&sigi=12tbalv8f&sigt=131u5jveg&sign=131u5jveg&.crumb=1A/YjAdBc4m&fr=yfp-t-901
When I looked up “Rainbow Pillar” I couldn’t find any of these. But I DID find images that are actually parhelia or sundogs, which can appear to be long and straight. They occur in different places than rainbows or rainbow pillars, and are due to iced crystals, rather than to raindrops. And, although they may not look it, they are curved, too.
Chronos once saw a 270-degree rainbow from his apartment nine floors up.
If you are driving along a road and see a rainbow, you will note that as you travel, the rainbow moves with you. But if you and a buddy are standing 100 feet apart looking at the same rainbow, then each of you will see the rainbow as being centered on yourself/himself. Thus, you aren’t even seeing the same rainbow. And if you are watching the same rainbow as you drive along that road, you’re actually seeing a different rainbow with every incremental move!
I actually tried to reach the end of a rainbow once. I was standing on a walkway of a building on a hillside, overlooking a football field (photo that I found on-line – Roosevelt High School, Honolulu). There was a rainbow that touched down right smack in the middle of that field. So I went down there. But when I got there, I found that the rainbow now touched down on the mountainside beyond (in the background in that photo).
Here is a photo of a full circle rainbow, as seen from an airplane.
You mean to tell me that the pot o’ gold was an illusion, too?
Tell it to this guy. He’s got pix so it happened. And that’ the last info from that other thread.
Why is a rainbow curved?
Because otherwise it would be a rainstripe.
Leo, who from that link “got pix”?
Sorry, I messed up the cite originally supplied by Colophon.
Rainbows have an end, until somehow they start moving.
Today I saw a rainbow inside a cloud. The cloud was very long, thin, and wispy/feathery/thready- not at all solid. The rest of the sky was clear and blue. Interestingly, the rainbow was actually straight across the part of the cloud nearest the sun. I’ve never seen anything like that!
